[appleburger]

I'm diagnosing an issue with my stock 325i. It'd started to idle rough then over a couple weeks went down on power. Felt like it was a cylinder down. I assumed I had a bad coil. I went to troubleshoot the coils and then it wouldn't start at all. I had the coils out so figured why not compression test? Well the results were confusing not only because they were low but because they were inconsistent. I tested three times and got wildly different results each time. I pulled the fuel pump fuse, removed all spark plugs, screwed a brand new compression tester into a hole and cranked 5-6 times (when the needle stopped going up) with the throttle wide open. Pressure held until I released it so I don't believe the tester is failing to make a good seal. I know its ideal to do it cold but since it won't start it was my only option. I figured if all the results were within a tolerable amount of each other it'd tell me what I need to know. Here are the results for each cylinder. Commas separate test 1, 2, and 3.

1 - 42, 60, 61
2 - 149, 111, 121
3 - 59, 70, 59
4 - 83, 105, 72
5 - 50, 151, 116
6 - 31, 100, 30

How is this possible? I assume I did something wrong. I did put a scope down to look at the combustion chambers. Couldn't see much. Some carbon buildup but no pieces of ring land bouncing around or anything.

Extra context: About 6 months ago I had the intake manifold off to do CCV, coolant hardlines, gaskets, etc. Been running great since until the DISA started rattling. Rebuilt the DISA with the German Auto Solutions kit. All was well for a few weeks until this started.

 

[Effduration]

I read a previous post....years ago...but it mentioned an overheat..Did the car overheat?

I am kind of surprised you went right to a compression test... a vacuum leak or a bad coil might be more common. Are there any codes?

I would not get too hung up on your cold compression test results for now. I might suggest you try a free tester from Autozone and preferably only when the engine is warm.

Does the car run and/or idle with MAF unplugged? If so, maybe you have a vacuum leak.

 

[appleburger]

Its done the standard E46 spring-a-leak-and-heat-up but always been caught quickly, resolved, and ran fine afterward. At this point every piece of the cooling system is new and has been holding temp fine.

I was definitely thinking I was going to be troubleshooting a vacuum leak or coil but had been curious about compression numbers so jumped straight to it. I've had an intermittent lean code (pointing toward vacuum leak). If the consensus that these compression numbers aren't something I should worry about I'll go on about that troubleshooting track. I kind of just stopped in my tracks when I saw those numbers.

 

[IdahoDoug]

Do a smoke test to find vacuum leaks would be my thought.

 

[TobyB]

It may have cold- flooded on you-

then the rings lose seal, and the thing won't start.
There have been more than a couple of erroneous engine rebuilds for that.
You get low compression reading and excessive leakdown numbers
when the fuel washes the oil off the ring pack.

Try a tablespoon of oil down each bore, and see if that helps.

t

 

[Sapote]

It may have cold- flooded on you-

then the rings lose seal, and the thing won't start.

OP should add a tea spoon of engine oil to each cylinder and test compression again. If same bad result then I think this maybe the cause: the splined shaft bolted to the cam might have slipped when the driver played the rev like a musician. This caused cam out of time with crank, hence low compression and high compression if the vanos activated.

 

[appleburger]

It may have cold- flooded on you-

then the rings lose seal, and the thing won't start.
There have been more than a couple of erroneous engine rebuilds for that.
You get low compression reading and excessive leakdown numbers
when the fuel washes the oil off the ring pack.

Try a tablespoon of oil down each bore, and see if that helps.

t

I didn't know this was a thing. Great info, thanks! The only time I've seen varying results like these was on an engine with broken ring lands hence why I got worried. I'll do this and go on about troubleshooting vacuum leaks.

 

[appleburger]

OP should add a tea spoon of engine oil to each cylinder and test compression again. If same bad result then I think this maybe the cause: the splined shaft bolted to the cam might have slipped when the driver played the rev like a musician. This caused cam out of time with crank, hence low compression and high compression if the vanos activated.

This guy assumes my VANOS works ha! J/k thanks for the info.

 

[TobyB]

Yeah, some cars and some oils seem to do it a lot more,
but BMW uses a weak ring and a LOT of fuel when cold,
so sometimes if you're doing odd things where the engine never
warms up, it happens.
Like, pull the car out of the drive, shut it off,
move another car, pull the car into the drive, shut it off again, and
then the third start, it'll 'chuff' once or twice- and then spin almost
freely, like the cams aren't turning.

Scared the pants off me the first time it did it, because on an E30,
that meant your timing belt had broken. And for a while, that's how
I was getting cheap E30's...

Hope that's it. I've had broken rings before, and usually
it was just low compression on one or more cylinders.

t

 

[Inlajnsiksbenzin]

Yeah, some cars and some oils seem to do it a lot more,
but BMW uses a weak ring and a LOT of fuel when cold,
so sometimes if you're doing odd things where the engine never
warms up, it happens.
Like, pull the car out of the drive, shut it off,
move another car, pull the car into the drive, shut it off again, and
then the third start, it'll 'chuff' once or twice- and then spin almost
freely, like the cams aren't turning.

Scared the pants off me the first time it did it, because on an E30,
that meant your timing belt had broken. And for a while, that's how
I was getting cheap E30's...

Hope that's it. I've had broken rings before, and usually
it was just low compression on one or more cylinders.

t

I can confirm! Cold flooding happened to my E46 like 1 and a half years ago when I was doing throttle body adaptations reset. It was cold and I started the car before this for a few seconds and shut it off. If I did a compression test that time it would have shown very low numbers because the engine was turning over as if there were no spark plugs, lol. So this can be the OP’s case.

 

[Effduration]

I didn't know this was a thing. Great info, thanks! The only time I've seen varying results like these was on an engine with broken ring lands hence why I got worried. I'll do this and go on about troubleshooting vacuum leaks.

Yeah but you said you pulled the fuel pump fuse (the right way to do it), so there should not be any fuel in your cylinders..

I still think your nonsensical, cold compression figures should be ignored for now..

I still think