Practical Pressure Transducer Diagnostics
by Bernie Thompson – Automotive Test Solutions
Perhaps one of the world’s greatest known detectives is Sherlock Holmes. Sherlock is known for his proficiency with observation, deduction, cutting edge equipment, and logical reasoning that borders on the fantastic. These are all attributes that one needs on a daily basis when diagnosing the modern vehicle. When Sherlock takes on an investigation he immerses himself in the available data that is present. This is what you must do when looking into your new investigation; for each and every vehicle diagnosis you take on is an investigation that will require your proficiency with observation, deduction, cutting edge equipment, and logical reasoning.
“The game is afoot”; our first investigation is a 2011 Jeep Cherokee Overland, with a 5.7 Liter V8 engine that has a misfire. This misfire is elusive, and only presents itself once the vehicle has reached operating temperature. Addi-tionally the misfire is intermittent and will only occur on heavy acceleration, such as that from a stop. The first observation is the vehicle is in good condition and has been maintained. The engine components are all intact and appear to be in good condition. The engine sounds normal without any unusual noise. Our first clue is provided from scan data. The data from the on-board engine control module has DTC’s set for misfire; P0300 random misfire, P0302 misfire on cylinder 2, P0305 misfire on cylinder 5. There is no direct connection between the cylinders 2 and 5, firing order 1-8-4-3-6-5-7-2. The cylinder arrangement is Bank (1) 1-3-5-7 – Bank (2) 2-4-6-8. These cylinders are not next to one another, nor do they proceed one another in the firing sequence, nor are they on the same crank plane.
The scan data also provides additional data on the fuel control for the engine. Upon monitoring the oxygen sensors for bank 1 and bank 2, they indicate that the fuel control is working correctly. The total fuel trims for both banks are less than +/- 10 across all load ranges. This indicates that the fuel control system is working within a control window that should not be contributing to that of an engine misfire.
Now we need a better understanding of which cylinder or cylinders are actually misfiring. A wiring diagram for the vehicle is analyzed. An oscilloscope is then connected to the crankshaft position sensor signal and the #1 ignition coil control wire. The vehicle is then driven until the misfire is present. The data is then processed with an advanced frequency plot so the crankshaft velocity changes can be analyzed. The only cylinder presently misfiring is that of cylinder number 2, as can be seen in Figure 1.
Fig 1
Figure 1 indicates the crankshaft speed changes. If the fuel stock is combusted the thermal energy expands, accel-erating the crankshaft. If the fuel stock does not combust then this thermal expansion does not occur, so the crankshaft speed slows down. These crankshaft slowdowns can be seen as the three drops at the top middle of the screen. Since the control for the #1 ignition coil is also monitored, a firing sequence can be established. Thus cylinder 2 can be identified for the misfire.
Now that the misfiring cylinder is known the oscilloscope is moved to the #2 ignition coil control. The vehicle is then driven until the misfire occurs. The ignition waveform is then analyzed. The waveform does not show that an ignition spark event was responsible for the misfire. The spark plug is then removed and a 300 PSI pressure trans-ducer is installed in its place. Without a spark event present within the cylinder the cylinder will misfire. This is acceptable because the pressure changes within the cylinder can help identify the cause of the misfire. Additional-ly a -30 Hg transducer is installed in the induction system behind the throttle plate. This allows the intake pressure pulls for each cylinder to be monitored. A +/-25” H2O transducer is also placed in the exhaust system at the tailpipe outlet. This will allow each cylinder’s exhaust pushes to be monitored. It is important to run the in-cylinder testing during Crank, Idle, and Snap/Decel, with all three transducers on the engine. The following data does not show each test, but each test was completed on each investigation. The results are shown based on which test was used to identify the problem.
The engine was disabled so it would not start, then the engine was cranked for a long period, about 8 seconds. Make sure to not open the throttle plate during this crank period. If the throttle plate is opened there will be less vacuum produced from each cylinder pull in the induction system, thus making the diagnoses harder. This in-cylin-der cranking pressure waveform (green trace) was then taken and is shown in Figure 2. The peak pressure is 191 PSI at TDC. The compression towers are not leaning but are even. The exhaust valve opening occurred at 54° BBDC. The intake valve opening occurred at 18° ATDC. The intake valve closing occurred at 26° ABDC. Additionally there is an induction pressure drop present at 45° ATDC.
Fig 2
The first indication that a problem exists is with the valve opening and closing. The exhaust valve opening is normal; rule of thumb the exhaust valve should open from 30° to 60° BBDC. However the intake valve should open as a rule of thumb at about TDC; and rule of thumb the intake valve should close 30° to 60° ABDC. The late intake valve opening at 18° ATDC, and the early intake valve closing at 26° ABDC indicates a problem. The pressure drop occurring at 45° ATDC is due to the piston moving down, creating a negative pressure within the cylinder. This negative pressure is established due to the cylinder still being sealed. Once the intake valve opens the pressure within the cylinder quickly rises as volume is moved from the induction system into the cylinder.
Fig 3
This can be seen in Figure 3 within the intake pulls (blue trace). This is the same data as is Figure 2 but the intake pulls are now turned on, and the data has been zoomed. Note the pull for cylinder 2 is different from the other intake pulls. When you inspect these pulls across the center (up to down) the pulls should be even across the middle of each pull. Additionally each intake pull should be formed like the other pulls. It can clearly be seen this is not the case with the cylinder 2 pull. First the transition point (marked with an arrow) from where cylinder 7 is ending its pull and cylinder 2 is starting its pull, falls to a point of less negative pressure (vacuum) than the other transition points. Since cylinder 2 intake valve is not open the intake pressure drops. Note that the transition point is very sharp. This occurs from the valve not opening slowly as it is designed to do, but opening rapidly. The quick rise in negative pressure is due to the high vacuum that was contained within the cylinder from the late intake valve opening. With high negative pressure within the cylinder when the valve does finally open, there is a greater force between the intake pressure and that of the cylinder pressure. Therefore a fast rise in negative pressure occurs within the intake pressure. This indicates that the intake valve opening mechanism is loose or worn. In this case the intake camshaft lobe and lifter roller are worn.
Our next investigation is a 2001 Ford Ranger with a 3.0 Liter V6 engine; this vehicle has a misfire. This misfire is elusive and is only present after running the engine hard with the engine at operating temperature. The first observation is the vehicle is in fair condition and is in need of maintenance. The engine components are all intact and appear to be in fair condition. The engine sounds normal without any unusual noise. Our first clue is provided from scan data. The data from the on-board engine control module has DTC’s set for misfire; P0302 misfire on cylinder 2. This 3.0 Liters firing order is 1-4-2-5-3-6. The cylinder arrangement is Bank (1) 1-2-3 Bank (2) 4-5-6. Mode 6 also has data indicating that cylinder 2 is the misfire.
The scan data also provides additional data on the fuel control for the engine. Upon monitoring the oxygen sensors for bank 1 and bank 2, they indicate that the fuel control is working correctly. The total fuel trims for both banks are less than +/- 10 across all load ranges. This indicates that the fuel control system is working within a control window that should not be contributing to that of an engine misfire.
An oscilloscope is then connected to all of the ignition coil control wires and the engine is run hard on the test drive until the misfire occurs. The ignition data shows turbulence on cylinder #2. The #2 cylinder’s sparkplug is then quickly removed and replaced with a 300 PSI pressure transducer. Additionally a -30 Hg transducer is installed in the induction system behind the throttle plate. This will allow the intake pressure pulls for each cylinder to be monitored. A +/-25” H2O transducer is also placed in the exhaust system at the tailpipe outlet. This will allow each cylinder’s exhaust pushes to be monitored.
Fig 4
The engine is started and allowed to idle. The pressure waveform is shown in green, Figure 4. The peak pressure is 38 PSI and the compression towers are leaning, which indicates the problem is mechanical. The exhaust valve opens at 52° BBDC, the intake valve opens at 6° BTDC, and closes at 52° ABDC. Note that the exhaust pocket is lacking definition and it appears very rounded. This indicates that the exhaust valve is not seating properly. Furthermore the exhaust pocket is lower than the intake pocket. This indicates that the cylinder lost volume during the compression stroke.
When the intake valve is open and the piston is moving downward increasing the volume within the cylinder, a negative pressure is produced. When the intake valve seats, this negative pressure is sealed within the cylinder. Under the compression stoke, if anything is leaking, the volume contained within the cylinder will be reduced thus lowing the exhaust pocket from that of the intake pressure. When the piston returns to the same position it was in when the intake valve closed, if there is no leak, the volume will return to the same volume contained within the cylinder when the intake valve closed so the same pressure will be present. In some cases it may be slightly higher (within 2 PSI). This difference is due to the intake valve closing and the exhaust valve opening occurring at different crankshaft degrees. If there is a leak the volume will be less thus the pressure in the exhaust pocket will be less; lower than the intake pressure. This always indicates a volume loss within the cylinder.
The intake pressure (blue trace) can be observed to be dropping pressure during the intake valve opening. So this clue is that the intake valve is not the leak. This is due to the intake valve being off of its seat during this pressure drop. This would indicate that the leak is the exhaust valve or the piston rings.
Fig 5
The throttle is then snapped open and closed as seen in Figure 5. This will allow the engine to go into decel. During the snap throttle event the RPM increases so that when the throttle is closed the engine RPM is about 3000. This increases the negative pressure (vacuum) within the intake system and the cylinder. With this greater pressure
differential leakage can be located. For example in Figure 6 the exhaust waveform has been zoomed in on, the exhaust indicates a high negative pressure during the intake stroke on cylinder 2. This indicates that the exhaust valve is not seating. With this high negative pressure within the cylinder there is a greater pressure differential between the cylinder and exhaust system. When the exhaust valve is not seating, the negative pressure being lower than that of the exhaust pressure pulls the exhaust into a lower pressure state. This indicates that the exhaust valve is leaking. This vehicle had the exhaust seat beat into the head. When the valve seat moves into the head the valve moves as well. Eventually the hydraulic lifter runs out of adjustment. When the valve gets hot it expands which, in turn, causes the valve stem to increase in length. The lifter can no longer properly adjust the valve lash so the valve does not seat properly.
Fig 6
Our next investigation is a 2015 Mercedes Benz C300 with a 2.0 Liter inline 4 cylinder engine; this vehicle has a misfire. This misfire is elusive and is only present upon first start with the engine temperature cold. The first observation is the vehicle is in good condition and has been maintained. The engine components are all intact and appear to be in good condition. The engine sounds normal without any unusual noise. Our first clue is provided from scan data. The data from the on-board engine control module has DTC’s set for misfire; P0300 random misfire. This 2.0 Liter firing order is 1-3-4-2. The cylinder arrangement is Bank (1) 1-3-4-2.
The scan data also provides additional data on the fuel control for the engine. Upon monitoring the Wide Range Air Fuel (WRAF) sensor for bank 1, it indicates that the fuel control is working correctly. The total fuel trim for bank 1 is less than +/- 10 across all load ranges. This indicates that the fuel control system is working within a control window that should not be contributing to that of an engine misfire.
Now we need a better understanding of which cylinder or cylinders are actually misfiring. A wiring diagram for the vehicle is analyzed. An oscilloscope is then connected to the crankshaft position sensor signal and the #1 ignition coil control wire. The vehicle is then started cold. The data is then processed with an advanced frequency plot so the crankshaft velocity changes can be analyzed. The only cylinder presently misfiring is that of cylinder number 1, as can be seen in Figure 7.
Fig 7
Now that the misfiring cylinder is known the oscilloscope is moved to the #1 ignition coil. The vehicle is started cold and while misfiring the ignition is monitored. The ignition waveform is then analyzed. The waveform shows turbulent air but the ignition spark event is not responsible for the misfire. The spark plug is then removed and a 300 PSI pressure transducer is installed in its place. Additionally a -30 Hg transducer is installed in the induction system behind the throttle plate. This will allow the intake pressure pulls for each cylinder to be monitored. A +/-25” H2O transducer is also placed in the exhaust system at the tailpipe outlet. This will allow each cylinder’s exhaust pushes to be monitored.
Fig 8
The engine is then started cold and the pressure data from the engine is analyzed, as shown in Figure 8. The first indication that a leak is present is the exhaust pocket is lower than the intake pocket. This is always a loss of volume during the compression stoke. The exhaust valve is opening at 36° BBDC, the intake valve is opening at 5° ATDC, and closing at 40° ABDC. Thus the valve timing is good. The next clue is that the exhaust pockets are all clean and all look like clones of each other. When a valve leak occurs the exhaust pockets usually have cyclic changes. These will be warping on the falling and rising edges and flat bottoms on the exhaust pocket itself.
Since these exhaust pockets have no changes within them, it is likely that the piston and or piston rings are where the leakage is occurring. The tailpipe sensor that reads +/- 25” H2O is put on the dipstick of the engine using a spark plug boot. Note before this dipstick can be used you must lightly blow into it to make sure that the tube is not under the oil level. If the tube is under the oil level you must use another location. The engine is disabled so it cannot start and then the engine is cranked for an extended period. The cranking waveform is shown in Figure 9.
Fig 9
The green trace is that from the in-cylinder pressure. The peak pressure is low at 66 PSI and the compression towers are leaning, indicating a mechanical problem. When measuring the difference between the intake pull and the exhaust pocket bottom, the exhaust pocket is large at 8 PSI. If the exhaust pocket is greater than 3 PSI during crank a leak is most likely present. The yellow trace is the crankcase pressure; note that the crankcase pressure increased after the compression of cylinder #1. Since the crankcase has a large volume contained within it, it will take the volume moving past the rings a period of time to actually change the crankcase pressure. Also the crankcase pressure hump for #1 cylinder is much wider as well. This indicates that the piston and or rings are leaking. A borescope was used to inspect the #1 cylinder. It was observed that there was scaring on the cylinder wall caused from overheating of the engine.
It will be important to always test the engine under the conditions that the problem is occurring under. For instance, if the cylinder on this MB would have been tested on a warm or hot engine the problem would not be present. It will be important during your investigation to keep proficient with observation, deduction, cutting edge equipment, and logical reasoning. By using this format you can become the sleuth of your shop.