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Old 06-20-2011, 01:10 AM   #1
318_IT
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Default Compression Test

I just did a compression test on my 95 318TI M42. It has about 148k on it and the PO said that he put a new head on about 20k ago due to overheat and cracking the original head.

Anyways, my results seemed kind of low..I got 120psi straight across...I helped my father-in-law replace the HG on his 98 318TI M44 and it seems like we were more like 160-180 across the cylinders.

Any thoughts?
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Old 06-20-2011, 01:43 AM   #2
xxxJohnBoyxxx
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Quote:
Originally Posted by 318_IT View Post
I just did a compression test on my 95 318TI M42. It has about 148k on it and the PO said that he put a new head on about 20k ago due to overheat and cracking the original head.

Anyways, my results seemed kind of low..I got 120psi straight across...I helped my father-in-law replace the HG on his 98 318TI M44 and it seems like we were more like 160-180 across the cylinders.

Any thoughts?
It is good. You look for even numbers accross all cylinders not the psi number. If you want to know the cylinder piston condition you need to do a leak down test with a leak down tester...
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Old 06-20-2011, 01:53 AM   #3
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Ok I feel pretty ignorant right now...what is the difference and how would I do a leak down test?
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Old 06-20-2011, 04:45 AM   #4
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you should see 135-185psi, but the fact that they are even is a good sign. the fact that they are low could be the gauge or user.

was the test done with the engine cold or warm. warm should bring the numbers up.

put a teaspoon of oil in the cylinder and try again. if the psi comes up, then the rings are worn.

a leak down test is when you use a controlled amount of air pressure with the piston at TDC on the ignition stroke. you read how much air escapes from the cylinder. 25% or more will require some repair.

with the cylinder pumped up to pressure, if the air leaks out of the intake manifold then a intake valve or valves are not seating. same thing with the exhaust, air out the tailpipe = exhaust valve. if the air escapes form the dipstick, then the rings are leaking.

so you can see haw a test like this is more important then a compression test. a compression test only measures the engine's ability to seal at low rpms. once a engine is spinning 6000rpms, there is a valve open or opening 100 times per second, so a good low speed compression is less important.
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Old 06-21-2011, 09:33 AM   #5
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Ok 10% or more on a leak down and the motor needs a rebuild. My stock M44 had 120psi on the compression test and 6-7% leak-down and had no issues when it was torn down at 173K. Anything over 155psi and you might want to run over 93 octane fuel. 220psi requires 110 race fuel.

You are just fine and if you're worried do a leak down test and it will tell you everything you need to know about your rings, cylinder walls and valves. I think you don't have anything to worry about...

John S
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