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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / September 2006compression readings, cold vs. hot
Can anyone tell me if it is normal to have widely varying compression readings if you check a cold motor vs. a hot one? This is an overhead valve pushrod motor with hydraulic lifters. With the engine cold, readings vary from as low as 30 to as high as 140 psi, but with the motor warmed up, readings are between 135 and 140 psi on all cylinders. Why is this? Is this a cause for concern? > Can anyone tell me if it is normal to have widely varying compression > readings if you check a cold motor vs. a hot one? [quoted text clipped - 6 lines] > > Why is this? Is this a cause for concern? 30-140 is too big of a range for a compression test - you should see no more than 10% variance. I suggest posting the results for all cylinders hot & cold, and telling us what kind of motor (Chevy V8?) it is. How does it run? I'm not qualified to tell you why the huge difference cold vs hot. The two things I can think of - head gasket if two adjoining cylinders are low, possibly misadjusted valvetrain. Ray AS long as the 140 is happening at the right time, and its running good, I dont see a problem. You have 140 hot and cold. The 30 is a throwaway number if its happening any other time other than on the compression stroke, but are you saying one of the cylinders is reading 30 MAX when old? that IS a problem. > Can anyone tell me if it is normal to have widely varying compression > readings if you check a cold motor vs. a hot one? [quoted text clipped - 6 lines] > > Why is this? Is this a cause for concern? It's the GM 2.5 L 4 cyl. Engine cold (sat unused a few days), I got: #1 - 140 psi, #2 - 82 psi, #3 - 52 psi, #4 - 32 psi. I triple checked the readings and used 2 different gauges to make sure I was right. Engine hot, I got: #1 - 140 psi, #2 - 135 psi, #3 - 135 psi, #4 - 135 psi. The car runs fine, has decent pickup, and gets good gas mileage, it is just that it has a miss/sputter at idle that I decided to track down. It is not a consistent, steady miss, it is more occasional. I thought I had a vacuum leak, but checked the hoses and intake gasket, and my plugs/wires are new. Someone was telling me that this is the reason you don't check compression cold- that you need the oil around the rings which would have drained away from the engine sitting a few days. Also that hydraulic lifter bleed down would contribute to this. Is this true? > AS long as the 140 is happening at the right time, and its running good, I > dont see a problem. You have 140 hot and cold. [quoted text clipped - 12 lines] > > > > Why is this? Is this a cause for concern? |
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