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International Automotive Technicians Network
How to Prove an Engine Misfire
Posted to Technical Discussion Forum on 12/24/2009 197 Replies

I am putting up my information here for all to see. It took a while to put all of this together. I hope I didn't make too many mistakes.

I am admitting that I could not prove what was causing a dead miss on cylinder #2 on a 95 Chevy S-10 Blazer with a 4.3 liter V-6. The vehicle has 153,000 miles and is a first time customer. What I am trying to accomplish with the help of everyone here is to develop a test that can be performed to isolate this type of failure without undue or unnecessary testing. This is the 4th vehicle in about 2 years that we have seen this happen on. We have a mechanical problem that I am unable to prove with any specific tests in my (limited) arsenal.

You can refer to my original post located


to see what many others thought about this particular situation.

This engine uses a single fuel injector that feeds 6 separate poppet nozzles which snap into the lower intake manifold, aimed at the back of the intake valve for each cylinder. The manifold is designed as such that the possibility of reversion feeding fuel from one cylinder to another seems impossible to me.

It has a single coil with a high voltage switch (distributor) and the spark is triggered from the crankshaft position sensor. I believe it was GM's first use of the low profile distributor cap that has become known to have a high incidence of crossfire internally.

The customer's comment was a rough idle and low power and a CHECK ENGINE light that was on. The vehicle exhibited a typical "dead miss" on the very short test drive into the shop. She stated it happened all at once. The misfire was evident at all engine speed and engine loads. The trouble code that was stored was a P0302 (misfire detected cylinder #2). Fuel trims were reportedly at +20% at idle after resetting and running for a while. No problem right. Easy diagnosis right?

Just a little background for clarifications sake. I am the shop owner, and I let my techs do all the work, unless it becomes a problem child, then I put them on something productive while I do the testing and show them what I found. This lends some reason to some of the redundancy of the tests that I am going to present.

This started out as a simple misfire. My tech swapped the plug and wires to see if the misfire moved to a different cylinder after checking for good spark at the plug wire with and ST125 spark tester. Nothing changed. He then did a compression test on that particular cylinder only and nothing was out of the ordinary. We then used some carburetor cleaner into the throttle body to see if the misfire would go away. One time we thought it did, the next time we thought it didn't. The test was inconclusive to us, so we called the customer and sold upper intake removal to inspect the fuel injector poppet nozzle. My techs performed the poppet nozzle test while I was away and said everything looked the same when comparing #2 cylinder with #6 cylinder. I then instructed them over the phone to swap the nozzles to a different cylinder to see if the misfire changed, as seeing a 10% difference would be impossible. After swapping the nozzles, the misfire remained on cylinder #2.

I talked to my techs about how they had performed the poppet comparison and found out that they were not allowing the fuel pump to run during the test and that they were doing only a visual test. During this time, the first tech became sick and since the shop was slow, I instructed my second tech to swap the injectors a second time, just in case something got mixed up the first time with the testing, multiple techs, etc. After the second time of swapping the poppet nozzles to different cylinders, the misfire remained on #2 cylinder. This rules out fuel delivery to me 100%.

Here is a picture of the intake manifold with the top off

I then proceeded to have tech #2 do a running compression test on the car, as the first tech was out sick. He checked the cranking and running compression on 3 different cylinders. #2 (the problem cylinder), #4 the neighboring cylinder, and #1 on the opposite bank. The cranking compression, in numeric order was:

#1 = 150

#2 = 155

#4 = 165

The running compression was:

#1 = 70

#2 = 60

#4 = 70

Using a manual gauge with the Schrader valve installed and taking the reading at the same rpm after the needle stabilizes, then bleeding the gauge and repeating the same pressure every time at least 5 times in a row with no change in the pressure. My thoughts at this time, was that we had a valve sealing problem. I was elated due to the fact that this is another one of those that were rare, the customer was in no hurry as the wife was out of town, and I wanted to try out my vacuum and pressure transducers to be able to narrow down what was causing the problem. My only concern was that I was going out of town.

Initially, performing secondary ignition testing with my scope, I could not isolate which cylinder was causing a problem. The secondary pattern was such that, not being one who uses secondary analysis a lot, I could not even isolate which cylinder was actually misfiring. I discussed procedures with my techs and they both assured me that cylinder identification was done manually by disabling spark to each individual cylinders and that the computer had identified the correct cylinder. The next day (Thursday) I was scheduled to leave town for a long weekend vacation, but I wanted to get some pressure waves with high resolution so I phoned a technician that owns a Pico scope and has and FLS sensor and explained my predicament. Due to some communication issues and my inability to be here while he was present at the shop, we didn't get any FLS waves of this engine. He did mention the fact that fuel trims can tend to cover up lean issues, so I cleared the codes the next morning and scoped this thing. The secondary confirmed the cylinder and showed what was happening in the cylinder. Since I have had problems with my scope in the past, I assumed a scope issue, because even after running a while I could still see the problem very readily.

Secondary parade pattern

Single cylinder view

Another Secondary parade pattern

Jason Parkin has seen and diagnosed this type of secondary pattern before here:


And confirmed an exhaust valve that was too tight.

I performed my own running compression tests and came up with:

#1 = 75 @ 680 rpm, and snap to 150

#2 = 70 @ 680 rpm, and snap to 150

Dang. Only 5 psi difference in idle pressures with a dead misfire. But my techs results showed 10 psi. I had recently talked to an automotive instructor about the problem I had on the previous three engines proving a valve sealing problem and he assured me that he has tested thousands, and his spec was anything over 5 psi difference in running compression indicated a valve sealing problem as long as engine rpm was within 50 rpm when compared. According to my scope, it was less than 10 rpm difference. Still shows a valve sealing problem, but I would never have thought that was enough to cause a dead misfire. Low power contribution maybe, but a dead miss?

I instructed my service writer to sell teardown and inspection of the cam and lifters with the potential of performing a valve job on this car. They were instructed to remove the RH valve cover and back off the rockers slowly first to see if they could make the misfire stop. But since my first tech was out sick the rest of the week, no repairs were performed before I came back. This was great for me as I wanted to get some waves to prove what was happening to this vehicle.

Tired of reading yet? You wanted the info right?

Anyway, Monday morning I'm back at this thing cold, after setting for 4 days. This thing has been at my shop approximately a week due to scheduling, sick tech, etc. and has missed ever single solitary revolution since it has been here. So I didn't even start the car after setting. I wanted to see if there was any difference between a hot and cold engine.

My thought process is this. Perform cranking compression and leak down tests cold, then proceed to check them hot. I performed the compression tests and leak down tests, then proceed to the running compression test on a cold engine.

The leak down test showed each cylinder holding

#1 = 80%

#2 = 90%

#4 = 85%

(This part was from memory as I didn't write it down). On a dead cold engine. What I am sure of is the 90% on cylinder #2 was the highest of all the cylinders tested and it was 5-10% higher than the others.

I start the thing up and it is running perfectly. Rev the engine, torque it, etc, and it runs great.

DAM. DAM. DAM. Just in case you are wondering, that is not a misspelled curse word, that DAM is there to help me hold back my anger VBG :^)

So now I have this thing in here with all this time spent. I know it has a mechanical problem, but I don't have enough data yet to prove to myself what it was except for the running compression tests, pressure waves, and scope patterns. I am not convinced the running compression tests are that conclusive, but it is all I have to go on. That and past experience. My tech's running compression test shows a 10 psi difference. My running compression test shows a 5 psi difference. We do have some running pressure waves that compares good and bad cylinders and most of them hover around the 5 psi difference between a good cylinder and a bad cylinder.

Missing Cylinder #2 before

Cylinder #2 After Valve Job and lifters

Good Cylinder Before

Good Cylinder After

Vacuum Wave Before

Vacuum Wave After

Another Fixed Vacuum Wave

Good Vacuum Wave After

So this begs the question. Do you sell a 4 figure repair based on 5 psi of running compression? Or was it 10 psi?

At this time, I removed the valve springs from both of the #2 valves to test for any sticking, binding or lack of sealing. Applying 10 psi of air and up to 120 psi of air. I can feel nothing wrong with either valve, they spin free and seal immediately regardless of the air pressure applied. Oh well, I know the problem was there so we sell a valve job and replace both lifters, just in case. The car runs great, but then again, it was running good just before the teardown, even though it had been running bad for a week prior.

It is my firm belief that we had one of two things happening. Either we had a lifter that was not collapsing properly or we had a high pressure valve sealing problem. If the valve could hold cranking and running compression but still had a dead miss, my thoughts are that it was not closing completely or that it could not hold the pressures of combustion and would leak under high pressure. For this we rely on prior experience outlined below.

We were working on a 2004 Chevy Impala with a 3.4 liter V-6. It had similar problems except that it would only miss at a slow idle. It would not miss if the engine was revved up even slightly above warm idle. That one, we could find nothing wrong with the original leak down and cranking tests. The only thing we could find was the running compression was 10 psi lower that two other good cylinders. We performed a valve job only to have the exact same conditions. We found on that one, that the intake lifter was stuck and not collapsing. Loosening the intake rocker arm 1/4 turn fixed the misfire on that vehicle, so we replaced all of the lifters and fixed the problem. We were able to confirm that the lifter would not collapse in a vise. After disassembly, it looked perfect. After reassembly and pumping it up manually, it would retract normally. We found the camshaft lobe was not perfect, but the customer declined repairs.

Here are some waves from that vehicle.

Missing #2 cylinder pressure

Missing #2 cylinder with BDC mark

Good cylinder pressure

Good cylinder with BDC mark

Vacuum Wave 1300 rpm #2 leaking intake valve at idle

Vacuum Wave 1481 rpm #2 leaking intake valve at idle

Vacuum Wave 1500 rpm #2 leaking intake valve at idle

Vacuum Wave 780 rpm #2 leaking intake valve at idle

So, what I am looking for from the greatest minds here is some type of test that can be performed that will prove to me what is wrong when I have a mechanical problem. Some say 5 psi difference running (at idle) compression is enough. I would like to believe that, but I haven't checked enough engines to know. Until about 2 years ago, I have never seen a mechanical problem that I could not prove. I am relatively new to the running compression test thing and now I have these high tech procedures at my disposal to analyze them, but I still can't seem to prove it. Before you answer, consider the fact that when this vehicle was fixed a repeat of the running compression test revealed that #1 cylinder which had 75 psi before the repairs, now has 70 psi running compression. The exact same pressure that was measured on cylinder #2 when it was missing prior to repairs. But #1 is running perfectly and not missing.

Is there a minimum pressure that is required when running? I am thinking this test as well is inconclusive. Anybody out there been doing a bunch of them? What have you come up with? I have loads of captures on this vehicle. Before and after. I know we had a mechanical problem. I don't know what it was.

I have some pressure waves on the 2004 Malibu, and some vacuum waves. We know for a fact that it passed all cranking compression tests and passed a leak down test. The only difference we could find was 10 psi running compression, measured at idle with a manual gauge screwed into the cylinder with the Schrader valve installed. There is a discrepancy between the manual gauge and the pressure transducers, not the least of which is caused by using different hoses and the schrader valve.

Scott from Missouri

Files Referenced:

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197 Replies Received (View Replies)


car Vehicle Data

1995 Chevrolet Blazer 4.3L

Engine4.3 L
Trans4-speed Automatic (Electronic)

car Vehicle Data

1995 Chevrolet Blazer 4.3L

Engine4.3 L
Trans4-speed Automatic (Electronic)