Compression testing. S14

# Performing a compression test. S14

Remove plastic covers from firewall. |
Remove main relay, located on right side of firewall. |
Remove the two 10mm nuts retaining spark plug wire housing. Pull straight up on plug wire boots. Twisting boots while pulling will ease removal. Use BMW special tool 12 1 160 from the tool kit in the trunk to remove spark plugs. |
Install a compression tester and follow instructions provided with the tester.
Press accelerator pedal to the floor while cranking engine during test.

### What to look for ?

An actual number is not required, although ideal compression will be above 175 psi, the main concern is equality between all cylinders, no more than a 10 percent variation should be present between the highest and lowest readings.
If you have a cylinder with low compression, a leak down test should also be performed. If the results are inconclusive, you can insert a small amount of oil in the cylinder and run a second compression test. If the compression raises, your rings are the primary suspect. If the reading stays the same, then you probably have leaking valves.

E30 M3 Repair Manual V2.7. Copyright ©1999-2001 Koala Motorsport & Brett Anderson