Compression Test

JMO · 06-02-2009, 06:01 PM

I just did a compression test on the car. I kept the engine cranking until the highest value was reached. Here are the results:

Cylinder 3 has a noticeably lower PSI value as well as taking more rev/crank to reach it. Cylinder 2 has a closer PSI value to the others but takes slightly more rev/crank to reach it.

Are the measured values correct? I will do a wet test on cylinders 2 and 3. What type of oil should I use in the test (anything specific)? Is there anything else can do to see what is wrong with the cylinders?

NB The test was done as part of the hunt for the cause of my rough idle.

Sox · 06-02-2009, 06:58 PM

That all looks fine, though you don't really need to crank more than 4-5 times.

JMO · 06-02-2009, 09:36 PM

I found it hard to get consistant readings by doing it 4-5 times, so I just did it until the max compression was reached. It still seems a little strange that the values of 2 and 3 differ. I'll do a wet test tomorrow and see what happens.

Cookie-XR8 · 06-02-2009, 09:56 PM

before i done the head on my turboed au the commpression was all over the shop and it ran fine, it differed from 150 down to 130 but 2 was the highest (compression is low becasue of the decompressed head)

mik · 06-02-2009, 10:01 PM

within 15 lb`s is the tolerance i was told.

FS5 · 07-02-2009, 01:11 AM

as long as it aint burnin oil its all good

Sox · 07-02-2009, 08:15 AM

Quote:

Originally Posted by JMO

I found it hard to get consistant readings by doing it 4-5 times, so I just did it until the max compression was reached.

It would still have been adequate. After 4-5 stroke, the readings will only increase by a few psi and you're just putting strain on the battery and starter motor for nothing.

Quote:

It still seems a little strange that the values of 2 and 3 differ. I'll do a wet test tomorrow and see what happens.

They will always differ, the kicker is how much do they differ, and 15psi is nothing.
When one cylinder is 50psi down you'll have something to worry about.

A wet test is only done to determine wether it's the rings or valves which are stuffed. I wouldn't bother as your readings clearly show that nothing is stuffed.

JMO · 07-02-2009, 10:10 AM

Ok then. Thanks for explaining it to me. Saved me the hassle of doing something that was unnecessary

justa6 · 07-02-2009, 06:47 PM

yeah seem's ok, as long as there is not 1 low as. but should only crank it a couple of times.

james22 · 09-02-2009, 08:48 AM

Just a bit of info for future reference. The way the test is usually analysed is by a percentage. As long as all cylinders are within 10% of eachother, thats is considered perfectly healthy. In this case, obviously theres nothing wrong with it.

Food for thought :-)