Checking compression

[KA1J]

I just can't remember, is this supposed to be done on a cold or warm engine?

I can't remember where I left my memory...

Gary

[dpotter58]

warm. Wide open throttle

[ViperRon]

Spec's say cold but to really determine the condition it should be done both on a cold and a warm engine. You can not depend on cold compression on an engine that has set for some time rings tend to stick rust builds in cylinders valves stick etc. So checking compression on an engine that has set for some time my be totally worthless for checking much more then stuck rings or valves. However on an engine that you use all the time it gives you a good idea of how tight it really is. If an engine has good compression cold reading it will start well and normally will run well. Now identifying if an engine can run well can be done while warm. If you get good readings there may be no need to do any work to the engine as long as you get good even numbers.

However if you get uneven numbers less than 145 warm and then you do a cold compression check and get numbers in 90's it won't start easy will probably run erratic burn some oil etc etc. Determining if the issue is valves or rings can be done with a little light oil added to the cylinders. If the rings are worn,sticking, or there is mild rust in cylinders the oil will bump the compression. If the valves are the problem most times it will have little effect

I prefeer to get cold and warm readings but be aware if you have cold compression readings even in the 170 range you do not want to take any chance with the engine firing on the warm test on any cylinder as it may blow your guage. Move wires away, take all plugs out and open the throttle.

[KA1J]

Thanks for that, it's just what I needed to know.

Have a happy 4th!

Gary