Mechanics

WHAT CAN CAUSE SMALL POOL OF OIL ON TOP OF PISTON (ENGINE COLD)?

2001 Chevrolet Prizm • 160,000 miles

Have owned vehicle for less than a year so know little about history. Runs very well except for excessive oil consumption (1qt/300miles or so). Assumed this was the sticking oil rings problem common to these cars (and Toyota Corollas) but decided to try to determine which cylinder(s) were the problem. After the car sat for more than 24 hours:

1. Removed spark plug from C4 and found everything looking good except for a bit too much white ash build up on the plug (and the plug past due for a change). Put a borescope into C4 and everything looked good to my inexperienced eye (can attach video of borescope and photo of plug if needed).

2. Removed spark plug wire from C3 and found oil in the plug well. Assume tappet cover seals are leaking a bit as top of cover is clean. Removed spark plug from C3 and found it in good shape except for being wet and past due for a change. Put the borescope into C3 and found oil filling the gap between the piston and cylinder and a pool of oil on the top of the piston - probably covering 1/8 of the piston surface (apparently piston top is not quite level). Again, can attach video of the borescope and a photo of the plug if that would be helpful.

Have done nothing further as I am wondering if I need to do compression tests (though not sure how to do the dry test on C3) or if the pool of oil is a conclusive indicator that the valve guides are leaking. Or is there some other possible source for the oil in the cylinder?

I'll have a look into C2 and C1 tomorrow but thought I'd ask the questions right away.
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Snoopy2
August 19, 2012.



Proberbly faulty valve seals and guides, as the engine is hot and oil thin, when shut down oil will work past faulty seals and down worn guides, but to get oil pooling on the piston crown is quite excessive, oil in ring lands can by from worn rings and oil control ring, not scraping excessive oil from bore walls, if this was the case I would expect to see lots of smoke and high oil consumption, a leak down test will give a good indication as to ring condition.

Mhpautos
Aug 19, 2012.
Worn oil control rings will cause a consumption issue- but only when engine is running. The oil level is well below these rings and cant drain up. Worn seals/Guides are your culprit. Oil remaining in the rocker area drains past the worn seals and guides - collecting in the cylinder bores. Viola, oil present on start-up after sitting. Another conformation for this condition is to watch for bluish/white smoke from the exhaust on decel.- The cat conv. May eliminate the smoke if it operating properly, but if this has been present for a while, the cat is probably not working well anyway. Let me know, Doc

DocDTC
Aug 22, 2012.
Thanks very much. Will check for oil smoke on decel. Smokes on start-up but didn't think of checking on decel. Will replace the valve (and other tappet cover) seals. Are 0.003"(i) and 0.004"(e) good limits for stem clearance or should tolerances be tighter on these engines?
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Tiny
Snoopy2
Aug 23, 2012.
Seals should rub the valve stems to wipe any oil off of them. In other words, no clearance on stem to seal.

Rivermikerat
Sep 7, 2012.

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