Will a compression test on a cold engine tell me anything?
 

I have my intake off as well as spark plugs out. Wanted to replace the plug wires and valve cover gaskets. Truck was running well then started missing, and could see plug wires arcing at night,....so decided to pull the intake to facilitate plug wire replacement and do the rocker box gaskets at the same time. Was about to begin re-assembly and thought I might as well check compression first while I had everything open,.....only did the driver's side and got 150,85,75,85. I know it's recommended that compression be checked when an engine is hot/warm,......but would a cold engine explain this much variance?.........or should I pull the heads while I have it partially dis-assembled?

I started to do a second test on the same bank, and the first cylinder yielded 120 (previously 150)..................decided it was time to stop and check with those that know about such things before proceeding with either re-assembly of the previously running OK engine or did-assembly.

Input appreciated:

BC

Warm test with all plugs removed, pump relay removed and the t/body cracked open.
Those first numbers were terrible. Get back to us when you get all 8 checked.

Mike, just want to make sure you read my entire post. I have the intake off, valve covers/rocker boxes off, A/C compressor off (moved to the side) and, of course, all the hoses and cables that are attached to that stuff, disconnected and out of the way,....so, for a warm test,.....I will have to re-assemble all, versus,....just move forward with pulling the heads from where I am now. I've already done this job (pulling the heads) on my last Disco,..so, know I'm about half way through the job at this point. Not a glutten for punishment, but if the consensus is that the cold test variations at least tell us there is SOME kind of problem that will require pulling the heads, then I will pull them,.....if, on the other hand, the cold compression test tells us nothing at all,.....then I will re-assemble and hope that I don't immediately have to tear all back down again.

BC

Cold compression test numbers are always low and inaccurate.
When you finally get them, there should not be more then 10% difference between the highest and lowest numbers.

testing cold is not ideal, but if you should get consist low numbers. Not swings of 60lbs.

That was my guess as well, that although low, they should still be in that 10-15% deviation range,......and that a 50-60 PSI cold variation might be shouting.................."Pull the heads off now while you are half way there"