M50 low compression in cyl 5

[wilkie3]

I've been fighting the notorious hunting idle for a few weeks now. I've addressed any vacuum leaks with a propane tank. The only sensor left to check is the ICV. I am also going to perform a pressure decay test tomorrow to look at the fuel pressure regulator and injector seals.

Tonight I performed a compression test with the following results:
Cyl 1 200
Cyl 2 210
Cyl 3 210
Cyl 4 210
Cyl 5 180 max, then falls to 0 after cranking stopped
Cyl 6 210

For a 14 year old car I was shocked to see the values that high. My big concern is obviously with number 5. It appears to be a valve issue because the pressure wants to increase, but it keeps oscillating between 120 and 180. As soon as I stop cranking the pressure decays to 0 nearly instantly.

Any ideas how I should go about determining if it is truly a valve issue.

Thanks in advance. This is by far the best bimmer forum!!!

[SnakeyesTx]

Sounds like maybe an exhaust valve isn't closing all the way, maybe you have a broken spring under the tappet cover that's not letting it pull the valve back up and closed after the cam lobe leaves the high spot. Maybe you could find someone with a boroscope to take a peek in there. I did this on my jag once, but I took the exhaust manifold off to get a better viewing angle.

[wilkie3]

That's a great idea. Any idea how to tell if it is an intake or exhaust valve other than removing the exhaust manifold? Thanks for the tip.

[winfred]

the numbers look a little high, you didn't oil the cylinders did you? as oil can mess up compression #s the cylinders don't really hold the pressure for very long as they don't need to, there could be some crud on the spark plug seat keeping the tester from sealing, if you are getting better then 120 outta the "bad" hole it's not going to be a dead miss, you may not even be able to notice it while the engine is running, if it's good enough to get to 180 theres no mechanical problem

[pyro]

well first re check the compression on that cylender, (like someone said clean the threads out) then after that to prove if its rings or valves, Pour some oil down the hole and then retest it. If the number goes up alot then that would mean you have a ring problem, if it dosn't change much then its a valve problem.

[SnakeyesTx]

Well.. typically lopey cams leave the exhaust manifold open slightly longer while the intake valve is opening. What I'm thinking is, you're able to build a little compression at the moment the intake valve is closed, but the fluctuation is caused by the exhaust being stuck slightly open (giving you the idle stumble since only 1 cylinder is doing this and not all 6 typically like the lopey cammed V8's you're used to seeing/hearing). The almost immediate zeroing out comfirms a leak there somewhere. Sometimes you can hear a broken spring with a stethoscope while the motor is running, so you might wanna try to prod around that area and try to hear for something out of the ordinary compared to other cylinders. If all else fails, take off that manifold and take a peek. If you're lucky you can eyeball it, if not, find a boroscope or someone with them and treat them to a sixer :D