Date: Fri, 15 Mar 2002 16:55:11 GMT
From: John Davies <>
Subject: Re: compression numbers?

On Thu, 14 Mar 2002 22:50:41 -0500, "Geo" <> wrote: >I had a mechanic look at a 93 9000 CSE. He says the following compression >results are good: 110, 110, 115, 110. Are they? Just double checking. You haven't given us enough info. How many miles on the engine? How does it run? Does it burn any oil? Was the test done with a warm engine or cold? What condition was the battery (how fast did the engine spin over)? Are the cams standard or modified? Was the throttle held wide open for each cylinder? The only correct way for a professional mechanic to do a compression test is with a differential compression tester (leak-down tester) on a warm engine. This entails manually setting each piston at TDC on the compression stroke, so all the valves are closed, and pumping in filtered compressed air thorough a calibrated oriface. The leak rate is measured on the test gage, and should be consistent from cylinder to cylinder and no more than 10% of the input air pressure. This method is not subject to the variables and uncertainties of simply cranking the engine over with a standard compression gage hooked up. In addition, the tech can listen at the tailpipe and inlet to see which valves are leaking, or check for air coming out through the oil filler opening to check the amount of ring leakage. Or even look for air bubbles in the radiator if the head gasket is suspect. This is also the only way acceptable to the FAA to check a piston type aircraft engine, for good reason - it is the _only_ method, short of a top end tear-down, that tells you something of value about its internal condition.. Here s an excellent source of additional info about using a leak-down tester (including a photo of the tester): As far as your readings go? I couldn't even begin to guess if they are ok. I suggest you either forget about them or have the engine rechecked properly. Do you _really_ want to know if your rings are shot? Sometimes ignorance is bliss. Drive the car and enjoy it! John Davies Monroe WA USA 2001 Saab 9-5 Aero Wagon 2000 Audi A4 quattro 1.8T 1991 Jeep Cherokee Laredo 4x4

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