Z4M S54 Cylinder Compression Pressures

J

jontZ4M

Member
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
 
jontZ4M said:
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
Click to expand...

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
 
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.
 
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.
 
jontZ4M said:
Zedebee said:
Which is 160-175psi near enough
Click to expand...

Thanks.

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

Cheers
Click to expand...
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
 
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.
 
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.
 
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… 🙃
 
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.
 
Argenta said:
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… 🙃
Click to expand...
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.
 
Argenta said:
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… 🙃
Click to expand...
Have you done a leak down test?
 
Dont have the equipment, a pro mechanic (on S54’s) I know does, tho. Will check later this week.
Still.. Can't be much else than rings by now.
 
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