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PRESSURE TRANSDUCER TRAINING
Pressure Transducer Articles
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• Hidden Pressure Hunt
James Garrido - Have Scanner Will Travel
This month we will use our in cylinder pressure transducer to take a look at a defect that many shops have trouble nailing down even by other more time consuming methods. This defect is a clogged catalyst on only ONE bank of a 2 bank engine equipped with a dual catalyst system.
When only one catalyst on one bank has become restricted the typical complaint is a lack of overall engine power. And if the restriction is severe enough miss firing on only one bank and not necessarily on the bank with the defect! This bank isolated miss fire is due to the effects on airflow through the engine with one side of the engine restricted. When one bank is flow restricted the result is less than 50% of the total air entering the engine ends up being passed through the restricted bank. Yet 50% of the fuel mass is still delivered via the injectors to that restricted bank. As a result one bank ends up very lean and the other bank over rich. The first telling sign of an unequal airflow due to restriction on a V-type engine are the fuel trims. If the fuel trims are moving in increasingly opposite directions bank to bank as engine speed and load increase you need to check for a restriction.
Figure 1 is a P0300 Failure Record captured on code set from a 2003 Chevrolet pick up with a 4.8L cam in block V8. Notice the fuel trims in the capture. Bank one is the side with the clogged catalyst and as such has a negative total fuel trim (STFT+LTFT) correction of -21% while bank 2 is showing a positive total fuel trim correction of +25%.
How would you diagnose this condition to be a clogged exhaust before purchasing that expensive catalyst? Drop the exhaust and test drive with an open system? And risk needless broken bolts, not a great idea in the rust belt states. A vacuum gauge reading at idle and 2500 RPMs is only useful on severely restricted catalyst systems and will not show a partial restriction on only one bank. You could install a back pressure gauge in place of the front oxygen sensor and then power brake the engine under load at a specific RPM. Then move the back pressure gauge to the other side and compare at the same load. My experience with this is unless the catalyst is severely restricted the gauge bounces around so quickly it is difficult to see if the average needle movement is higher side to side. Pulling hard to get at O2 sensors on a hot engine is no picnic either.
I prefer removing one spark plug from each bank in turn and installing a pressure transducer. Then look at the saved pressure waveforms so that you can get an easily acquired and highly accurate comparison of in cylinder conditions bank to bank.
To do this test start at idle speed so that you have a relatively slow crankshaft speed and a plenty of engine vacuum. Then snap the throttle open. The engine vacuum drops as outside air rushes in to fill the low pressure area in the intake. Initially the crankshaft has not yet begun to spin faster so the extra air has lots of time to fill the cylinders before it attempts to exit the exhaust ports.
In figure 2 I have captured bank 2, cylinder 6, the good side of this engine. Compare these pressures waves seen from the good side to the clogged bank 1, cylinder 7 shown in figure 3. In the zoomed out capture shown in the lower right corner of the images you can see that the cylinder from the restricted bank built up more pressure on average than the cylinder from the unrestricted bank.
. If we zoom in on the highest in cylinder pressure peak section of the wave we can actually see the cycle by cycle difference between the cylinders on the different banks. On the bad cylinder you can see that the end of the exhaust stroke pressure wave plateau peaks at a high of 0.78V even after the pressure drops slightly as the intake valve opens during the overlap period. On the good cylinder you can see the exhaust stroke pressure wave plateau peaks at only 0.53V and then permanently drops off as the intake valve opens during the overlap period. The scaling on this sensor is 1.0V = 75 PSI so the variation between the restricted and unrestricted cylinders is 0.25V or 18.75 PSI! It is impossible to get this kind of accuracy and detail using older methods for testing for back pressure. Even a slightly clogged catalyst will show up by using in cylinder pressure measurements.
• The Pressure is On
James Garrido - Have Scanner Will Travel
While engine mechanical diagnostics dates back to, well, ever since the first engines were made, the way we diagnose those defects continues to evolve. The toughest aspect of engine mechanical diagnostics may not be as sophisticated as electronic control systems diagnostics but due to disassembly requirements inherent in mechanical inspection it can be just as time consuming. However there are some relatively new techniques we can use to pinpoint engine mechanical defects in both a timely and accurate manner without disassembly by using pressure transducers. A transducer is anything that senses one type of physical quantity and outputs another physical quantity in proportion to the first.
In this case I am speaking of an “In Cylinder Pressure Transducer” which reacts to cylinder pressure by outputting a corresponding electrical signal to an oscilloscope. This pressure transducer is installed in the cylinder in the place of a removed spark plug. Then the engine is cranked, started and run while the transducer is used to graph pressure changes in the cylinder as the pistons is moving and the valves are opening and closing the way they would during normal combustion. Only there is no combustion due to the spark plug energy being shorted to ground to facilitate use of the tester. In this way we can watch each of the mechanical aspects of the engine and determine if any part of the equation is defective with out disassembling the engine!
In figure 1 we will take a brief look at what a good in cylinder pressure waveform should look like relative to possible cam timing issues. For a much more detailed explanation of the information contained these types of waveforms visit see the following article: "Anatomy of a Compression Waveform"
On a normal engine as the piston begins to rise from the 180 degrees of rotation the BDC marker should dissect the exhaust stroke pressure rise ramp at approximately 50% of the total height of the waveform. If the 180 degree marker falls as much as 10° below or 15° above the 50% point, exhaust cam timing is within a normal range.
When viewing the intake stroke pressure drop ramp, a cursor placed 20 degrees after the 360 degree TDC marker (or 380 degrees). This cursor should intersect the downward slope of the waveform at the 50% point, give or take 10°, to be within a normal range.
Normal cam timing design tends to fall in these ranges due to the influences of ambient pressures and the requirements of fuel mileage and emission control priorities. Non-normally aspirated engines, vario-cam and high performance engines may vary further from these general specifications but not drastically.
Now let’s look at a 1995 Toyota Tacoma with a complaint of poor idle and low power on acceleration. This truck has a 3.4L DOHC V6 engine. There were no DTCs stored and all the fuel trim values were a negative 5-8%. I put a vacuum gauge on the engine and found that at idle we had only 8”Hg of vacuum. At 3000 RPMs the vacuum just barely improved but did not drop so I did not believe the problem was an exhaust restriction. I asked the shop if they checked the camshaft timing. They stated that they had pulled to timing covers and checked the cam and crank gear timing marks and that they were exactly where they should be. I decided to check the CMP and CKP sensor waveforms to see if at least the bank #2 camshaft was lined up properly. The bank one cams had no CMP sensor to look at for comparison. The waveform shown in figure 2 matched perfectly with 2 known good waveforms I found on iATN. Still believing the cams may be out of time I installed a pressure transducer in the bank #1 cylinder #5 spark plug hole and ran the engine. As you can see in figure 3 the pressure rise is occurring late (retarded) completely after the BDC mark. Installing the pressure transducer in the bank #2 cylinder #2 spark plug produced exactly the same retarded pressure waveform figure 4.
With both sets of cams being out of time on both cylinder heads the exact same amount and a CMP/CKP waveform that was in alignment, there was only one possible explanation. The crank shaft had advanced independently of the crankshaft timing gear and the entire rest of the valve train. As you can see in the photo in figure 5 the crankshaft key way gouged into the crankshaft gear allowing the crankshaft to advance without the rest of the valve train.
When the pressure is on to come up with an accurate diagnoses in a short amount of time nothing beats pressure transducer waveforms! We’ll explore different engine mechanical defects using in cylinder pressure waveforms in future columns.
• Anatomy of the Compression Waveform
by Bernie Thompson - ATS
The compression waveform produced from the internal combustion engine holds the key to determine if the mechanical condition of the cylinder is in good working order or if there is a deficiency within the mechanical condition of the cylinder.
It is necessary to break the waveform down into several divisions in order to make a determination of the cylinder condition. In figure 1 (taken with a 300psi transducer) and in figure 2 (taken with a -30hg transducer to increase the resolution of the exhaust plateau) a compression waveform is shown from a good spark ignition engine at idle and has key points of the waveform marked. Note that the compression waveforms, figure 1 and figure 2, were taken from the same spark ignition engine. At the top of the waveform point A marks the peak pressure that occurred. This point will correspond to the point at which the piston position came within the closest distance to the cylinder head. This pressure is the sum of compression from point K to point A. The amount of pressure built will depend on the volume between the cylinder head and the piston, when the distance from the piston to the head is at its closest point. This peak pressure point will represent the top dead center (TDC 0º) position of the piston’s movement. This is the point at which the piston has come to rest and is no longer in movement. This occurs when the crankshaft has reached the end of its stroke. This pressure will change due to the operating condition the engine is running under.
When the engine is in a cranking condition the compression on a spark ignition engine should be about 130lbs/square inch (psi). When this cranking pressure drops below 90psi it is an indication that the pressure within the cylinder is no longer adequate to support combustion of the hydrocarbon chains. When the engine is in a running condition the compression at idle should be about 70psi. When this running pressure drops below 40psi a misfire will occur. This is an indication that the pressure within the cylinder is no longer adequate to support the combustion of the hydrocarbon chains.
During a snap throttle compression test the idle compression pressure should increase by about 3 times. As the crankshaft rotates past the top dead center position the piston starts to move away from the cylinder head. This allows the volume between the cylinder head and the piston to increase. Under this condition the peak pressure that has been produced will start to decrease.
If the compression tower is measured from its lowest point, D, to its highest point, A, and this pressure is divided in half; then this point should occur at 30º after top dead center. This point is indicated by point B, halfway down the compression tower. The piston will then continue to move away from the cylinder head increasing the volume between the head and piston. The piston velocity will continue to increase until the crankshaft has reached the 90º position. The piston was at rest at top dead center and, as the crankshaft rotation continues, the piston speed increased until it obtained its maximum velocity at the 90º point. From this 90º position to the bottom dead center (BDC) point the piston will slow its velocity down until it reaches BDC and stops. The piston movement has now reached its half way position at 90º of crankshaft rotation.
In this first 90º of crankshaft rotation the cylinder has totally decompressed and now enters into a negative pressure or vacuum state. The piston continues its downward travel building more negative pressure within the cylinder. At the point the exhaust valve opens, point D, the piston travel is still moving in a downward direction but, the cylinder pressure starts to rise. This is due to the pressure in the exhaust being higher than the pressure in the cylinder. The cylinder pressure will continue to rise until it is equal with the exhaust pressure, point F. This exhaust pressure change should occur at the point the piston has decelerated to a stop or has obtained BDC 180º. The pressure change from point D to point F is referred to as the exhaust ramp. The target point is for the center of the exhaust ramp to be equal with the BDC 180º point, figure 3. At this point the exhaust camshaft timing is correctly timed to the crankshaft. If this exhaust ramp crosses the BDC 180º position within -10º to +15º of this target the camshaft is in proper time of the piston position.
On some performance based engines it is normal for the exhaust cam timing to be advanced and can still be in proper time with a +20º of this target. The piston being at bottom dead center is not in movement. The crankshaft continues to rotate which in turn moves the piston. The piston now starts to accelerate in an upward direction on the exhaust stroke. This forces the contents of the cylinder out of the cylinder into the exhaust system. The piston crosses the half way point, 90º position, reaching its maximum velocity and then starts to slow down and stop as it reaches the TDC 360º position. Approximately 15º to 30º before TDC 360º the intake valve will open. This pressure change can be seen at point G however, in different engines this pressure change may not be apparent.
When the piston is coming to a stop and the intake valve opens the piston has very low velocity. The exhaust valve is still open at this point and will equalize the cylinder pressure to the higher pressure that is within the exhaust system. When the piston reaches TDC 360º and then starts to move away from the TDC 360º position in a downward movement, the negative pressure will overcome the exhaust pressure within the cylinder and the cylinder pressure will decrease. The pressure decreases until it equalizes with the intake manifold pressure. The intake manifold is in a negative state of pressure or a vacuum. This intake pressure change creates the intake ramp, point G to point I. The exhaust valve will now close at approximately point I. This intake ramp should start to drop at the TDC 360º position and equalize with the intake pressure by the 60º mark after TDC 360º, point I. If the pressure from point G to point I is divided in half this target point should occur at 20º after TDC 360º, figure 4. This indicates that the intake camshaft is in time with the crankshaft. If the intake ramp crosses the TDC 360º plus 20º position within -10º to +10º of this target the intake camshaft is in proper time with the piston position.
On variable valve timed (VVT) engines the target for the center of the intake ramp is TDC 360º +30º within +/-10º. The intake pressure at point J should be approximately equal to the exhaust pressure at point D. This is due to the intake manifold pressure, point J, being compressed to the peak pressure and then decompressed to this starting pressure, which should be equal to point D.
The exhaust plateau, point D to point I, is created by the pressure differential within the intake manifold or the vacuum that is contained in the intake manifold. As this intake vacuum changes so will this exhaust plateau. For example, figure 5, when the engine is in a cranking condition the engine can only produce 1 inch hg to 3 inches hg of intake manifold cranking vacuum. With this reduced intake manifold vacuum the exhaust plateau will also be reduced or will decrease in its definition. With this decrease in the exhaust plateau’s definition the exhaust plateau will change in the way that it appears and is used. Since the height of the plateau is based on only 1 to 3 inches hg, this plateau will no longer cross the bottom dead center 180º mark or the TDC 360º +20 mark. The intake manifold vacuum will need to be much greater in order for the exhaust plateau to have enough height or pressure change for these exhaust and intake ramps to cross their targets. Since the exhaust and intake ramps cannot be used to check cam timing during a cranking condition, the valve openings must be checked instead. The exhaust valve opening should occur 30º to 50º before BDC 180º. The intake valve opening should occur just after TDC 360º. The intake valve closing should occur 30º to 60º after BDC 540º. If these targets are met the camshafts are timed closely enough for the engine to start however, the camshaft timing could still be up to 1 tooth out of time. In order for the cam timing to be known the engine must be at a steady idle state. The piston then continues to increase its velocity in a downward direction until it reaches the 90º position. At this point the piston has reached its maximum velocity. The piston then continues to move downward, slowing until it reaches the stopping point or BDC 540º. The crankshaft continues to rotate and the piston starts to move in an upward direction but, the piston velocity is low. At this point the intake valve is still open so the pressure is equalized by the pressure within the intake manifold. The intake valve closes at point K and the cylinder pressure begins to rise. This intake valve closing should occur at approximately 50º after BDC 540º. The piston continues to travel in an upward direction, gaining velocity until it reaches its maximum velocity at 90º. The compression ramp at this point is clearly increasing in pressure. The piston continues to travel upward and is now slowing its velocity as it approaches 30º before TDC 720º point. At this point the compression should be halfway between the minimum pressure and the maximum pressure, point M. The compression then continues to build until the piston slows down and reaches a stopping point at TDC 720º. It is important to note that most of the compression pressure is produced in the last 30º of crankshaft rotation.
• Pressure Transducers
by Bernie Thompson - ATS
It was black with a two inch brim; the inside had a red satin liner. By all accounts it looked like a normal top hat that anyone could be wearing. The man placed the top hat on the table where, in an instant, he had reached into the hat and out came a white rabbit! How did the rabbit appear, was it magic or mechanics? Once there is an understanding of what has happened it is no longer magic; but only physics.
For over one hundred years mechanics have been diagnosing the internal combustion engine. Over the years many tools have been developed to help with this process and with the advent of the modern automobile have come modern high tech diagnostics. Now, let us pull a rabbit out of the hat and examine the magic behind one of these high tech diagnostic tools; the modern pressure transducer.
A pressure transducer is a device that takes a physical quantity and changes it into an electrical signal. The pressure transducer can measure physical quantities such as; oil pressure, fuel pressure, engine compression, exhaust pressure, intake pressure, crankcase pressure, and radiator pressures to just name a few. By viewing this electrical signal on an oscilloscope, a large amount of information can quickly be conveyed to the technician. These devices will change the way that the modern technician will diagnose the internal combustion engine.
Now let us examine a Dodge Caravan with a 3 liter V-6 engine with overhead camshafts. This vehicle was brought in exhibiting a rough idle condition. The complaint was verified and the PCM codes were pulled. There were no pending or mature DTCs recorded and all of the monitors had run. A pressure transducer was placed into the exhaust tailpipe (Figure 1). This pressure transducer is a special type of transducer called a differential pressure transducer which can read the exhaust pulses from the tailpipe. For years technicians have used their hand or a dollar bill to feel or see these exhaust pulses in order to determine whether the exhaust pulses were even. This can help with the diagnosis of the engine. If the differential pressure transducer is connected to an oscilloscope, these exhaust fluctuations can be viewed as a waveform, which will help the modern technician in diagnosing the engine.
This waveform, however, cannot be understood without a trigger to locate the exhaust pulsations. If the ignition is used as the trigger, the exhaust pulsations can be related to each individual cylinder. To accomplish this, the firing order must first be known (Figure 2). There will also be a timing issue when applying the trigger to the exhaust waveform. In a four cycle engine, the ignition spark occurs at the end of the compression stroke. During the compression stroke and power stroke both the intake and exhaust valves are closed. At the point the spark ionized the spark plug electrodes; the air/fuel mixture is ignited. In turn, the burning air/fuel mixture creates an expanding force that drives the piston away from the cylinder head. As the piston approaches the bottom of its stroke the exhaust valve opens. The high pressure inside the cylinder moves to the low pressure area outside of the cylinder which creates a pulse as it moves through the exhaust pipe.
The piston now starts to move toward the head on the exhaust stroke, pushing out the remaining content of the cylinder into the exhaust system. If you are using the ignition as the trigger for the exhaust pulse there will be a delay between the spark ionizing the spark plug electrodes and the exhaust stroke. To compensate for this delay, the trigger will need to be moved from cylinder 1 to cylinder 3. By moving the trigger, two firing events after the firing event in cylinder 1, the exhaust pulse for the number 1 cylinder will align with the triggered event. Therefore, on a 4 cylinder engine, the trigger is moved 1 cylinder after number 1.
On a six cylinder engine, the trigger is moved 2 cylinders after number 1. On an 8 cylinder engine, the trigger is moved 3 cylinders after number 1.
In Figure 3, the yellow trace is the waveform produced by the differential pressure transducer. The red trace is the waveform produced by an inductive clamp around cylinder number 3 spark plug wire. The green trace is the waveform produced by the ignition coil primary signal. With the addition of the ignition triggers, this will divide the exhaust waveform into individual cylinders. Once the waveform can be isolated into individual units, the waveform can be analyzed to determine where the problem cylinder or cylinders are located. At this point the firing order must be known so an association can be made between the exhaust pulse and the cylinder that created it. When examining the exhaust waveform, two things will need to be checked; the amplitude of the signal and the timing placement of the exhaust pulse.
Of these two items, the timing placement is the most important. When analyzing Figure 3 the peak amplitude (vertical) on cylinders 1-3-5 are greater than the peak amplitude on cylinders 2-4-6. Now check the timing placement of the peaks on cylinder 1 and 2.
The peak on cylinder 1 comes very close in time (horizontal) to the green primary ignition turn on signal. The time between the cylinder 1 peak and the green primary falling edge is 1.69ms. On cylinder 2 the peak is much further away from the green primary falling edge at 6.76ms. Now check the other cylinders. Upon further analysis, it becomes clear that cylinders 1-3-5 are very close in time to the primary falling edge, whereas, cylinders 2-4-6 are further away from the primary falling edge. In figure 2, the firing order is given as 1-2-3-4-5-6. 1-3-5 are all from bank 1 and 2-4-6 are all from bank 2. These data would indicate that there is a difference from bank to bank. One complete bank has a problem.
Many things could affect a complete bank and create a problem. To narrow down the problem very quickly, we will install the differential pressure transducer in the brake booster hose (figure 4). This will allow us to view the intake pressure pulses (figure 5). It will be necessary to use the ignition triggers so the intake waveform can be divided. Once the intake waveform has been broken down into individual cylinders, the pulses can then be analyzed. However, there is a timing issue between the ignition ionizing the sparkplug electrodes and the intake valve opening. The intake stroke occurs before the ignition event. In order to time the intake pulse to the cylinder that created it, the trigger must be installed around the cylinder 5 ignition wire. This will align the inductive red trace with the cylinder 1 intake valve pulse (yellow trace).
Therefore, on a 4 cylinder engine the trigger is moved one cylinder before cylinder 1. On a 6 cylinder engine the trigger is moved two cylinders before cylinder 1. On an 8 cylinder engine the trigger is moved three cylinders before cylinder 1.
In figure 5, the engine is at idle and the intake waveform is divided into individual cylinders by the ignition. Let us examine cylinder 1 and cylinder 2. In cylinder 1 there are three distinct positive pulses between the primary green trace of cylinder 1 and cylinder 2. In cylinder 2 there are two distinct positive pulses between the ignition primary on cylinder 2 and cylinder 3. The amplitude of cylinder 1 is also greater than cylinder 2. Now examine the other cylinders in figure 5. It becomes clear that cylinders 1-3-5 all have 3 distinct positive peaks with higher amplitudes. Whereas, cylinders 1-4-6 have 1 or 2 peaks of lower amplitude and a different waveform shape. In figure 2, the firing order is shown as 1-2-3-4-5-6. This confirms the exhaust data that we had previously gathered; bank 1 is different from bank 2. These data would indicate that a camshaft is out of time with the crankshaft.
It can take hours to remove the camshaft timing covers to confirm this finding. The problem with this is that if the camshaft has moved from the gear it may be hard to confirm the camshaft timing by only checking the gear timing marks. There is an easier, faster and more accurate way to confirm camshaft to crankshaft timing. To accomplish this, remove the spark plug from cylinder 1. Install a compression adapter into the sparkplug hole. Before installing the compression adapter remove the check valve from the adapter. This will allow the air pressure to move freely in and out of the adapter hose. Now install a 300psi pressure transducer on the compression hose. The oscilloscope will now display a waveform of the pressure changes within the cylinder. Before starting the engine, install a spark tester on the #1 ignition wire. It will only be necessary to allow the engine to run a very short time to capture the data. Once the data is captured, turn off the engine. Install the sparkplug and ignition wire back into the #1 cylinder. Remove the #2 sparkplug and install the spark tester on the ignition wire. Install the compression adapter and 300psi pressure transducer into the #2 sparkplug hole. Start the engine and capture the data. Now shut off the engine so the data can be analyzed. Figure 7 is from cylinder 1 and figure 8 is from cylinder 2. Let us examine these cylinder pressure waveforms.
The first thing that will need to be done is to measure from compression peak to compression peak. In figure 7 the peak to peak time is 145.34ms. This is equal to 2 crankshaft revolutions or 720° of revolution. By dividing 145.34ms by 4 the time for each stroke can be calculated. This time is 36.33ms, which is equal to 180° of crankshaft rotation. Now move the cursor 36.33ms from the 1st compression peak. This is when the piston has reached the bottom of its stroke or bottom dead center (BDC) after the power stroke. The exhaust valve will open at the end of the power stroke before BDC. This is where the waveform changes due to the exhaust valve movement. This pressure change will cause the waveform to rise until it hits its peak. This peak should be very close to the BDC mark. On most engines, the BDC mark will fall between half way up the ramp and close to the top of the ramp when the cam timing is correct. By measuring the time from BDC to the exhaust waveform peak, the crankshaft degrees can be calculated. 720° of crankshaft rotation divided by 145.34ms of time will equal 4.95° of crankshaft rotation for 1ms of time. In figure 7, the time from BDC to the peak of the exhaust waveform is 2.42ms. To calculate the crankshaft degrees, take the time (2.42ms) and multiply it by 4.95° and it will equal 11.97° or 12° of crankshaft rotation.
Now let us examine figure 8. First measure the compression peak to peak. This is equal to 150ms of time. Now divide 150ms by 4 which equals 37.5ms. This will give you the time for each stroke. Now move the cursor 37.5ms from the 1st compression peak. Notice that the BDC mark occurs below the half way point on the exhaust ramp waveform. There is a delay in building the pressure of the exhaust ramp so it takes much longer for the peak to form. Also notice that the waveform before the BDC mark is much more rounded. This is due to the exhaust valve opening later which is an indication of a retarded camshaft. Once the cursor is in place; measure the time from the BDC mark to the exhaust waveform peak. This time on cylinder 2 is 6.99ms. To calculate the time per degree, divide 720° by 150ms.
• Wheel of Fortune
John Anello - Auto Tech on Wheels
I was called to a shop with a complaint of a no start condition on a 1996 Dodge Caravan with a 3.3L engine (figure 1). This shop only had a scan tool in its arsenal of equipment. No scope, no graphing meter combined with a component tester or even a repair information system. With no codes in memory the shop resorted to the feels like tactics. They used their best instincts to replace parts associated with their visual inspections and old school test procedures. The spark tester they were using only showed spark on one coil so the list included coil pack, crank sensor, cam sensor and an ECM. The old ECM was bad due to a damaged coil driver that was holding one coil primary constantly on. This I have seen many times in the field caused by a bad coil assembly with
a shorted coil primary winding. So I can justify their replacement of the coil assembly and the ECM but the vehicle at this point still had a problem with a no start. This shop is not alone in the way they diagnose cars. All too often I cater to these type of shops and I try my best to educate on site how important it is to keep themselves up to date on training and have the proper equipment to perform tasks that will only save them time and unwanted parts. It will only make them a better technician in the long run. It is never too late to step up to the plate to take it to the next level. The worst thing in any business is to let technology surpass you. You will only find yourself stuck in the past with not much of a future to keep afloat. When I arrived at the shop I attempted to start the car but it only spit back a few times like it wanted to start. The shop tech told me that the #2/5 ignition coil was the only coil firing during cranking. The other coils did not fire at all. At this point I decided to hook up my scope to some selected signal lines to get a visual concept of what was exactly going on. I used my EScope Limited 4-trace scope and placed my channels on the cam sensor and all three coil drivers. As I cranked the engine you could see (figure 2) that the cam sensor was providing the proper signal pattern but the ECM had a problem controlling the coil drivers. One driver attempted to ground the coil while another driver was held on for as long as 600mS of on time. By current ramping the one working coil against the crank and cam signals (figure 3) you could see that the coil driver was maintaining about 8.5 amps of current for almost one camshaft revolution. There is no way any driver would hold a coil primary that long unless that driver was in love and just did not want to let the coil go. It was a relationship that just went bad with no one to give advice to just let go. Someone was just not telling the ECM to let go of the coil. Now I am wondering if this caused the failure of the coil driver within the old ECM.
This erratic coil operation could only be caused by a defective ECM with an internal driver failure, a corrupted crank/cam signal input or a crank/cam sensor correlation problem. The cam and crank sensors seem to be producing the proper patterns with correct amplitude but I needed to compare their synch correlation. I used my Ace Misfire crank/cam waveform database and pulled up a good known crank/cam pattern for this vehicle (figure 4). You can see how the cam sensor pattern repeats the 1-2-3-1-2 pattern while the crank signal repeats the 4-4-4-4-4-4 pattern. It is between where the cam pattern ends and begins that there should be equidistant 4-4 patterns. I imported the cam sensor signal into the measuring section of the scope (figure 5) and placed cursors to show one complete event of the cam sensor. By hitting the Mark Cam Shaft button, the program automatically placed 5 purple cursors on the screen creating 4 divisions each representing 180 degrees of crankshaft rotation and 5 smaller purple cursors creating 6 sub-divisions each representing 30 degrees within each 180 division. I superimposed the crank signal on top of the cam signal and zoomed into the end and beginning event of the cam signal (figure 6). Notice how the crank signal has shifted at lest 30 degrees to the left. This indicated that the crank and cam sensors were not properly synched. The cam sensor indexed off the front of the engine while the crank sensor indexed off the torque convertor at the rear of the engine. Now I had to decide whether this was caused by a problem in the front or rear of the engine. With 108,000 miles on the clock it was a better
sell from a maintenance point of view to disassemble the front of the engine to inspect timing chain components then it was to pull the transmission to check the flywheel assembly. The cause at this point could be a jumped timing chain, sheared crank key way, sheared cam gear roll pin or even a damaged flywheel. There was no flywheel noise while cranking the engine so where do you start with this dilemma without having to spend unwanted labor? The answer to this question is a new test procedure I have been using to check valve train problems by simply using a 300 P.S.I. pressure transducer. You remove the spark plug from a selected cylinder and install a hose adapter to accommodate the pressure transducer. Next you place a spark tester on the spark plug wire representing that same cylinder, place an inductive clamp around the plug wire and just crank the engine. The resulting patterns (figure 7) will show you a peak to peak cylinder pressure rise representing a 720 degree event of a cylinder and an induction square wave representing the spark event in the cylinder. You then place your cursors on the compression peaks and select the Mark Cam Shaft button and look at where the purple cursors are laid out. The lowest point (u-curve) before BDC of the power stoke
is where the exhaust valve begins to open. This I have found to be about 30-45 degrees before BDC of the power stroke on most of the cars I have been checking. The waveform pattern (figure 7) shows the exhaust valve to be opening close to 30 degrees before BDC of the power stroke. The spark was occurring only once in the cylinder when it should have occurred twice. The spark only fired the waste spark and fired wrong at about 40 degrees after TDC of the exhaust stroke. This was due to the incorrect coil primary control caused by the crank/cam correlation problem.
By viewing a good known pattern of a 1997 Dodge Caravan with a 3.3L (figure 8) you can see the exhaust valve opening is happening at about 30 degrees before BDC of the power stroke. This file was from a running engine so the advance timing was added to the base timing to bring the timing to about 25 degrees (always use the first rise of the inductive clamp square waveform) before TDC of the compression stroke for ignition spark and 25 degrees before TDC of the exhaust stoke for waste spark. This method is such a great way to learn about combustion strategy and at the same time give you another tool in your arsenal to fine tune your diagnostics to which direction you want to head in. Even if this was a dual cam set-up you would have no problem in finding out which cam was off from spec without even pulling a timing cover. Just by having this information validated from doing other good known vehicles it was safe for me to instruct the garage to pull the transmission to inspect the flywheel
About an hour later the shop had called to tell me that the transmission was pulled and I needed to comeback to see the damage they found. When I arrived there at the end of my day I was amazed at the damage. The center of the flywheel was completely cut out from the rest of the flywheel like a cookie would be cut out of rolled dough. The center piece spun slightly and wedged itself in place (figure 9). The amazing thing was that there was no noise associated with this flywheel while cranking. I helped the garage remove the flywheel and placed it on the ground. I pushed on the center piece and it fell out. The only thing holding these pieces together were the center thrust plate and flywheel bolts.This flywheel was like a wheel of fortune just making the shop spend time & money without a cure in site and at the same time sending them on a wild goose chase. It held the torque convertor and provided the crank triggers necessary to start the engine. Who would ever think a flywheel could break clean and spin but yet go undetected in its journey of unforgiving charades. I hope this story sheds some light on the value of a scope which allows you to have a window to see beyond the normal reach of a scanner.
• Hocus Focus
John Anello - Auto Tech on Wheels
I was called to a shop for a complaint of no power on a hard acceleration on a 2002 Ford Focus with 2.0L with only about 53,000 miles (figure 1). The vehicle idled fine with no misfires or codes stored in memory. The shop had already replaced the fuel filter and performed a back pressure check to rule out a partially clogged catalyst convertor. The vehicle just did not have the horsepower it should for this 4 cylinder. The owner was told by the garage that there was a possibility that the fuel pump was bad and suggested that he seek the dealership to have it changed under warrantee. When the vehicle was checked by the Ford dealer they had a different opinion of what was needed to fix the car. The Ford dealer had recommended a transmission due to the lack of driveline torque and a condition of engine flare-up on acceleration. The torque multiplication at the wheels just did not seem enough. The customer at this point called the garage back to let them know about the outcome and the garage recommended a local transmission shop to get a second opinion. A visit to the transmission shop proved the dealer wrong and now the vehicle was back at the garage for further investigation. This poor car was like a carnival
ride where everybody wanted to take a turn in the driver seat. There were too many formed opinions with no solid answers to remedy the problem.
When I arrived at the shop I had to drive the vehicle to get a feel for the problem others had experienced. I warmed the car up and it seemed to rev up fine with no hesitation problems. The engine idled fine with no misfires or apparent roughness. I placed the vehicle in drive and accelerated normally from a dead stop and it did not seem too bad at all. Then I tried it again but this time on a hard acceleration. The vehicle seemed to accelerate fine without holding back but it took too long to come out of first gear due to the throttling needed to get the vehicle going. By holding the throttle down further it created an engine flared up which was followed by a bang into second gear. It felt like a bad transmission but then again it seemed like the engine did not have the top end torque it was designed to have. At this point I needed to run some tests to determine if the engine was experiencing a fuel, air/exhaust flow or a timing problem.I started my diagnosis by checking for codes in memory and there were none present (figure2). The EVAP monitor did not run yet indicating that someone had erased prior codes and stored freeze frame
information but the vehicle did run all the other monitors proving out any fuel trim or misfire conditions. When dealing with a no power condition it is important to know that most failures I have seen in the field usually fall into the common categories of lack of fuel, restricted air intake, clogged exhaust, miscalculation of volumetric efficiency or an ignition/valve timing issue. It was easy to just watch data parameters on acceleration to determine if the problem was fuel related so I accelerated the engine again from a dead stop to wide open throttle while viewing some selected PIDs (figure3). You can quickly see that the ECM maintained fuel control during the whole time indicating that the power loss was not do to a fuel problem. Had it been a fuel problem I would have seen fuel
trims maxed out with an O2 sensor value constantly below 500mV. Providing that the O2 is working properly, this is a quick test to eliminate the need to get too intrusive by hooking up a fuel analyzer to check fuel pressure and volume.
Now that I eliminated a fuel delivery problem, my next test was to perform a volumetric efficiency test to check air flow through the engine. The engine is nothing more than an air pump and the ECM will use the MAF calculation to determine the proper fuel and ignition timing mapping to keep the engine at peak performance. The correct air flow reading is dependant on the proper calibration of the MAF, amount of restriction in the air inlet and exhaust system or valve train integrity. This test has to be done at wide open throttle while driving to be accurate. You also need to take into consideration the ambient
air temp, altitude and engine size. This can be done by recording your captured scan data into a VE calculator or by simply plugging your engine and ambient conditions into the Escan VE program and let it do all the work for you while you concentrate on your driving skills to maintain a steady wide open throttle acceleration to capture the data you need. Once the proper information was entered into the VE program I accelerated to wide open throttle from a dead stop and captured the air flow data (figure4). The actual volume of air was 30 percent below the calculated specification. There were no fuel trim issues
which indicated that the MAF was correct in its findings. The exhaust back pressure was already checked by the garage and found to be below 3 P.S.I. and the air inlet and air filter were checked for restrictions and proved to be okay. This problem was valve train related.
I next hooked up my scope and tagged the crank and cam sensors to perform a correlation check by viewing the signals in a superimposed format (figure5) to prove my suspicions. By pulling up a good known pattern from my Ace Misfire database (figure6) you can see that the crank and cam correlation was off by about 60 degrees. This test is a quick and easy check to validate mechanical timing issues with trigger points but it alone still can not prove out a jumped timing chain or belt. There have been too many situations where I have found loose trigger wheels that lost their indexing, damaged flywheels or worn crank pulley keyways. The only true way for me to quickly prove out a valve train problem would be with the use of a 300 P.S.I. pressure transducer in a cylinder chamber to get a true indication of piston and valve correlation.
I removed the spark plug and placed a spark tester on the wire using a spark tester. I next screwed the pressure transducer and adapter assembly into the cylinder and started the engine to capture a waveform. I shut down the engine and zoomed into the pressure pattern to view my peak to peak pressure rises indicating one combustion event of 720 degrees. I placed my cursors on my peak to peak compression rises and then hit the Cam Timing button within the EScope program (figure7). The program automatically placed 5 large purple cursors creating 4 divisions of 180 degree of crankshaft rotation and 5 smaller purple cursors creating 6 sub-divisions of 30 degrees on the screen. You could now see that the exhaust valve was opening at about 75 degrees before BDC of the power stroke (take note the lowest fall after the compression rise is where the exhaust valve begins to open). Having seen many waveforms showing exhaust
valve openings between 30-45 degrees before BDC of the power stroke, this engine's valve timing was definitely off by 30 degrees or better. It was now safe to instruct the garage to pull apart the front timing cover which involved supporting the engine with a jack, removing an upper engine support and removing an upper metal timing housing cover just to expose the timing belt.
Once the timing belt cover was removed I marked the gears with white out (figure8). The cam gear was found to be off about two teeth. The belt also had some slop indicating that the timing belt self adjuster was not doing its job to compensate for timing belt stretching. The fix here was to sell the customer a timing belt and new belt tensionor. I also recommended replacing the water pump that ran off the belt as a preventative maintenance measure. Prior to being a mobile technician for the last 15 years I worked in an engine rebuilding shop for 5 years and as a dealer technician for another 10 years and I have seen so many guys in the field skim on many types of jobs just to save a customer money by not replacing a component that may involve an overlap in labor time. An experienced mechanic or technician should understand that educating a customer is the best way to sell needed work on a vehicle. This can only help to prevent a return visit of the vehicle on a hook because you decided to do a good deed by keeping operating costs low on a job to be competitive with shops in the area. I find that whenever you try to save a customer money you always loose in the end.
It’s amazing to me how this Focus timing belt went undetected by the ECM or even a dealer tech that knew and understood the Ford product well. But I can see this happening to any tech because there were no obvious signs that you would normally encounter with others engines such as poor engine vacuum, rough idle, engine misfire, total loss of power, erratic electronic spark operation or even popping through the intake. This was a Hocus Focus that performed a trickery act of deceiving and fooling those into thinking that anything could have been at fault except for the timing belt. Even when I drove the car I thought the engine was alright and I was leaning towards performing a transmission stall speed test because it did seem like a transmission problem. I believe that the key to resolving power issues are to keep it simple by performing pinpoint tests to validate engine performance. Engine performance can have an effect on transmission
performance and using the “feels like” or failure pattern tactic will only lead you down a dead end street with no resolution to the initial problem. I always strive to find new ways to fine tune my diagnostics in order to cut down on time wasted by performing intrusive tests or component removal that leads to unwanted labor tasks. As technology in equipment advances it can only help us to achieve new levels in our diagnostic strategies. I hope this story can only enlighten you to help you choose the right path when you hit that diagnostic fork in the road.
• Introduction to In-Cylinder Pressure Testing Part 1
by Bernie Thompson - ATS
Deck: This exciting diagnostic method opens new possibilities in driveability troubleshooting.
Every decade or so a new automotive technology is discovered that is truly game changing. The use of pressure transducers in automotive service bays is one of the most exciting discoveries of the 21st century. This innovative technology saves repair shops time and money on a grand scale.
This technology can be used to check engines, transmissions, power steering, brake systems, EVAP systems, and air conditioning systems. Basically any system that uses pressure can be analyzed by the use of a pressure transducer. These transducers measure physical pressure changes, negative or positive, and convert these changes to an electrical output signal. Pressure transducers need a power source, ground source, and they will produce a voltage signal that is proportional to the physical quantity applied. An oscilloscope is used to display and analyze the signal output produced from the pressure transducer by graphing the pressure changes over time thereby identifying changes that occur within the system.
Pressure transducers allow the technician to see the inner working of the internal combustion engine without disassembly. In order to check the spark ignition internal combustion engine, three pressure transducers are used; one in the cylinder, one in the intake, and one in the exhaust. To place the one in the cylinder; remove the spark plug from the cylinder head (be sure to ground the spark), then install a compression test hose with the one way check valve removed, and place a 300 psi transducer on the compression hose. The -30 Hg vacuum transducer on the intake manifold will be centrally located on the vacuum port close to the throttle body. Place the exhaust 25 inch/H20 transducer hose in the end of the tailpipe. With these transducers in place, the engine will be operated in three different modes; crank no start, idle, snap throttle and each of these engine operating conditions will produce different pressure waveforms on the oscilloscope and will use different techniques to diagnose them with.
The engine under these three operating conditions can be checked for: camshaft to crankshaft timing problems; variable camshaft timing problems; intake and exhaust valve sealing problems, both consistent or intermittent; valve spring problems; piston ring sealing problems; worn camshaft lobes; restricted exhaust problems; ignition timing problems; or cylinder misfire identification. As you can see, this list includes some of the toughest problem to diagnose. These difficult diagnoses will become routine in your service bay with just a little understanding of the pressure changes that occur within the engine.
Let us start by analyzing the idle compression waveform as seen in figures 1 and 2. Figure 1 is a camshaft chart with the compression waveform overlaid on the cam card. Figure 2 is a basic compression waveform produced at closed throttle at low RPM. The large pink lines divide the compression waveform into 180 degree divisions of crankshaft rotation or the strokes (intake, compression, power, exhaust) of the engine, and the small pink lines divide the crankshaft rotation into 30 degree divisions as seen in figure 2. The large pink line in the middle of figure 2 represents when the piston is at 360 degrees of crankshaft rotation at Top Dead Center (TDC). The intake valve opens just before this point. The crankshaft is in rotation so the piston is in motion, the piston moving away from the cylinder head increasing the volume within the cylinder. This in turn creates a low pressure area contained within the cylinder which pulls a negative pressure (vacuum) against the closed throttle plate. This reduction in pressure can be seen from “G” which is atmospheric pressure to “I” which is negative pressure. This drop in pressure should start at the TDC point and fall rapidly to “I” and this pressure change should occur before the two small pink markers after TDC or 60 degree after TDC. “I” indicates the lowest pressure obtained during the intake stroke, whereas “J” indicates the average pressure during the intake stroke. The intake duration is from “G” to “K”, note that “K” occurs after the intake stroke ends at the Bottom Dead Center (BDC) mark. The intake pressure stays low after the BDC mark occurs even through the piston is in an upward movement. One would think that this upward piston movement would create an increase in pressure within the cylinder; however, since the intake manifold has volume under low pressure, the intake manifold acts as an accumulator storing negative pressure. As long as the intake valve is open, it is exposed to this low pressure area contained within the intake manifold. This accumulator effect stabilizes the low pressure area in the cylinder which in turn keeps the low pressure in the cylinder even with the upward rising piston. When the intake valve closes, the pressure will start to rise which occurs at ”K”. The intake valve should close at 40 to 60 degrees after the BDC mark.
The piston is now in an upward movement in the cylinder and both valves, intake and exhaust, are closed. The volume contained within the cylinder is now trapped. The crankshaft is rotating and thus is moving the piston toward the cylinder head. As the piston comes closer to the cylinder head, the area within the cylinder is diminished. This reduction in cylinder area creates less space for the volume contained within the cylinder; this in turn increases the pressure within the cylinder. Peak pressure occurs where the piston comes as close to the cylinder head as mechanically possible. This is the compression TDC point which is “A”. This peak pressure at “A” can be used to identify the TDC position for such things as checking the ignition timing, injector timing, and checking the crankshaft or camshaft position sensor. It is interesting to note that 60% to 70% of the compression pressure within the cylinder is created within the last 30 degrees of crankshaft rotation before TDC (BTDC) during which time the piston is slowing down and will stop though momentarily, at the TDC point. Although the piston velocity is slow the pressure is rising due to a decrease in the area contained above the piston crown. Since the volume contained within the cylinder works against the area contained within the cylinder, any volume loss due to cylinder leakage during the compression stroke will affect the peak pressure within the cylinder. It is important to check the peak pressure points over multiple cylinder cycles as they should be the same. If one peak is high and the next peak is low by just a few pounds (PSI), and then the next peak is high again, the cylinder is leaking. The air flow into the cylinder cannot change fast enough to allow a high/low/high pressure change. This is due to a volume change or leakage problem within the cylinder.
Since the crankshaft is in rotational motion, the piston is then pulled downward by the connecting rod. The downward movement of the piston allows the area between the cylinder head and piston to increase, thus the cylinder pressure decreases. Since there is no spark present within the cylinder (the spark plug is removed), this stroke is not the power stroke but instead is a decompression stroke. The compression tower has an upward ramp and a downward ramp, if the tower is measured from “K” to “A” and the pressure is divided in half there is a point on both sides of the tower that represent the point of half mast. Half mast is identified by “B” and “M”. These points will be measured in crankshaft degrees to the TDC mark, and must be within 20 degrees of each other. If the compression tower has more than a 20 degree differential between the rising and falling ramps, there is a mechanical failure. When this occurs the compression tower will look like it is leaning, one side will have a lot more space between the ramp and the TDC mark, compared to the other ramp and the TDC mark. The piston continues its downward movement and at 90 degree after TDC the waveform has returned to a negative pressure state. This point is denoted by “C”. The piston continues downward increasing the area within the cylinder and the compression waveform also continues downward to a point “D”; this is the point where the exhaust valve opens. There should be a clear point of definition at point “D” that indicates the valve seal is intact. The point at “D” should look like clones cycle to cycle with very little change occurring to the exhaust pocket. If point “D” is changing cycle to cycle this is an indication that the valve has a seating problem. In figure 3 it is clear that none of the exhaust pockets look alike and therefore indicates that there is a valve seating problem. It is important to understand that either valve, intake or exhaust, can cause the exhaust pocket to change. It will be necessary to check the intake manifold pressure and exhaust pressure to determine which valve is not seating properly.
The pressure within the cylinder starts to rise at point “D”; however, the piston is still moving downward. It would seem that because the piston is moving downward and increasing the area within the cylinder that there would be a corresponding decrease in pressure. The exhaust pressure is near atmospheric pressure and the pressure within the cylinder is in a negative state. Since a high pressure area always moves to a low pressure area, the exhaust pressure rushes into the cylinder as soon as the exhaust valves opens. This pressure rise within the cylinder from ”D” to ”F” is the pressure equalizing to the atmospheric pressure within the exhaust system. Point “D” is where the exhaust valve opens. This valve opening event should occur at 30 to 50 degrees before BDC (BBDC) and it is used to check camshaft timing. The exhaust ramp from “D” to “F” will also be used to check the exhaust camshaft timing. If the pressure is measured at “D” and then at “F”, and this pressure ramp is divided in half (this is shown at point “E”) this point should fall on the BDC mark. If the BDC mark falls between “E” and “F” the exhaust cam timing is correct. If the BDC mark falls below the “E” mark then the camshaft timing is retarded. If the BDC mark falls to the right of the “F” mark, the exhaust camshaft timing is advanced. The BDC mark on newer engines may fall several degrees to the right of the “F” mark and be timed correctly. It is important to measure the exhaust ramp and find point “E” and mark it with a vertical cursor. This cursor will now cross the pink grid that represents the crankshaft degrees. On older engines this cursor should fall between the 15 degree BBDC and the BDC marks. On newer engines this cursor should fall between the 23 degree BBDC and the 12 degree BBDC marks.
The piston will rise from the BDC mark to the TDC mark and during this time the exhaust valve will be open. As the piston moves upward the area within the cylinder is decreasing thus creating a higher pressure than the slightly elevated atmospheric pressure within the exhaust. This in turn forces the volume contained within the cylinder into the exhaust system. The ripples between “F” and “G” represent the exhaust pressure resonance within the exhaust system. Since the exhaust valve is open, the pressure within the exhaust system can be seen within the cylinder. The area between points “D” and “I” is referred to as the exhaust plateau. This plateau is created by the intake manifold vacuum. The intake stroke pulls the cylinder into a negative pressure area and the intake valve is then closed, trapping the negative pressure within the cylinder. The piston then moves up compressing the volume within the cylinder to its peak pressure, and then moves down decompressing the volume within the cylinder. At the point the piston returns to the same position within the cylinder as it was when the intake valve closed, the pressure within the cylinder will also return to the same pressure as it was when the intake valve closed, which is negative (vacuum). Since the intake stroke changed the cylinder pressure to a vacuum relative to the exhaust pressure and the exhaust valve opened when the cylinder returned to the same point and then rises back to the exhaust pressure, thus the exhaust plateau is created by vacuum. The points “D” and “I” should be the same. If point “D” is lower than point “I” the cylinder is leaking volume, if point “D” is slightly higher than point “I” at about 2 psi or less the cylinder leakage is ok. If this is greater than 2 psi the cylinder is leaking volume.
The intake ramp will be used to check the intake camshaft timing. Since the intake valve will need to open in order for the intake pressure to drop rapidly, the intake valve opening can be calculated using the intake ramp which is “G” to ”I”. If the pressure is measured at “G” and then at “I” and this pressure ramp is divided in half (this point shown at point “H”), this point should fall 20 degrees after the TDC mark. The intake camshaft timing is correct if the 20 degrees after the TDC mark falls within + or - 5 degrees of “H”. If the 20 degrees after the TDC mark falls below the “H” mark the camshaft timing is advanced. If the 20 degrees after the TDC mark falls to the right of the “H” mark, the exhaust camshaft timing is retarded. On newer engines with variable camshaft timing (VVT) on the intake cam, the 20 degrees after the TDC mark will be adjusted to 30 degrees after the TDC mark. It will be important to measure the intake ramp and find point “H”, then mark point “H” with a vertical cursor. This cursor will now cross the pink grid that represents the crankshaft degrees. On older engines this cursor should fall between 10 and 20 degrees after TDC. On newer engines this cursor should fall between 20 and 30 degrees ATDC. The point at which the intake valve closes can also be used to check intake camshaft timing. This point is marked as ”K”, and should occur between 40 and 60 degrees after BDC.
Now let’s look at figures 4 and 5 which are camshaft to crankshaft timing problems. We will first analyze figure 4 in which it is quite easy to see that the compression waveform is not like figure 2. Let’s start with the location of the exhaust pocket. In figure 2, the exhaust pocket is located at 35 degrees before BDC whereas in figure 4, the exhaust pocket is at 65 degrees before BDC. Next the exhaust ramp at “E” in figure 2 is at 12 degrees before BDC, and in figure 4 the exhaust ramp at what would be “E” is located at 45 degrees before BDC. On the intake ramp in figure 2, “H” is located at 18 degrees after TDC, and in figure 4 the intake ramp at what would be “H” is located at TDC. The intake valve closes in figure 2 at 45 degrees after BDC, in figure 4 the intake valve closes at 30 degrees after BDC. Whether you look at the exhaust cam or intake cam, it is quite apparent that this camshaft is advanced.
Now we will analyze figure 5. In figure 5 again it is quite easy to see that the compression waveform is not like figure 2. Let’s start with the location of the exhaust pocket. In figure 2, the exhaust pocket is at 35 degrees before BDC and in figure 5, the exhaust pocket is at 0 degrees TDC. Next, the exhaust ramp at “E” in figure 2 is at 12 degrees before BDC, in figure 5 the exhaust ramp at what would be “E” is located at 13 degrees after BDC. On the intake ramp in figure 2 the point at “H” is located at 18 degrees after TDC, in figure 5 the intake ramp at what would be “H” is located at 35 degrees after TDC. In figure 2, the point at “G” is located just before the TDC mark whereas in figure 5 this point is at 25 degrees after TDC. The intake valve closes in figure 2 at 45 degrees after BDC; however, in figure 5 the intake valve closes at 70 degrees after BDC. Whether you look at the exhaust cam or intake cam, it is quite apparent that this camshaft is retarded.
Be aware that these compression waveforms that have been explained above are idle compression waveforms, and some of these techniques do not work on cranking or snap throttle waveforms. With a little practice, these compression waveforms will start to provide your shop with fast actuate diagnoses. These diagnostic techniques will bring your shop into the 21st century, and provide your shop with the winning edge over the competition.
• Introduction to In-Cylinder Pressure Testing Part 2
The impossible is upon us, the ability to see into the internal combustion engine while it is running. Could this really be true, can this really happen? Modern technology is ever expanding helping us with every facet of our daily lives from our homes to our phones to our vehicles; advancements in technology are just out right amazing! With such advancements in technology it is an exciting time to be an automotive technician. The modern vehicle has more computer power than the space shuttle while carrying the appearance of a sophisticated aerodynamic road machine. These advancements in the modern automotive industry are moving at such a rapid pace that it seems like we are being left behind. With all of the advancements in the modern vehicle where is the technology that can help us keep pace with the repair of these sophisticated machines? Just like the rapid advancement in the modern vehicle, rapid advancements have recently been made in automotive tools. Now the tools that we can use in our service bays have reached the next level and have finely caught up with the advancements of the automotive industry. These tools can make a huge difference in your shop’s ability to quickly and accurately diagnose modern vehicle systems.
I would like to cover one of these modern tools that allows you to see into the internal combustion engine! Just several years ago this would have been impossible; but today for many shops, this is routine and is accomplished with the use of pressure transducers. A pressure transducer is a device that measures a physical quantity of pressure (negative or positive) and converts it to an electrical output that is proportional to the applied pressure. In order to check the in-cylinder pressure the spark plug is removed and a compression testing hose with a pressure transducer is attached to the hose and inserted into the cylinder head as seen in figure 1. Since the internal combustion engine pumps air volume into and out of a cylinder, pressure changes will occur that will be proportional to the air volume being pumped. By using pressure transducers to monitor this volume-to-pressure change, one can “see” into the internal combustion engine.
In order to monitor the voltage output from these pressure transducers an oscilloscope will be used. The oscilloscope will trace the pressure transducer’s voltage output over time. This will allow one to see the inner workings of the engine such as the intake and exhaust valves opening and closing. When diagnosing using pressure transducers there are three different distinct in-cylinder waveforms that will need to be analyzed; (1) Cranking, (2) idle and (3) Snap Throttle. Additionally each of these in-cylinder pressure waveforms come with different intake vacuum waveforms and different exhaust pressure waveforms that will need to be analyzed as well.
Now let us analyze a cranking in-cylinder pressure waveform as shown in figure 1. The cranking pressure waveform is designated by the green trace while the engine strokes are designated by the pink vertical lines. These pink vertical lines are broken down in 2 degree marks, the large vertical lines are 180 crankshaft degrees, while the small vertical lines are 30 crankshaft degrees. The pink mark on the left of the screen marked TDC or Top Dead Center shows the point where the piston came as close to the cylinder head as mechanically possible. This occurs on the compression stroke where both the intake and exhaust valve are closed. The peak pressure can be checked by the scale seen on the left side of the screen, which currently indicates that the peak pressure is 120 PSI. You will need 7 to 10 seconds of cranking data obtained with the throttle in the closed position. It will be important to check the peak pressure on all the cylinders during the crank. These pressure peaks should be less than 1 psi from each other. Make sure that the engine is not trying to start thus changing the cranking RPM. If the RPM is changing, the peak pressure will also be changing. You need a steady crank RPM so the peak pressure will not change. If a large leak is present in one of the cylinders the RPM cannot be stable, additionally this leak can usually be heard as the starter spins the engine faster during the low pressure cylinder event and then slows down on the next high pressure cylinder event. If a steady crank RPM is obtained and the peak pressure is changing cycle to cycle then the cylinder volume is changing, this is usually caused by a leak within the cylinder.
After the TDC compression event at “A” the piston, after being stopped momentarily, will start to be pulled away from the cylinder head by the connecting rod which is connected to the rotating crankshaft. This will increase the volume within the cylinder thereby decreasing the cylinder pressure. This would normally be the power stroke, but remember there is not a spark plug present in the cylinder to start the point of ignition (On cylinders with dual spark plugs the second spark plug must be disabled). Since this stroke does not have a point of ignition this will be referred to as the decompression stoke. As the piston moves further away from the cylinder head, the pressure within the cylinder follows the volume change and continues to decrease. Point “B” is the point at half mast, which is the point half way between point “A” and point “D”. This Point “B” on the decompression ramp as well as point “J” on the compression ramp should be within 20 crankshaft degrees of the TDC mark. If these half mast points fall greater than 20 crankshaft degrees of the TDC mark then the cylinder is most likely leaking. When this happens the compression tower looks like it is leaning. These leaning towers can be caused by cylinder leakage from valve sealing issues, piston sealing issues, head gasket sealing issues, or camshaft timing problems.
As the cylinder pressure decreases further point “C” is reached. Point “C” is at 90 crankshaft degrees after TDC compression. As the decompression of the cylinder continues point “D” is reached. Point “D” is the point of exhaust valve opening and this should occur between 30 and 60 crankshaft degrees Before Bottom Dead Center (BBDC) exhaust. The exhaust valve pocket at “D” should be formed with a definition point that is clear, concise, and repeats over and over. This shows the exhaust valve seal is intact. Right after point “D” the waveform starts to rise up until the exhaust plateau is reached. It is interesting to note that the piston is still moving down when the pressure waveform is rising right after point “D”. This is caused by the vacuum created during the intake stroke and seal within the cylinder when the intake valve closed. When the exhaust valve opens the higher atmospheric pressure in the exhaust system rushes into the cylinder to fill the low pressure area contained in the cylinder. This allows the pressure to rise even though the piston is still moving down, creating more volume which in turn should create a lower pressure. The point at “D” is how you can see if the exhaust camshaft timing is off during engine crank. This point can be up to 1 tooth off and you may not be able to identify the exhaust camshaft timing is off; however, you will be able to identify if it is 2 teeth off. This is very helpful because most engines will still start with the camshaft timing being 1 tooth off. If the engine will start it is better to check the idle compression waveform for camshaft timing which will identify a 1 tooth camshaft timing error. During the idle compression waveform the exhaust plateau ramps will be used to check camshaft timing. It is important to understand these exhaust plateau ramps will not be used to check camshaft timing for a cranking waveform.
Point “D” will rise into the exhaust plateau which is at atmospheric pressure and is shown as the average pressure at point “E”. At point “F” the intake valve opens allowing the air to flow into the cylinder. As the piston moves away from the cylinder head the cylinder volume is increased, creating a lower cylinder pressure. This pressure drop should occur at the TDC intake mark and should fall rapidly to the lower intake pressure, shown as an average at point ”G”, and is a lower pressure than the atmospheric pressure shown at point “F”. This pressure differential between “E” and “F” is at engine crank and is low compared to engine idle. At engine crank the pressure difference is 1-2 PSI, and at engine idle this pressure is 8-11 PSI. The faster the engine is turning the more energy is available to pull against the close throttle. This is why the exhaust plateau is larger at engine idle and smaller at engine crank. Additionally the pressure differential at point “D” and at point “G” should be within 2 psi of each other. If the pressure at point at “D” is lower than 2 psi from point “G”, there has been volume loss and you should suspect a leak within the cylinder. During the compression stoke the in-cylinder pressure is increased which allows a larger volume to escape past a leak point. As the cylinder decompresses the decrease in air volume within the cylinder will show greater vacuum at point “D” than the intake pull pressure at point “G”. This allows the exhaust pocket at point ”D” to have more vacuum than the intake vacuum pull had at point “G”.
At point “H” the intake valve closes as can be seen by the rapid rise in pressure after point “H”. This pressure rise should occur between 40 and 60 crankshaft degrees After Bottom Dead Center (ABDC) compression. This point is where the intake camshaft timing will be checked. It is interesting to note that as the piston is rising, reducing the volume in the cylinder, the pressure within the cylinder remains the same and does not increase as one might expect. This is due to the intake valve still being open during the piston’s upward movement, thus exposing the cylinder pressure to the pressure from the intake manifold. The intake manifold having volume area will store other cylinders intake pulls, which act like an accumulator storing a lower pressure. This lower pressure will stabilize the in-cylinder pressure from rising until the intake valve is closed, which makes this an ideal point to check the intake camshaft timing.
The piston continues its upward movement lowering the volume thus increasing the in-cylinder pressure. The pressure will increase and come to point “I” which is at 90 crankshaft degrees. As the pressure increases it will come to point “J” which is the point of half mast on the compression tower. The piston will continue its upward travel to the point where the smallest volume and highest pressure in the cylinder is reached, which is TDC. It will help to understand that the majority of the in-cylinder pressure is made in the last 30 degrees of crankshaft rotation and very little piston travel occurs at this point. If any of the cylinder volume leaks out of the cylinder it will have a great impact on the overall pressure at TDC within the cylinder.
In order to fully understand the in-cylinder pressure waveform we will need to look at the intake and exhaust waveform that is produced during engine crank as well, as seen in figure 2. The green waveform is the same compression waveform that we have analyzed thus far. It will be very helpful to overlay the intake waveform (blue trace) and the exhaust waveform (yellow trace) over the compression waveform (green trace). When analyzing the in-cylinder pressure waveform this overlay will help show where the cylinder leak is located.
We will start by analyzing the intake waveform (blue trace) which is read in inches of mercury. The cylinder that is producing the in-cylinder pressure waveform will have the intake pull located right after the TDC 360 crankshaft degree mark. The intake pull indicated by the blue trace hump marked number 1 is produced from the in-cylinder pressure waveform (green trace). As we have covered previously, the point at “F” is where the intake valve opened first which then affects the manifold pressure. The intake manifold pressure is falling across the TDC mark and then starts to rise; this is the point that the cylinder intake flow is great enough to change the accumulated intake pressure. It will be important to check this point on each of the cylinder pulls. This point where the intake pressure is falling and then rising is the point where the vacuum pull is transferring from one cylinder to the next. These transfer points should all be very even, as well as the overall pressure increase located next to the cylinder number. If they are not even a cylinder leak is the likely cause. To locate the cylinder vacuum pull, first locate the pull from the cylinder that is producing the in-cylinder pressure waveform as described above and then apply the firing order for the engine you are working on. As can be seen on the blue trace each of the cylinder vacuum pulls is marked by the firing order, indicating the cylinder that created it.
Now we will analyze the exhaust pressure waveform (yellow trace) which is read in inches of water column. The cylinder that is producing the in-cylinder pressure waveform will have the exhaust push located right before the TDC intake 360 crankshaft degree mark. As we have covered previously, the point at “D” is where the exhaust valve opened and this exhaust valve opening will affect the exhaust manifold pressure. The yellow trace where the number 1 is located is where the exhaust valve opening, allowed the vacuum contained within the cylinder to pull the exhaust manifold pressure into the cylinder, thus creating a low pressure area in the exhaust. This low pressure, indicated by the number 1 mark, is the transfer area or the point that one cylinder exhaust is transferring to the next cylinder’s exhaust. These exhaust transfer points should be even from one cylinder to the next cylinder. Just after the BBDC exhaust mark the piston starts to move upward creating a pressure push which in turn increases pressure in the exhaust manifold. As the piston moves upward this pressure will increase until it reaches a level just above atmospheric pressure. This pressure level will have slight ripples which is normal. At the point the exhaust valve is closing and the next cylinder exhaust valve is opening the pressure drops into the next exhaust pressure transfer point.
Now we will look at a problem cylinder as seen in figure 4; the green trace is the in-cylinder pressure waveform, while the blue trace is the intake waveform, and the yellow trace is the exhaust waveform. When analyzing the in-cylinder waveform several things stand out, such as the leaning compression towers, and the deep exhaust pocket. These items clearly show the cylinder is leaking. Now look at the intake waveform in blue. The cylinder intake pull marked 1 is from the cylinder we are currently testing. The intake pull marked 3 as we can see is narrow and the transfer point at the TDC mark has moved to a positive pressure. This is caused from an intake valve sealing problem. As the piston moves up ward on the compression stroke the air volume in the cylinder is push into the intake manifold past the leaking intake valve. This creates the narrow intake pull on 3 and then the positive pressure in the intake manifold.
Now we will look at a problem cylinder as seen in figure 5; the green trace is the in-cylinder pressure waveform, while the blue trace is the intake waveform, and the yellow trace is the exhaust waveform. When analyzing the in-cylinder waveform several things stand out, such as the leaning compression towers, and the deep exhaust pocket. These items clearly show the cylinder is leaking. Now look at the intake waveform in blue. The cylinder intake pull marked 1 is from the cylinder we are currently testing. Notice that the intake pulls from 1 and 3 are slightly lower, additionally the transfer point after 1 is lower. If the exhaust valve is leaking when the intake valve opens, some of the air is pulled from the intake manifold and some of the air is pulled from the exhaust manifold past the leaking valve. This lowers the intake manifold pressure until the intake valve closes. To be sure that this is not a piston sealing issue, the engine will be run and the exhaust pockets will be check. If the exhaust pockets are changing cycle to cycle the exhaust valve is likely the problem. If the exhaust pockets are good the piston seal is likely the problem. The crank case pressure can also be tested to see if pressure is building in the crank case when the compression stoke is made indicating the piston is not sealing.
Technology is a great asset in our lives, but one must use these assets in order to gain from them. Could you imagine not using a cell phone? Not that long ago we didn’t have cell phones but now the cell phone serves a purpose and helps us in our daily lives so now most people carry one of these high tech units. Once you start to use pressure transducers in your service bays you will wonder how you ever got along without these high tech marvels!
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