Compression Check

Compression Check

I will be checking the compression in my 86 32-V shark soon. How do I disable the ignition so as not to harm the computer? I propose to ground out ignition leads from coil to a ground on the car. Also, should I pull the fuel pump relay?

Pete L
1986 928S 5-speed, 150,000 miles

I personally prefer to pull the fuel pump fuse instead. Also make sure that all the spark plugs are off before you do the test and preferably with wide-open throttle.

Vince Yu
'83 928S
'78 928

>I have searched to no avail through the shop manuals for a reference on
>the expected cylinder compression values on a 1986.5 32V engine.

Check page 10-02, the first one, there are two pages marked "10-02".
According to the book the wear limit is 6.5 bar or just over 95psi per cylinder.
I live at 7000 feet and my compression was right at 140 for all but one which was at 135psi.
My understanding is that the most important thing is that they all be within 10 to 15% of each other.

Ken Postma '86 928S Au

I've had a compression check done on the 88 S4 that I have all but
purchased. The car has almost 90k miles on it. Values were:
185, 185, 180, 175, 180, 180, 180, 175

I've been told that this is an acceptable compression level, but how does one account for the variance, which in my (underinformed) opinion, seems to be more than I might have imagined?

Don Davis

10% variation between the highest and lowest value is acceptable. Though generally, this amount of variation causes the engine to idle on the rough side. Was the compression check done while the engine is hot? Potential causes could be due to ring wear, valves not seating properly, or even heavy deposits on the piston. Only a leakdown test will tell for sure.

Vince Yu
'83 928S
'78 928

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