Low compression on 1 side
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Joined: May 2008
Posts: 50
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From: Port Washington Wi
Low compression on 1 side
While researching another problem I did a compression check and found #2,4,6,8 all around 132 lbs while the other side were all about 145. I shot a small amount of oil into #4 and retested, the compression jumped up to 150. Based on other posts I found this suggests bad rings, but the symptoms of all 4 cylinders being low indicates bad head gasket. Do I have both? I replaced the intake manifold gasket to solve excessive oil consumption, but the head gasket might have been the problem. All of the even plugs had crud on them. Any insights would be appreciated.
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Racer
Joined: Apr 2010
Posts: 409
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From: New York
How many miles on your car. Did the car ever over heat ? When yoiu say crud what kind of crud exactly are we talking about; black oily sooty, white. rusty orange. Also did you do the comression test hot or cold?
Drifting
Joined: Aug 2005
Posts: 1,682
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From: League City TX
Since you have fairly consistent readings on each bank, I wouldnt think that a failed component is the cause.
I would bet that either your heads are two different casting numbers, or your head gaskets are different thicknesses.
I would bet that either your heads are two different casting numbers, or your head gaskets are different thicknesses.
Thread Starter
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Joined: May 2008
Posts: 50
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From: Port Washington Wi
The P.O. had the engine rebuilt, it has about 36 K on it. The plugs have a a baked on soot, the electrode was pretty caked. The car has not overheated since I've had it, always runs at a nice 195 degrees. The P.O. was a Bubba and I've found some questionable workmanship through out the car so it's possible the headgasket on the passenger side was not installed with care. I don't see white smoke and I don't see oil in the antifreeze, I just drained it to replace the water pump. Since the oil trick boosted the compression I'm thinking bad rings, possibly all four. I'm trying to assess the problem so I know if I'm going to replace the head gasket soon or start saving for a motor rebuild.
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Race Director
Joined: May 2005
Posts: 10,674
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From: Millington Illinois
St. Jude Donor '08-'09
The pressure is consistant on both sides but vary side to side. The pressure increasing when adding oil to the cylinder sounds like rings. Check the other side for an increase adding oil. The problem could well be a difference in deck height. My original deck block needed a lot more cut on one side than the other to get zero deck. It was also off from the inside to outside of the deck surface and front to rear. I cut .008 before it was cutting all the way across the deck. GM was not real concerned about this on the assembly line and their tolerances were not real precise. Modern technology and performance machining tolerances are way more exacting than these were off the line.
Le Mans Master
Joined: Sep 2001
Posts: 7,739
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From: Unreconstructed, South Carolina
intake gasket was replaced ... sometimes they'll leak along one bank but not the other
here's a stretch ...
perhaps ... before gasket ... left bank was sucking oil into ports from leaking intake gasket ... perhaps scorched oil /carbon buildup on left bank pistons ... perhaps left side int gasket was leaking much more than 2,4,6,8 side ... more buildup on left might lead to higher comp on left bank.
I said it's a stretch ... got a better one?
all in all ... it's roughly 10% diff ... I don't see enough bank-to-bank %diff to sweat it anyway.
the oil trick will usually increase comp in even a fresh motor
here's a stretch ...
perhaps ... before gasket ... left bank was sucking oil into ports from leaking intake gasket ... perhaps scorched oil /carbon buildup on left bank pistons ... perhaps left side int gasket was leaking much more than 2,4,6,8 side ... more buildup on left might lead to higher comp on left bank.
I said it's a stretch ... got a better one?
all in all ... it's roughly 10% diff ... I don't see enough bank-to-bank %diff to sweat it anyway.
the oil trick will usually increase comp in even a fresh motor
