Cold compression check...need to know quickly
Cold compression check...need to know quickly
I know that ideally a compression check should be done on a warmed up engine. However, I sincerely doubt that the engine I am having compression checked will be warmed up first (besides, I really dont want them running it that long anyway). Since the engine will most likely not be up to operating temperature, how will this affect the compression numbers? I dont know what the expansion properties of aluminum vs. cast iron are...but I would assume that tolerances tighten up when the engine is warm, hence increasing the compression. ie: I would get higher compression if the engine was warm. I'm suprised this hasnt been covered before, but I couldnt turn anything up on the subject and it's something that ideally I would like to know quickly. I posted this in 240 General too but so far there has been no help there so I perhaps one of you guys knows.
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roundrocktom
- Posts: 63
- Joined: Mon Apr 21, 2003 4:13 am
Go ahead and check the compression with the cold engine, as this will still give you a pretty good idea of ring and valve condition.
You will discover the compression readings are down a little, but compare all the cylinders to each other.
Typically I've found COLD compression checks to show the start of a blown head gasket (once warmed up, everything was fine for a few months until the gasket blew).
Valves needing adjustment might show leakage at hot, but fine at cold. Burned valves will leak, regardless of hot or cold!
Piston slap is more noticeable at cold temp. Oddly the only time I ever hear it is a on Chevy Vortec 6.0 (2500HD tow vehicle) first start in the morning.
Remember to do a dry/wet readings. Just add less than a teaspoon of oil to the cylinder, THEN crank about a half dozen times (most excess oil is pushed out the open spark plug hole, THEN take the compression reading. With a cold engine you'll probably notice more of an improvement with oil, but once again you want to compare all the reading to each other to see if there is a problem. Usually as I am about to test cylinder #6 dry, having completed 1-5, I'll squirt the oil into #1 to check after I do #6... so on. That fine mist of oil is flammable, so use care with any spark!
If you are inspecting an engine to buy, pull off the valve cover and take a look! With proper oil changes everything should look almost new, just a light coat of "light brown" shellac is normal.
Good luck!
Tom
You will discover the compression readings are down a little, but compare all the cylinders to each other.
Typically I've found COLD compression checks to show the start of a blown head gasket (once warmed up, everything was fine for a few months until the gasket blew).
Valves needing adjustment might show leakage at hot, but fine at cold. Burned valves will leak, regardless of hot or cold!
Piston slap is more noticeable at cold temp. Oddly the only time I ever hear it is a on Chevy Vortec 6.0 (2500HD tow vehicle) first start in the morning.
Remember to do a dry/wet readings. Just add less than a teaspoon of oil to the cylinder, THEN crank about a half dozen times (most excess oil is pushed out the open spark plug hole, THEN take the compression reading. With a cold engine you'll probably notice more of an improvement with oil, but once again you want to compare all the reading to each other to see if there is a problem. Usually as I am about to test cylinder #6 dry, having completed 1-5, I'll squirt the oil into #1 to check after I do #6... so on. That fine mist of oil is flammable, so use care with any spark!
If you are inspecting an engine to buy, pull off the valve cover and take a look! With proper oil changes everything should look almost new, just a light coat of "light brown" shellac is normal.
Good luck!
Tom
Thanks a lot for all that advice, that was the information I was looking for 
It's impossible for me to do the check myself right now, but I know that Venus-Auto is not going to let the engine warm up and do the check, hence I wanted to know if I should expect slightly lower than normal compression numbers on a cold engine. I turned down one engine with 150ish compression across the board so far because I felt it was too low for a 40,000 mile engine that should have compression around 174 psi from the factory. I'll be doing my own THOROUGH compression check when the engine arrives and if it doesn't check out within 10% of their compression check then they'll be getting a call 
Thanks again for that response!
It's impossible for me to do the check myself right now, but I know that Venus-Auto is not going to let the engine warm up and do the check, hence I wanted to know if I should expect slightly lower than normal compression numbers on a cold engine. I turned down one engine with 150ish compression across the board so far because I felt it was too low for a 40,000 mile engine that should have compression around 174 psi from the factory. I'll be doing my own THOROUGH compression check when the engine arrives and if it doesn't check out within 10% of their compression check then they'll be getting a call Thanks again for that response!