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PRESSURE TRANSDUCER TRAINING
Pressure Transducer Case Studies
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• Cam Gear Boogie - 2002 Dodge Stratus 2.7 Vin R
Bob Powell - Bob Powell's Auto Repair
A local shop brought this car in after a remain engine was installed. The complaint was that the car would not start unless the Cam sensor was unplugged. Otherwise, it ran OK. They figured that much out but were unable to pinpoint the cause of the fault.
The first thing I wanted to do was see if the cam and crank sensors were in sync. I was able to start the car with the sensor unplugged, then plug it back in and get some signals. Here is what I saw (Figure 1) Note the cam waveform pattern is 1-2-3-2-1-3 which will be important information. I could not find any known good patterns that matched my cam and crank signatures so at this point the jury was still out.
Next I decided to hook up a pressure transducer in one spark plug hole on each bank to verify whether or not the cams were physically in sync. The patterns indicated that things were in proper sync mechanically. (Figures 2 & 3)
OK, now I'm getting somewhere. The engine is mechanically synced so I must have a sensor problem. The crank sensor reluctor on this car is on the flywheel. The tech told me the same flywheel was used from the original engine. I next asked him about the cam gear reluctor. This is where things got real interesting. He informed me that the engine came with new chain and gears in a box and had to be installed. I asked if he had the original gear. He did and agreed to drop it off at the shop so I could inspect it.
The original reluctor slot pattern was 1-2-3-1-3-2. This did not match the cam pattern I saw on my scope. At this point I knew the problem was an incorrect cam gear for this application. I instructed the tech to install the correct cam gear/ and his problem would be solved.
One week later he comes back and says, "I replaced the gear but I have the SAME problem". I can't believe it so I hook up my scope again to see what's what. Here is what I see from the cam sensor now. (Figure 4) Big chunks of the signal are totally missing. Time to quiz the tech again. After some discussion and checking, the "new" problem was discovered. I found if I took the cam sensor hold down bolt out and tilted the sensor toward the reluctor ring, the car would start and the cam signature was correct.
This picture tells the story. (Figure 5) Rather than put a new correct gear on the motor the customer ground off the rivets from the original gear, took the reluctor ring off and put it on the new gear that came with the motor. Because it was not centered properly it rotated in an elliptical pattern that caused it to move away from the cam sensor every 180 degrees. That's why the cam signal was missing lots of pulses. This is what the proper cam and crank sync looks like for this motor. (Figure 6)
The tech had installed all the mechanical components correctly but unfortunately he did not understand the significance of the slots in the relutor ring. After some show and tell with the scope he now understands completely.
• 1995 Jetta Crank / no start
by Rusty Flake - Beck Service Center
Complaint: Car will turn over, but wont start.
Verification: Indeed, did crank, would not start. Nothing else noted by technician looking at car.
This Jetta has a 2.0L engine with a little over 90 K miles on it. I did not start out working on this vehicle, so I do not know exactly what all was checked or replaced on it.
I know this car was in the shop for two days before I was asked to do a compression wave form on it to see if it was either in or out of cam timing. It also had an ignition rotor and cap installed prior to determining that there surely has to be a possible timing belt jump concern or something along that line.
The technician that was working on this car is very good at diagnostics normally, and I don’t know what the process was that led to the point of him asking me to do the compression waveform analysis, but even though I’m not what I consider great, I do have some cool equipment and am glad he asked so that I get yet another chance to look at some interesting waveforms and analyze them as to what I believe is the problem.
Pressure transducers are fairly new to me, and I am still in the learning curve, but these things are great. I connected to cylinder 1 since it was already with out the spark plug installed. And I synced off of that cylinder also. This waveform was interesting to me and only took about 30 seconds to capture on this car.
I called the technician working on the car over and told him I know that the ignition firing event is happening at the wrong time which would indicate to me that something in the timing components on this engine wasn’t correct.
At this point he talked to the service writer working with this cars owner to get approval for the inspection of the timing components.
The approval was given, and the car was taken down to our heavy line tech for disassembly and inspection. Once the cover was removed, the tech came and got me since he thought everything was in time or at least really close and wanted a second opinion, along with knowing I wanted to see it anyway.
I thought that the cam sprocket mark and the rotor position in the distributor were both off by a tooth.That however did not make sense when compared to where the firing event was taking place in the compression waveform.
I then suggested that the sprockets be removed to make sure that the dowel and or woodruff key had not broken or elongated the slots they go in. Figures 2 &3 show the main problem with this car.
A new crankshaft sprocket was ordered and installed with a new timing belt, and the car started.
• 1999 Ranger 2.5L std. trans, 117K miles
by Rusty Flake - Beck Service Center
Complaint: Starts and idles fine, but surges when put into gear. When I went to the truck and started it, it bucked and jerked and died several times before it got to where it would even idle. After letting it run long enough to stabilize at idle(several minutes) I tried to drive it into the shop, it could not be throttled up or it would sputter and jerk and die. I had to let it idle and slowly release the clutch to get it rolling enough to coax it into the shop. After finally getting into my bay, Ipulled codes and retrieved P0171 sys lean, I then connected my gas analyzer to look at lambda out the tail pipe. The exhaust showedbetween 25-30% lean when Idling fairly smoothly, I still could not bring it off idle, fast or slow, when trying too, the exhaust lambda reading went to 80% lean, when it didn't die out completely before it could be kept running. The fuel system on this truck is returnless, the fuel spec., when converted in alldata was 63 psi. I connected my flow meter with the outlet hose into ma container and restricted, to allow some type of a measurement of flow. The pressure showed 61 psi and flow was .2 gpm, when the restriction was opened to allow .5 gpm the pressure dropped to 50 psi. At this time I recommend a fuel pump and filter with a potential check of pwr. and ground through the sender and suggest sending the inj.'s out for cleaning.The job was approved and completed, with a result that wasn't good. Basically the truck ran the same with the exception that it took less time for it to be able to idle and could be snap throttled now without dying. Looking at fuel trims, O2's and lambda, this truck was still running very lean. Since I can now snap throttle I look at MAF sensor waveform, not good, replaced MAF sensor and finally the truck is fixed.
• 1994 Volvo 940 2.3 L 4Cyl. No Start 148,447 Miles
by Peter Landry - Landry’s Brookfield BP
I was called to a shop to see if I could lend a hand with a no start. At this point I’m told the car has spark, fuel, compression and wet spark plugs. Sounds like timing, right???? They told me they pulled the timing cover and the marks are spot on.
My first test was confusing. I did a cranking current test with a sync probe hooked to number 1 spark plug wire. The results suggest the engine is in time or at least spark is being fired when A cyl. is at TDC. Figures 1 and 2
Second test, I hooked up my 5 gas analyzer and the readings shocked me. On one hand I have wet spark plugs, and on the other I have very little, less than 500ppm, HC out the tail pipe. What can possibly cause this issue???
I figure this engine is not breathing. At this time I recall being taught that an engine is just an air pump. The best test I’m aware of to test the engines ability to breathe or pump air is a cranking vacuum test using a vacuum transducer. Since ATS transducers along with the Escope does the conversion of voltage to inches of mercury for me there is no need to use a gauge here. Notice the pulses are even and the pattern is what I would expect to see. However note the scale. The pulses are going above and below zero inches of mercury. Figure 3
Now we’re getting somewhere. My next test compares cranking compression and the firing of number 1 spark plug. As you can see there is a problem here. There are a few points in this waveform that stick out like a sore thumb. Figure 4
Let’s take a closer look at the pressure waveform. Figure 5
1. Spark should occur at or before TDC and it most certainly doesn’t. Figure 4
2. The towers aren’t even they are leaning to the right.
3. There is no exhaust ramp
The cam sprocket locating pin was sheared, allowing the camshaft to turn roughly 90 degrees out of time.
On this car, the distributor is driven by a separate sprocket by the timing belt, which is why the ignition and crank timing was still a match in figure 1 and 2. Figure 6
Looking at cylinder pressure dynamically is both a quick and accurate way to diagnose mechanical problems without engine disassembly. If you’re like me, you’re not going to take anything apart unless you’re quite confident you have to.
Thanks Bernie and ATS gang for your great tools and support.
• Impossible Dodge
by Bernie Thompson - ATS
The white Dodge Stratus pulled into the parking lot not under its own power, but behind the tow truck. An elderly gentleman walked into the office with the tow driver. He explained that the Dodge dealer had diagnosed his 2003 2.4 liter DOHC Stratus for a no start condition and that the valves had been damaged due to a timing belt problem. A close friend of his had recommended our shop for a second opinion. We exchanged information and I told him that I would call him later that day.
We pushed the vehicle in and cranked it over. At first it sounded like they were right but cranking over the engine will only indicate the RPM is high, not the cause of the problem. If there is a compression problem the engine will increase its cranking speed. In turn one could associate this rapid RPM with a timing belt problem or bent valves. However; additional testing will need to be preformed to determine the cause of the problem.
Until recently there was no testing sequence that could determine the exact cause of such a problem. With advancements in technology have come new diagnostic techniques that will astound you. These techniques use pressure transducers to make the impossible into the possible. With this new technology the technician can look inside the engine to determine the mechanical operating condition through the spark plug hole. How is this done? By removing the spark plug form the cylinder head and threading a compression test hose into spark plug hole a waveform can be generated by a pressure transducer attached to this compression hose. Figure 1 shows this compression waveform as the red trace. The yellow trace is produced from a pressure transducer connected to the intake manifold and represents the vacuum in the intake manifold. The green trace is used as a reference of 0 PSI. By analyzing these waveforms the engine condition can be determined
Now let us analyze the compression waveform in figure 2. Starting at the left side of the chart the red trace starts to rise. This pressure rise is due to the piston moving toward the cylinder head. The closer the piston comes to the cylinder head the higher the pressure within the cylinder becomes. At the point the piston comes as close to the cylinder head as possible; this is where the peak pressure will occur and represents top dead center (TDC). This peak pressure during cranking should be greater than 95 PSI and is usually about 140PSI in a good engine. The peak pressure in this Dodge 2.4 liter engine is very low at 40PSI. If a conventional compression test were done using a gauge this peak pressure would be all the data that would be given. It indicates that the cylinder’s ability to produce pressure is compromised but does not indicate why. Perhaps the Dodge dealer used this test to determine their conclusion. The problem with a traditional compression test is that there is not enough data to make a diagnostic conclusion.
As the crankshaft continues to rotate, after TDC the red trace starts to drop indicating a decrease in pressure. This is caused by the piston moving away from the cylinder head and increasing the volume within the cylinder. As the pressure continues to drop it is important to look at the pressure half way down the compression tower. At this point the two compression tower halves should be even (right and left of the pink TDC mark). The Dodge 2.4 liter compression towers are uneven, indicating a leak within the cylinder. This usually indicates a mechanical problem in the engine such as valves that do not seat or valves that do not open. However, incorrect intake camshaft timing can also cause the compression towers to become uneven.
As the piston continues its downward travel it starts to pull a negative pressure at about 50 degrees after TDC (0 deg). The negative pressure has a steady increase until the exhaust valve opens at about 30 degrees before BDC (180 deg). The exhaust valve should open at approximately 40 deg +/-10 deg before BDC (180 deg). This shows the exhaust camshaft is properly timed to the crankshaft. The pressure then starts to rise as the higher exhaust pressure flows into the lower cylinder pressure. At the BDC (180 deg) mark the pressure has equalized to the exhaust pressure. The piston now starts its upward travel creating a high pressure area in the cylinder. This high pressure moves to the low pressure area in the exhaust system. At the TDC (360 deg) mark the intake valve should open causing a drop in pressure as the piston moves down. The Dodge 2.4 liter has a slight pressure drop at the TDC (360 deg) mark as the piston moves down. There is a larger pressure drop at the 60 deg ATDC (360 deg) mark which could indicate that the intake valve opened late.
With the intake pressure not changing it is hard to determine exactly when the intake valve opened. A much better place to check for the intake cam timing is at the point the intake valve closed. The intake valve closing will start the cylinder pressure to rise. This usually occurs at 50 deg +/-10 deg after BDC (540 deg mark). The Dodge 2.4 liter intake valve closes at 93 deg after BDC (540 deg mark). This indicates that the intake camshaft is over 40 degrees retarded. The Dodge dealer was correct that the camshaft timing was off; but are the valves bent?
To determine this we will need to analyze the intake vacuum waveform in figure 3. The yellow trace shows the pressure changes in the intake manifold. As the yellow trace increases (goes up) the piston is moving down. This creates a negative pressure (vacuum) in the intake manifold. Normally the vacuum is 1 to 3 inches of mercury. The Dodge 2.4 liter is low at 0.8 inches of mercury. It is important to notice that all of the cylinders have equal vacuum pulls and that they all can pull vacuum. This would indicate the valves are not damaged. It is also important to notice that on each stroke of the intake the pressure goes positive. This usually indicates that the intake valve is not seating. If the intake valve is not seating and the piston is rising on the compression stroke; the high pressure created under compression is forced into the intake manifold. This creates the positive pressure in the intake manifold.
Now let us put the data that we have collected together so we can diagnose the Dodge 2.4 liter engine. We now know that the exhaust camshaft is in time but the intake camshaft is out of time by an amount of 40 degrees in the retarded position. This would mean that the piston is moving up to compress the air within the cylinder; although the intake valve does not close due to the retarded intake camshaft timing. As the piston continues to move upward the air is forced into the intake manifold. This creates the positive pressure on the intake waveform that we see. This pressure is not caused by a bent valve but solely by the intake camshaft timing error.
The timing belt was replaced and the Dodge 2.4 liter engine once again roared to life. This entire diagnosis took just under 15 minutes. Just a few years ago this would have seemed impossible, but with today’s technology the impossible has become the possible!
• The Misfiring Ranger Mystery
by Rick Layton - Diagnostics on Wheels
I received a call early one morning from a service manager I had dealt with many times in the past who had just moved to another shop that I hadn’t dealt with before. It turns out that when he switched shops he brought one of the top technicians with him who is a very thorough diagnostician. The new boss greeted them with a mystery misfire that was plaguing a fleet of 2005 Ford Rangers that this shop serviced and it had stumped all of his previous technicians. Knowing how most of the fleet vehicles in our area are maintained; I gritted my teeth and agreed to schedule a visit.
Upon arrival, I was pleasantly surprised to find a clean and well-maintained 2005 ford Ranger with a 3.0l V6. I commented on the condition of the vehicle and the technician explained that the fleet manager for this company got a brand new fleet of trucks all at once on the condition that they make them last. To accomplish this the fleet manager serviced all of the fleet on the severe duty service schedule and only used the best quality parts for repairs. That is why he was so perplexed when the Rangers started developing misfires that no one could solve.
I reviewed the service history of the vehicle, which had 119,000 miles, and spoke with the technician about the tests he had performed on the vehicle. The vehicle had started with an intermittent misfire on cylinder 6 (P0306) about 2,000 miles ago which had progressively gotten worse. Since the vehicle was almost due for its second tune up the fleet manager brought it in and had a full tune up along with a decarbon and injector clean performed. This had no effect on the intermittent misfire at all. Then two other trucks in the fleet started developing the same symptoms on different cylinders. The technician had checked the TSBs and had found that Ford had issued a bulletin 05-26-03 for misfires being caused by valve seat recession. Ford recommend performing a compression test looking for any cylinders whose compression was outside of spec and replacing the cylinder head with a revised unit if a problem was found. The technician had performed a compression test and found all cylinders within 10 PSI of each other with the misfiring cylinder being the lowest on compression. Even though the difference was less than 10% between cylinders, the fact that lowest cylinder was number 6 led him to follow up with a leakdown test. The leakdown test showed all cylinders to be within 1% of each other and the best cylinder was the problem cylinder. Convinced that the cylinder integrity was intact the technician proceeded to check for a vacuum leak, swap spark plugs, spark plug wires, ignition coils and injectors with other cylinders to try to locate the problem; with no luck.
I started by verifying that the P0306 was present and by test driving the truck to duplicate the problem. The miss got better when the vehicle warmed up but was still noticeable at idle. I then decided to verify cylinder integrity quickly by using my ATS EScope Pro with a 300 PSI pressure transducer installed in place of the spark plug to record a waveform of the pressure within the cylinder. This technology takes the dynamic (also known as running) compression test to the next level. I started with a known good cylinder and recorded this pattern (figure 1). I then proceeded to our problem cylinder and recorded another pattern (figure 2). Right away I noticed that the compression peak in the problem cylinder was lower than the good cylinder and was also below the 50 PSI minimum pressure that Bernie Thompson, founder of ATS, says is necessary for combustion. I was expecting to see something in the 60 to 80 PSI range as is shown in figure 1. How is it possible to have a good compression test and a good leakdown test with a bad dynamic compression result? If you look at figures 3 and 4 you will see that I have blown up the same section of the pattern from figures 1 and 2 for comparison. If you notice in our good pattern (figure 3) that the intake and exhaust pressure pulses are almost even while on our bad cylinder the exhaust pulse is much lower than the intake pulse indicating that we have an issue with the exhaust event on cylinder 6. I recommended that the shop remove the cylinder head for inspection and they found that the valve seat had receded into the cylinder head but was still sealing. A removal of the heads to replace the valve seats cured the misfires on all three vehicles.
• 1997 Cavalier 2.4L DOHC, Auto Trans, 88,701 miles
by Rusty Flake - Beck Service Center
Customer Concern: Fuel pump and water pump replaced, Hard to start, Have to hit the remote start button or key 3 times to get the vehicle to start, surges after it starts. The running problem started after the noted work was completed.
Verification: I went out to pull the vehicle. In or go for a test drive, which ever seemed to be best for the current situation on this car. Upon starting the car I noticed it did surge, and when put into gear to drive the car, it had an extreme lack of power, so a test drive was skipped.
After I got the car into my bay, I wanted to pull codes just to see if anything was pending.
The code number was not logged on the work order and I don’t remember the number, but it is noted on the invoice that a ckp/cmp correlation code was present.
Given the running condition of the car and the code, I chose to put a compression pressure transducer in one of the cylinders to see if maybe when the water pump was changed somehow the cam timing changed.
Figure 1 shows cylinder 1 compression and sync right after start up. Figure 2 shows another waveform after idling for a little bit longer. Notice that the compression peak pressure is high and the base pressure is high along with not having a defined exhaust valve opening, or visible intake event when first started, better a little later on.
Although the exhaust valve opening time doesn’t look way off (is out though) the increased cylinder pressure and lack of definition of exhaust and intake valve opening led me to believe that the timing between crank and at least one of the cams must be off.
We have different techs for different areas of work in our shop, so this car went down to the engine end for tear down and inspection. It was noted on the invoice that some of the timing cover bolts were missing and the seal had not been changed on the cover in the earlier performed work. The timing components were replaced and a tune up completed along with sending the injectors out for cleaning.
Figure 3 shows cylinder 1 after the repairs were completed.
As you can see from the change in cylinder pressures and the definition of the ramps, everything would appear to be normal, and indeed the car now runs good.
• 1997 Chevrolet S10, 2.2L engine, Standard Transmission, 181,529 miles
by Rusty Flake - Beck Service Center
Customer concern: Stumbles and backfires through the intake when accelerating, runs better if you ease into the throttle.
This truck came into the shop when I was on vacation, and was looked at by one of the back up drivability techs.
The truck definitely ran poorly and popped back through the intake, the misfiring cylinder was located and then the valve cover removed to see if there was anything visible causing the problem.
After the valve cover was removed, the technician noticed that the exhaust valve rocker arm nut had come off. With the nut off the rocker arm stud, naturally the valve will not open thus the poor running and popping back through the intake.
Two nuts were ordered to replace both of the nuts on cylinder 4. The nuts were installed and all of the others were re-torqued while the cover was off, the cover was re-installed with a new seal and only a couple of bolts put in to keep oil sling down while verifying the running condition.
With the cover back on the technician started the truck, and it ran good at idle. He let the truck idle for a short time and then decided to snap the throttle to make sure there was not a problem still with popping through the intake.
The truck appeared to be running normally, it was time to shut it off and fully assemble the truck. While on the way to shut the truck off, the engine began to run rough again.
The technician pulled off the valve cover again to see if maybe the stud on the exhaust valve had pulled out or the nut came off again or any thing else obvious. Nothing was noticed this time upon inspection and the parties that were now involved with this vehicle and its repair decided that it surely had an internal problem. The service writer was now going to work on an estimate to repair or preferably replace this engine due to the mileage on it. This is on a Friday, I came back the following Monday.
Monday morning my backup tech informs me about the jobs that are in the shop carried over from the Friday of the week I was gone. This is how and when I find out about the truck in this case study. At first I don't want to look at the truck since I am assuming that the estimate has already been written, the customer called, and we are waiting for approval to replace the engine.
I think about it and finally my curiosity gets the best of me and I can no longer let this go without at least hooking up my cool tools from ATS. I found it hard to believe that it ran good for a few minutes and then bad and didn’t have something simple as opposed to a major engine problem. I like to use pressure transducers for compression waveforms and analysis, and this was a really good opportunity to do just that.
Figure 1 is the compression waveform of cylinder 4.
Just a note…when I went to move the truck to my area of work, it would barely run and could not accelerate it at all.
After I captured the waveform and analyzed it I felt I had a problem with the intake valve on cylinder 4, I was not sure what it was, but the only thing that made sense to me was that the spring must have broken. I decided to take the valve cover off and inspect. I had to use a mirror to see the spring, and indeed it was broken. I reported what I found to the service writer and the two other drivability techs and then replaced the intake spring. Figure 2 is the compression waveform afterwards.
With the wave form looking normal and the truck running good, a test drive was taken after complete re-assembly, and the truck returned to the customer.
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