This is mostly a general knowledge question, one that’s got me stuck.
When checking compression on an engine, my understanding is that it must be fully warmed up to normal operating temperature. However, I’ve always been told that spark plugs should be removed ONLY from a cold engine, or else the threads in the head could get sheared away, especially if it’s an aluminum head.
Is there some sort of middle ground, such as let the engine cool some first? Or can compression checks be done on a cold engine without a problem?
In theory, a compression test should be performed on a warm engine.
In practice, it doesn’t make that much of a difference. If a problem exists it will be present hot or cold.
Regarding the spark plug removal, I’ve also never found it to make much of a difference as to whether the engine is hot or cold.
If they’re stuck, they’re stuck.
What I would recommend if you run across one that is apparently frozen is that you do not attempt to simply unscrew it in one shot. Work it back and forth in a see-saw motion while occassionally backing it out a half a turn. This can be a bit tedious at times but will usually bring them out without ruining the threads.
Use anti-seize on the new plugs.
When running a comp. test. make sure the throttle plate is wide open and write down the readings. It’s very, very easy to forget them or get them crossed up.
If a problem seems to exist then go back and retest the cylinders again to weed out a gauge error, etc.
Hope that helps.
I usually do it with the engine cold and then if anything is funny, I’ll redo it hot. For what it’s worth, on the engines I’ve done this on the readings have been pretty much the same cold or hot, but I’ve also never really done one on a car with ring problems, which I think is where the temperature difference is important.
I think the textbook answer to your question is that you should take all the plugs out while the engine is cold to make sure they’re not stuck and then reinstall them and warm it up before doing the test.
Since many people over-tighten spark plugs, when the engine is cold, loosen the spark plugs and, then, SNUG them in. That’s all that is need: snug. Then, if you do a hot engine compression check, you’ll be less likely to strip a spark plug’s threads.