procedure for engine compression test

[quickster]

I have an 1987 944 turbo with 117000 miles. Although the engine seems strong with no obvious signs or problems I would like to do a compression check. Ive done a lot of the work on this car but have never done a compression check and would like to know the procedure or steps.

thanks

quickster
West point, Va
1987 951

[soxnail]

There is a quick way of doing it, but requires a digital storage scope.
Sox

[DannoXYZ]

Originally Posted by soxnail

There is a quick way of doing it, but requires a digital storage scope.
Sox

Eh? Checking compression with a 'scope?

The way I've always done it is:

1. pull DME relay so it won't try to inject fuel or spark
2. pull all plugs
3. grease threads of compression-tester and insert into #1
4. hold WOT and crank for 5-7 revolutions
5. repeat for remaining cylinders.

[quickster]

for some reason I thought you pulled plugs one at a time while testing, thanks for the tip
I'll get a compression kit from harbor freight since I wont use it very often

quickster
west point, va
951

[soxnail]

Yah its a quick check to see if all cylinders have approximately the same compression, does not give you actual numbers for each.
Attach scope to the battery, gnd to negative and probe to positive.
Set to trigger on a negative edge.
Pull the dme relay.
Crank the car.
Scope will capture the dips in the battery voltage as the pistons hit max compression. Each dip showing consecutive compression peaks.
You can then compare them and see if one or more did not make the battery dip as much.
You get relative (comparative ) reading.
See pic
Sox

[quickster]

thanks sox,

but I will have to use the old compression tester...

[DannoXYZ]

Originally Posted by soxnail

Yah its a quick check to see if all cylinders have approximately the same compression, does not give you actual numbers for each.
Attach scope to the battery, gnd to negative and probe to positive.
Set to trigger on a negative edge.
Pull the dme relay.
Crank the car.
Scope will capture the dips in the battery voltage as the pistons hit max compression. Each dip showing consecutive compression peaks.
You can then compare them and see if one or more did not make the battery dip as much.
You get relative (comparative ) reading.
See pic
Sox

Interesting, I'll have to give that a try.