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Compression Test

Posted: Tue Jun 15, 2021 11:40 pm
by joseph548694
I have a question on how to do a compression test. I have seen it done both ways and want to know which is better. Should I have my battery connected to external power when performing a compression test? I think the question is what is causing the failure. So if the accurate measurement is a result of the battery not being able to turn the starter over anymore, then we would want to not connect it to a charger. However, if the accurate measure is that the starter cannot overcome the compression then it would not be a problem to connect the batter to power. I am assuming that it is the latter, but wanted to ask the experts.

Re: Compression Test

Posted: Wed Jun 16, 2021 3:16 am
by Rambozo
If your battery is good and all the plugs are out, there should be no problem. However, if your battery is old, external power might be necessary. What you want is consistency. It needs to spin over at the same speed for each cylinder. Also make sure your throttle is open. I like to record what it gets to on the first stroke and the final after about 5 cycles. Then compare to the other cylinders and they should be within 10% of each other. This is not a test for the battery or starter. There are other ways to check them.
FWIW, A cylinder leakage test will tell you a lot more than a compression test for about the same labor.