Odd compression test results

[acolella76]

So just for the heck of it, i went and did a dry compression test on my b27. Here are the results:

1- 180psi
2- 160psi
3- 125psi
4- 160psi
5- 155psi
6- 155psi

the bentley says compression should be from 142-156psi. So cyl 5 & 6 are good.. but what about 1-4? For cylinder 3 i'm going to blow compressed air into the cylinder, do i need a gauge to see how much is leaking or could i just listen for it? What is up with 1,2, and 4? 2 and 4 doesn't seem TOO bad, Why is the compression so high for cylinder 1?

[jlevie]

To figure out what is going on you need to also do a wet compression test and a leak down test.

[BMWZ318is]

yah i would def do a leak down test that way if you have a leake you will know exactly where its at. what was the car doing to make you check the compression?

[acolella76]

To figure out what is going on you need to also do a wet compression test and a leak down test.

What should I look for with the wet compression test? I plan on doing a leak down test anyways.

yah i would def do a leak down test that way if you have a leake you will know exactly where its at. what was the car doing to make you check the compression?

I will do that. Basically the previous owner claimed the engine was rebuilt, but had no documentation of it. Long story short i found out that the engine hadn't been touched for 300k miles so i thought it would be a good idea to test the compression haha

[hurttech]

I thought 180 was standard? I'm running:

1- 180psi
2- 180psi
3- 180psi
4- 180psi
5- 155psi
6- 155psi

Is that not good haha

[Lee Harvey]

perform wet compession after dry, squirt a small amount of oil intot he cylinder to see if this aids in sealing, higher wet test results than dry indicate worn piston rings causing low compression/blowby.

Next perform a leak down test, yes you need a gauge to observe the amount of flow passing by and you also need each piston at tdc compression when you test for leak down.

Regardless, if you don't know how to perform the base engine diagnostics you are attempting, it's likely a moot point since you probably also are not qualified or capable to make neccessary repairs. No offense intended.

[acolella76]

perform wet compession after dry, squirt a small amount of oil intot he cylinder to see if this aids in sealing, higher wet test results than dry indicate worn piston rings causing low compression/blowby.

Next perform a leak down test, yes you need a gauge to observe the amount of flow passing by and you also need each piston at tdc compression when you test for leak down.

Regardless, if you don't know how to perform the base engine diagnostics you are attempting, it's likely a moot point since you probably also are not qualified or capable to make neccessary repairs. No offense intended.

None taken. I know what I'm doing, i'm just lazy. :) I will follow up with a wet compression test and leak down test. I have done these tests before, and have torn down and rebuilt an engine before, but hopefully since it's my daily it's only a gasket or valve problem. Don't have the funds for a rebuild :(

[TrentW]

I know this is an old thread, but just did a dry compression test om my E34 and came up with similar readings. #3 was 140, so figured I'd do a wet test next. Is it as simple as using an oil can and squirting some oil into the cylinder or how exactly should it be performed?

[bmwstudent]

Yes it is pretty much that simple. Don't pour a cup in the cylinder, only about a tablespoon. Then crank away with throttle all the way open and watch the gauge.

[TrentW]

Thanks--performed wet compression test and got 180. Dry was 140. Damn.

[TobyB]

Compression tests can lie- drive it a bit, then try, try again.

It only takes a flek of carbon or grit in a valve to make the cylinder look
like death- but then, it clears itself, and viola, your engine is healed!

t