Re: E28 Compression Check

[Pat Donahue <pdonahue@xxxxxxxxx>]

Barthe wrote:
> 
> I have a 1986 model 528e with 130,000 miles on the odometer.  I performed a
> compression pressure test on the engine when at ambient temperature (90
> degrees) and obtained some erratic values.  I repeated the compression
> pressure test with the engine at operating temperature or just a little bit
> below operating temperature and obtained quite different results.
> 
> Can anyone comment on the difference in values recorded at ambient and
> operating temperature?
> 
> It looks like good news that all cylinders at operating temperature yielded
> a consistent pressure.  However, the 115 psi is about 35 psi below the
> Bentley manual recommendation.  Is this low compression pressure to be
> expected on an engine this old?
> 
> Can anyone share with me the specific compression test pressures they have
> recorded on a 528e?
> 
> 
> Engine at Ambient Temperature:
> 
> #1    210 psi
> #2    140 psi
> #3    140 psi
> #4    210 psi
> #5    205 psi
> #6    205 psi
> 
> 
> Engine at Operating Temperature:
> 
> #1    115 psi
> #2    115 psi
> #3    115 psi
> #4    115 psi
> #5    115 psi
> #6    115 psi

I'm surprised that the operating temp readings are that low.  The
cylinders should be well oiled from having run recently and hence
sealing better, not worse.  I wonder if the valves aren't adjusted way
too tight and expansion isn't letting them close to seal properly.  Did
you have all the plugs out when you cranked it both times?  If you left
them in except for the one you're checking it will slow down the
cranking speed and allow more pressure to be lost.

Next guess is up to someone else.
- -- 
Pat Donahue CCA 125031
Vienna, VA
'91 M5 4GTFUL
'72 MGB 8LYFGR&

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