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More testing
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Thu, 16 May 2002 08:53:19 Share Post by Email
In Reply to: what does it mean if...., AP, Thu, 16 May 2002 07:45:28
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Uneven and low numbers indicate a worn engine. The trick is to know where the wear is.

I'm going to assume you did a 'dry test' - remove the spark plug, stick in the compression tester, and turn the engine over about four times. That's half the story.

I don't know if you did the test yourself, or had someone else do it. There are some very important tips:
You MUST do it on a warm engine. Clearances in a cold engine are different.
You MUST PROP OPEN THE THROTTLE. Doing a compression test with a closed throttle plate will give low readings.
WATCH THE GAUGE. Notice how high the needle goes on the first pulse - it should be about 75 or 80% of the final reading.

The next thing to do is a 'wet test' - drop a tablespoon or so of engine oil into the spark plug hole. Turn the engine over a couple of turns, and check the compression again. The number will be a little higher, because the oil acts as a sealant between the piston rings and cylinder wall.

If the compression didn't change much in a cylinder between the wet and dry tests, the problem is in the valves or head gasket. Most likely worn valves/valve seats. But if one cylinder is particularly low, it can indicate a head gasket problem.

If the compression changed a lot (>15 psi - more like 20 or 30) with the addition of oil, then most of the problem is due to worn rings.

It would be nice for all the cylinders to be the same reading, but a 5 to 10% variation in an older engine isn't out of the ordinary. When you see one cylinder 30 psi lower than the rest, that's a sign something may have to be done.

Good luck!

posted by 140.157.4...
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