Misfires only at idle

Ray Diagnostician Ontario Posted   Latest  
Tech Tip
Driveability
2005 GMC Canyon Z71 Fleet 3.5L (6) 4-spd (4L60-E)
P0300 — Random/Multiple Cylinder Misfire Detected
Misfires

This is an inline 5 cylinder engine.

The scanner misfire data shows cylinder 2 misfiring at idle only. The shop did a compression test, replaced plugs and a COP and they called me in.

The red CH B is a First Look Sensor connected to the intake manifold, to compare each cylinder's intake stroke's vacuum pulls, while cranking and cylinder 2 shows a repeating pressure pulse.

The tan CH D is the starter amps with a repeating low compression in cyl 2

Ray 

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Mario Diagnostician
Florida
Mario Default
 

Try an in cylinder transducer test both cranking and running

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Geoff Diagnostician
Hawaii
Geoff Default
 

Another beautiful capture Ray. What's sad here, is that they had the right idea doing a compression check, but they either did it wrong, or their gage doesn't work. ;-) I don't have an FLS but my leak-down tester would have found that bad valve, and I bet a 50 year old vacuum gage, connected with the engine running, would have found it too. And fast!

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Mario Diagnostician
Florida
Mario Default
   

In order I always do clear flood test, relative compression test, in-cylinder transducer waveform analysis compared to good cylinder. Great convenience when the cylinder is under a plenum(the FLS). Good share Ray 👍 maybe they didn't keep an eye on how many compression strokes it took to reach its max compression. I keep a very close eye the first two strokes, even a leaky cylinder can reach

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Ray Diagnostician
Ontario
Ray Default
   

Your right, they must have done something wrong with their compression test or the compression gauge isn't working right. They have a vacuum gauge, but they couldn't find a small vacuum fitting on the intake. I didn't have to remove the air intake assembly and the spark plug to do a cylinder leakage test, because the transducer in the intake shows a valve leak. If I suspected a cylinder with

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Chris Diagnostician
New York
Chris Default
 

EXACTLY!!!!! When I speak to people on the phone that can't find a misfire, they often tell me that the compression test is "good". I don't remember "good" being on the face of the gauge. I try to explain to them that an engine only had one chance to make compression. the cycles aren't suck, squeeze. squeeze, squeeze, squeeze, squeeze, squeeze, squeeze, squeeze, bang, blow. So why test like

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Bob Diagnostician
Massachusetts
Bob Default
 

That's a nice capture Ray. There is a lot of information in those two waves.

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Mark Mobile Technician
Vermont
Mark Default
 

leaking intake valve?

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Bob Owner/Technician
Massachusetts
Bob Default
 

The positive pressure spike in the intake waveform as cylinder 2 comes up on compression does indicate an intake valve leak.

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