Thread: compression testing a cold motor

Tried to search with no luck.
What is the common procedure to test compression on a cold motor?
If you were gonna buy a JDM motor or used motor,how do you check compression before buying?
I know the FSM says to do it warm,but if the engine is not in the car what do you do??

if you've got air i'd do a leakdown, short of that though you shouldn't have any issues with a cold test

you wont have any issues if you want to see if all cylinders are running around the same psi.When metal gets warm, it expands,tightening all the clearances in the motor. That's how it creates more pressure. Thats why you have to do it when its warm

He's wanting to know how to check it without being able to start the motor since it's not in a car wired up.

You should be able to turn the crank right?

You can still slap a starter on and crank it. Even on an engine stand you can do this.

Bolt a starter to it and just use a battery. Its fine to compression check a motor cold. The compression numbers from what ive seen will come out a tiny bit higher than when warm but not much of a difference at all.

Make sure there is oil in it and so on. Hold the throttle wide open as well when its cranking. Allow it to crank over 5-6 compression cycles at least to get a full accurate reading.

arent they supposed to be lower when cold?

This is what I planned thanks.I was hoping would confirm the starter idea.I appreciate that.

From my experience compression testing a cold motor the rings are usually a little stiffer when cold which is just common metal properties and the oil is thicker on the walls creating a higher reading. This is testing a motor that was run the day prior as well. If you have a cold motor thats been sitting a long time you might see lower numbers if the walls are dry. But after cranking enough during the checks the walls should become lubricated and might still see higher numbers. Again its not much. If anything maybe 10-15psi higher on average.