Compression readings
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Joined: Mar 2004
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From: Norristown, PA
Compression readings
I was wondering if any of you can tell me what these compression readings mean? I tested the compression on my dad's '94 Mercury Marquis w/4.6L V8. The car has about 95K miles and is burning oil. The car still runs great. The oil has been changed at the dealer every 3000 miles. When I removed the spark plugs, none of them were oil fouled!! I still can't understand that. Anyway, here are the readings: Drivers side, front to back: 155 dry/175 wet, 130 dry/175 wet, 175 dry/200 wet, 170 dry/185 wet. Passenger side front to back: 170 dry/195 wet, 165 dry/180 wet, 180 dry/230 wet, 175 dry/225 wet. The "wet" reading means that I added oil to the cylinder. I also cranked the engine over until the compression gauge stopped at it's highest point.
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Joined: May 2002
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From: Chicago Area
I'd have to go back to the books to be sure, but since nobody else has replied, I'll give you an opinion.
As I recall, readings should be within 10%. The absolute values are not usually considered important. Your dry low is 73% of your dry high. Your wet low is 76% of your wet high. Not good.
Adding oil (wet readings) helps seal the rings. This allows the technician to test the valves and head gaskets separately from the bottom end. Since your compression goes up significantly wet, I believe this indicates worn rings. However, since there is still a big variance, you may also have valve wear or bad head gaskets.
Looking specifically at your numbers, #6 is the real bad boy and the adjacent #5 is almost as bad. It could be that the head gasket is leaking between them. You might be able to improve the engine significantly by just replacing that, but (especially with an overhead cam engine) I don't think it would make sense to do that without also doing the valves.
As I recall, readings should be within 10%. The absolute values are not usually considered important. Your dry low is 73% of your dry high. Your wet low is 76% of your wet high. Not good.
Adding oil (wet readings) helps seal the rings. This allows the technician to test the valves and head gaskets separately from the bottom end. Since your compression goes up significantly wet, I believe this indicates worn rings. However, since there is still a big variance, you may also have valve wear or bad head gaskets.
Looking specifically at your numbers, #6 is the real bad boy and the adjacent #5 is almost as bad. It could be that the head gasket is leaking between them. You might be able to improve the engine significantly by just replacing that, but (especially with an overhead cam engine) I don't think it would make sense to do that without also doing the valves.
Posting Guru
Joined: May 2002
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From: Chicago Area
Not sure, but I would suggest that if the level of oil consumption is not too bad, it may be burning with the gas and consequently not be left in the liquid state to deposit. I think that the O2 sensors may cut back on the gas somewhat to keep the mixture in the proper ratio to burn everything. Other cylinders might then be running lean, which could be hard on their valves.
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