Compression test, warm or cold
Thread Starter
Burning Brakes
Joined: Aug 2006
Posts: 793
Likes: 10
From: Grafton MA
Compression test, warm or cold
I'm planning on doing a compression test on my '71. From what i've read you're supposed to do it with the engine warmed up to operating temperature. The problem is it looks like it's going to be a nightmare removing all the spark plugs as they're especially tough to get to on the driver's side. I'll need to let the car sit for a little while to let the manifolds cool off somewhat so i don't burn my hands...then add probably an hour on top of that to get all the plugs out. How inaccurate will the compression test be if i do this with a cold engine? Also, any tricks to getting the plugs out quicker? I'm thinking of doing the passenger side from the top and maybe try driver's side from underneath.
Paul
Paul
Racer
Joined: Jan 2011
Posts: 266
Likes: 0
From: York Pa.
These old cars are tough to remove the plugs..what I would suggest before you warm up the car is remove the shielding and anything that is in the way..loosen all the plugs to make sure they will come out easy..put them back in not so tight and do the warm up..make sure you disconnect the fuel and spark before you do the test..a leak down may be easier to do and actually would be the next move if anything comes up off with the compression test.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
Other than having the oil be thicker because of the lower temperature, I fail to see what difference the engine temp makes to a compression test. It may affect a 'leak down' test some, as it will take more time for compressed air to force through some colder oil in the rings, but on a compression test? Someone else will have to explain to me what difference it makes.
Race Director
Joined: Dec 1999
Posts: 11,094
Likes: 990
From: Sierra Foothills CA
Every mechanic or engine builder I've ever talked to has said to warm up the engine first. From what I recall it's to get oil hot but also to heat up the internals to get optimum ring seal and therefore most accurate results. Just do the best you can to pull the plugs quickly and use gloves to prevent burning yourself if you need to.
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
If you want an accurate PSI rating then warm it up first, not sure it makes much difference.
I go it cold:
1) So I don't burn a hole through my gloves and more importantly...
2) The reason you are doing the test in the first place, you are looking for a cylinder that is not making the same pressure as the rest and you feel it might be a bad valve or piston,
You aren't ( in most cases ) looking
looking for what the PSI is in a certain cylinder you are checking to see if they are all the same or within about 10% of each other whether it's got 170psi or 230psi as long as they are all within 10% of each other
*This goes on all my posts: Please report back to those who helped you the final outcome of your initial post
I go it cold:
1) So I don't burn a hole through my gloves and more importantly...
2) The reason you are doing the test in the first place, you are looking for a cylinder that is not making the same pressure as the rest and you feel it might be a bad valve or piston,
You aren't ( in most cases ) looking
looking for what the PSI is in a certain cylinder you are checking to see if they are all the same or within about 10% of each other whether it's got 170psi or 230psi as long as they are all within 10% of each other
*This goes on all my posts: Please report back to those who helped you the final outcome of your initial post
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