I've got this truck with a fairly new engine, about 2500 miles and it's smoking out one side of the dual exhaust. So I decided to do a compression test on it to try to find the bad cylinder. The first seven cylinders gave me somewhere between 135-145. And the number 8 cylinder gave me 10. 10!!! It seemed way too low, so to see if this reading was realistic I put my thumb over the spark plug hole to feel for compression. Compared to the other 7 cylinders it felt the same. Yet the compression gauge kept giving me a reading of 10. I unscrewed and reinstalled the gauge and got the same reading every time. I tried it in a different cylinder and got a reading of 140. So the problem isn't the gauge. Wouldn't I be able to feel a difference with my thumb between 10 and 140 on two different cylinders? So what's the deal? The threads on the spark plug hole seem fine, both the gauge and spark plug thread in perfectly. And all the spark plugs in the engine look the same (equally fouled) and it isn't backfiring or anything, it just smokes out one side.
Help!
Now I'm more confused than ever.

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91 Z28
Come see The Vicious...

 

possible broken rings in cylinder. or cracked piston. squirt some oil in the cylinder then do a compression check. if it goes up, then a ring problem.



[This message has been edited by mrr23 (edited November 18, 2001).]

 

Yeah, do what he said to see if it is a ring problem. Is the engine misfiring? If you really have that much of a compression problem on that one cylinder, it would have to be just along for the ride and not be firing....... If the oil on the rings test doesn't change anything with your readings, then you could have a damaged valve.

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1987 GTA L98 MD8
355, TFS Heads, LT4 Hot Cam
My GTA

The Minnesota F-body Club

 

Or bad head gasket.

 

Its very likely that you have a burnt valve. The results you describe with the (highly technical finger test) would indicate to be that you have a hole in a valve and its not holding compression.
SSC

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85 Camaro, 355/400 "drag car" current DD 13.1@ 101mph on its last season run.
Plans: Possibly add N20 for next season.
82 Firbird, rebuilt 355 for it and slaped a 700r4 in. The 700r4 crapped out so back to the tb350

 

I thougth about the burned valve theorey. But if I was losing that much compression through a valve wouldn't I have some sort of problem with it backfiring, either through the intake or the headers?
And if there's really that little compression in the cylinder, then why when I covered the hole with my finger did there seem to be so much compression? This is what is confusing me.

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91 Z28
Come see The Vicious...

 

Don't try and rule things out by theory, cause when you work on alot of cars, you will always have one that will kick your **** and it turns out to be something stupid because you ruled it out as a possibility in your head, instead of testing.

Put some oil in the cylinder and re-check your readings.

When you pulled the plugs out did any of them look oil fouled, or any worse than the others?

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South East Thirdgen

1987 Trans Am 305
14.88 @ 92.81

 

You should be able to feel the difference between 10 and 140. 10 PSI should not be able to unseat your finger from the plug hole!

Are you SURE you got the hose screwed in all the way? Spray some soapy water around the plug threads after you screw in the gague- see if it blows bubbles. If it does, it's not sealed up.

If you've got a burned valved (especially one THAT bad) you should be able to hear the "puffing" past the valve either through the carb or the exhaust system.