Z4M S54 Cylinder Compression Pressures

jontZ4M · Post by **jontZ4M** » Thu Jan 12, 2023 11:13 pm

Hi guys

I've been investigating some white scum that I found on the oil filler cap.

I did a cylinder compression test today with the engine luke warm with a trusted tester.

All cylinders read 180-190psi.

This seems decent but I can't seem to find anywhere what the values should be from factory.

Does anyone know?

Thanks

Smartbear · Post by **Smartbear** » Fri Jan 13, 2023 12:35 am

jontZ4M wrote: ↑Thu Jan 12, 2023 11:13 pm Hi guys

I've been investigating some white scum that I found on the oil filler cap.

I did a cylinder compression test today with the engine luke warm with a trusted tester.

All cylinders read 180-190psi.

This seems decent but I can't seem to find anywhere what the values should be from factory.

Does anyone know?

Thanks

That sounds healthy enough to me, the most important thing is that all cylinders are closely matched.
It’s not going to cause condensation problems for the engine, short journeys are normally to blame for that.
Rob

maupineda · Post by **maupineda** » Fri Jan 13, 2023 3:41 am

Spec is 11-12 bar

Zedebee · Post by **Zedebee** » Fri Jan 13, 2023 8:37 am

Which is 160-175psi near enough

Rockhopper · Post by **Rockhopper** » Fri Jan 13, 2023 8:49 am

You are bound to get mayo on the filler cap at this time of year unless you are flogging it up and down the motorway all day.

jontZ4M · Post by **jontZ4M** » Fri Jan 13, 2023 11:50 am

Zedebee wrote: ↑Fri Jan 13, 2023 8:37 am Which is 160-175psi near enough

Thanks.

Strange that it's slightly over-achieving but maybe the tester device is a little ambitious.

Cheers

R60BBA · Post by **R60BBA** » Fri Jan 13, 2023 12:19 pm

Agreed with Rob’s comments - compression sounds healthy mate, wouldn’t worry about it.

If the car has been sat for a while then you may want to take it for an Italian tune up, but this is totally up to you.

grannyknot · Post by **grannyknot** » Fri Jan 13, 2023 4:41 pm

jontZ4M wrote: ↑Fri Jan 13, 2023 11:50 am
Zedebee wrote: ↑Fri Jan 13, 2023 8:37 am Which is 160-175psi near enough
Thanks.

Strange that it's slightly over-achieving but maybe the tester device is a little ambitious.

Cheers

Might not be your tester, when I did mine I got 185-190 psi across the board on a dry test with hot engine and I know my tester is accurate.
I searched high and low for those compression numbers from the factory and all I found was the 160-175psi.
Doesn't make sense to me, I know a couple of other guys with Z4MCs and they both reported in the 180s as well.
160psi sounds like a dead engine engine to me, 174psi is where I would be thinking about a rebuild.

My old S38 from the e24 M6 produced 200-205psi on a stone cold engine in winter. You could think of the S38 as the grandfather to the S54, very similar set up.
https://youtu.be/GJCCZLUqnDM

Rockhopper · Post by **Rockhopper** » Fri Jan 13, 2023 5:23 pm

Consistency across all cylinders is more important than the absolute reading.
Your readings could be affected by many things, temperature of the engine, atmospheric temp and ambient air pressure are just a few.
But if you've got one at 180 and another at 190 then you need to investigate further.

jontZ4M · Post by **jontZ4M** » Sat Jan 14, 2023 4:58 pm

Cheers guys.

I personally think a variation of 180 to 190psi seems acceptable, especially as it is a slow steady transition from Cyl 1 to 6.

I come from older turbo Jap engines, but 5% error across the engine way always considered OK.

Beedub · Post by **Beedub** » Wed Jan 18, 2023 5:05 pm

do you guys test with the other plugs in or out?

Rockhopper · Post by **Rockhopper** » Wed Jan 18, 2023 8:38 pm

All plugs out, engine warm, throttle wide open.

Argenta · Post by **Argenta** » Mon Jan 23, 2023 8:41 pm

Sorry to hijack thread, but I got 11.8 on all cylinders but nr.2, 10bar. Got some oil in there, and bam up to 11.9bar. So….
Snapped pistonrings..?
Engine has been really cared for with calm cold starts etc, but yeah over 200000km’s now.
A few trackdays but never mistreated.

I only tested cos I have a SLIGHT misfire on stone cold startups for 1-2min.

I sense a whiff of something expensive…

Rockhopper · Post by **Rockhopper** » Mon Jan 23, 2023 8:51 pm

That’s a big variation in pressure. Yes you need to inspect further. Stick a camera down there first but ultimately it’ll all have to come apart I think.

grannyknot · Post by **grannyknot** » Mon Jan 23, 2023 11:14 pm

Argenta wrote: ↑Mon Jan 23, 2023 8:41 pm Sorry to hijack thread, but I got 11.8 on all cylinders but nr.2, 10bar. Got some oil in there, and bam up to 11.9bar. So….
Snapped pistonrings..?
Engine has been really cared for with calm cold starts etc, but yeah over 200000km’s now.
A few trackdays but never mistreated.

I only tested cos I have a SLIGHT misfire on stone cold startups for 1-2min.

I sense a whiff of something expensive…

Yeah, probably something going on with the rings on that cyl. If it doesn't bother you then you can drive it like that but it could the foul that spark plug and screw up the catalytic converter.
S54 rebuilds aren't cheap, you might be able to get away reusing the original pistons with new rings and deglasing cyl walls but probably not.

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