Compression Test Results - BMW M5 Forum and M6 Forums
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post #1 of 12 Old 19th January 2013, 02:35 PM Thread Starter
AseanAero
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Compression Test Results

Delving into Taffy's M5 oil loss problem on GA25331 we took a compression test on his engine and got the following results

GA25331 is a UK spec high compression engine. Comp numbers with the engine warm (standing for 1.5hrs while we had our usual long lunch) and throttle closed (I forgot to open the throttle)

Cyls 1 to 6 - 180, 180, 180, 180, 180, 190

Then we took the compression on GA25324 (my M5) which has low compression tropical version 9.2:1

Cyls 1 to 6 - 175, 170, 165, 170, 170, 175

What's a normal compression reading for a normal and low compression version of the S38B36 ?

It looks like GA25331s oil leak is a combination of a poor sealing oil filler cap and dipstick and a stripped acorn nut on the cam cover



Tony, Aussie expat in Jakarta Indonesia

1990 E34 M5 S38B36 GA25324
GA25324
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post #2 of 12 Old 19th January 2013, 03:56 PM
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Finally found what we think is the cause of the oil leak ! YEAH !
Gotta order oil cap from the internet and wait 10 days


Welcome to coconut island

It is said that if you line up all the cars in the world end to end, someone would be stupid enough to try and pass them.

Last edited by BlueBiturbo; 19th January 2013 at 04:16 PM.
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post #3 of 12 Old 19th January 2013, 04:31 PM
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I changed the oil cap on mine when investigating oil consumption. Made no difference.

The system should be negative pressure there, it could be there is some splash (what I thought before buying a cap) but it was a red herring in my case,

Incidentally I got my cap from the BMW dealer... Stupidly cheap price, as it sometimes the case with BMW parts.
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post #4 of 12 Old 19th January 2013, 04:57 PM
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Mine was definitely leaking from there.
I wiped the surrounding area clean yesterday, it is oily again today after 6500 rpm run.

It is said that if you line up all the cars in the world end to end, someone would be stupid enough to try and pass them.
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post #5 of 12 Old 19th January 2013, 05:02 PM
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I hope it is that... Would be a great, easy, fix.

When I remove my cap (quite often for top ups) there is oil presence underneath the sealing faces.
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post #6 of 12 Old 19th January 2013, 05:08 PM
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My fingers are crossed

It is said that if you line up all the cars in the world end to end, someone would be stupid enough to try and pass them.
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post #7 of 12 Old 19th January 2013, 08:17 PM
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Compression Test Results

It wouldn't just be damp there at the amount of oil loss you're experiencing.

Pull the intake elbow off the plenum. Is it oily inside the plenum?

Last edited by Nikosx; 19th January 2013 at 08:18 PM.
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post #8 of 12 Old 20th January 2013, 03:04 AM
Roland H
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There aren't any real numbers that you should be striving for, since all compression tests are different. Density Altitude plays a huge effect in compression testing. Consistency across cylinders is more important. I usually say each cylinder within 5-10%. The testing gauge is also important, it's good to use an Aviation Grade tester vs the one you get at the tool store, as many sold at the tool stores aren't built to the same, if any, standards.

Leakdown tests are much better for determining engine health though.

Last edited by Roland H; 20th January 2013 at 03:05 AM.
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post #9 of 12 Old 20th January 2013, 05:50 PM Thread Starter
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Quote:
Originally Posted by Nikosx View Post
It wouldn't just be damp there at the amount of oil loss you're experiencing.

Pull the intake elbow off the plenum. Is it oily inside the plenum?
No significant quantities of oil in the plenum, just a very slight thin film of oil



Tony, Aussie expat in Jakarta Indonesia

1990 E34 M5 S38B36 GA25324
GA25324
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post #10 of 12 Old 21st January 2013, 12:50 AM
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Turns out that I DO HAVE a new spare oil filler cap !
Just installed it today. It is VERY tight seal.
I will drive it hard tomorrow and see the difference.

It is said that if you line up all the cars in the world end to end, someone would be stupid enough to try and pass them.
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