Compression testing process

[JZiggy]

Looking for some advice on best practices for running a compression test.

I always do this with the vehicle warm (ie, just after driving and at full operating temp). Pull plugs and then test each cylinder with pedal at WOT.

What I am unsure about is this... should I:

- Crank the engine over a set # of times (~7 is what I have seen recommended) and check gauge pressure
- Crank the engine over until the gauge settles at a maximum value

The times I have done this I've seen that at 7 cranks the value may be, let's say 170 psi but if I crank more like 15 times it will settle out to 190 psi and not increase further.

Thoughts? Thank you!

 

[Cujet]

I simply crank until the number is highest.

 

[thescreensavers]

Yea dont over think this just crank it till it stops moving up. Hot vs Cold numbers will also be different so make sure the spec you are reading is related to the condition you are testing.

 

[Lubener]

I often wondered if the ignition system should have a discharge path to ground as I have heard the ignition system can be damaged by not doing that.

 

[MattPersman]

I often wondered if the ignition system should have a discharge path to ground as I have heard the ignition system can be damaged by not doing that.

If you were extremely worried about this you could just unplug coils

I actually just jump the starter relay and not use the key/button. That way I’m taking everything out of the equation fuel and ignition related

 

[Kira]

What year, what vehicle, what engine? Someone might have some familiarity with your specific application.

 

[JZiggy]

Forgot to mention: I also pull the EFI fuse so no fuel gets injected, and unplug something to stop spark if accessible (like unplug the distributor, coils, etc.)

 

[Eddie]

Hold the intake open while testing. Usually we like to see 90% at three pumps.