How to Perform a Compression Test

[87MUSL]

Stupid question.....but I've never performed a compression test on my GN b4. Trying to diagnose a blown headgasket.

Do I....

(1) remove all spark plugs and from engine
(2) install pressure gauge in cyliner being tested and crank over engine for 4-5 seconds with throttle in WOT position.
(3) record value...all cylinders should be within 10-15% of one another.

Does this sound correct?

Thanks in advance.

-Mark

 

[DKREDT]

I think you also want to disconnect the orange ecm wire close to the battery and take off the up pipe and put a screw driver in the tb to hold the "butterfly" open to duplicate wot.

HTH

 

[Just a Six??]

Why does throttle have to be at WOT? What if you do it without touching the Throttle??

 

[1badTTA]

Originally posted by Just a Six??
Why does throttle have to be at WOT? What if you do it without touching the Throttle??

You will create vacuum in the intake and the numbers will be lower. Not a big deal since you are looking for constincy between the cylinders and not a specific number.

 

[Lee Thompson]

test

Remove all plugs---- disconnect ecm---hold throttle wide open---crank until it HITS 4 times on the cyl. you are testing. The warmer the eng. the better.

 

[turborayjay]

Not trying to hijack ya, however same questions...


how much of a difference does it make if you don't take out all of the other plugs? I recently did mine and seen 155 - 165 psi? I didn't realize I had to take the other plugs out. I did have the throttle open though.

 

[S10xGN]

Originally posted by turborayjay
Not trying to hijack ya, however same questions...


how much of a difference does it make if you don't take out all of the other plugs? I recently did mine and seen 155 - 165 psi? I didn't realize I had to take the other plugs out. I did have the throttle open though.

It just makes sense. You're gonna do all the cylinders anyway, and the empty plug holes make it easier on the battery...