Correct way to do a Compression Test?

[Goad]

What is the correct way to do a compression test on a LB7? I have the tester from harbor freight, but I want to make sure I get the correct numbers.

Do I do a cold crank on a cold block? or warm it up? if so, what is the ideal temp? where should the temp be taken from?

 

[2004LB7]

It can be done hot but working on a hot engine sucks. Plus, doing it cold will normally give you enough information to act on

Test through the glow plug hole. Crank it over a few times. Should be 350+ psi and preferably less then 10% difference between cylinders.

Make sure you disable the FICM so it doesn't fire. And it is also a good idea to hook up a good charger to keep the batteries topped up well cranking

 

[Dozerboy]

I don’t believe harbor freight has the right adapter to do a compression test on the Duramax. Make sure you tighten down the adapter with a wrench don’t just do it in by hand or it might leak.

 

[Chevy1925]

The HF one will work for 01-05, newer it won’t thread more than 1 or so threads in due to the glow plug change. I never trusted the gauge, used it more for variance between cylinders and rough idea of compression

 

[TheBac]

Moved to the General Engine Building section. Good info to have.

 

[CarolinaHD]

Adapter for 06+

Amazon.com: Lang Diesel Compression Test Adapt (LNG-TU-15-38) : Automotive

Buy Lang Diesel Compression Test Adapt (LNG-TU-15-38): Compression Gauges - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

 

[JoshH]

It has been a while since I've looked at the actual procedure, but I believe they call for you to remove all 8 glow plugs and hook the batteries up to a charger to keep them hot during the procedure and to crank the engine without the ignition being on to prevent the injector from firing and causing the cylinder being tested to give a false reading (or blowing up your gauge).

 

[Goad]

Thanks guys....I am going to give this a shot today. Should I unplug the 4 plugs that go to the injectors, or just unplug the harness that goes to the FICM?

 

[Chevy1925]

Thanks guys....I am going to give this a shot today. Should I unplug the 4 plugs that go to the injectors, or just unplug the harness that goes to the FICM?

just unplug the two connectors at the FICM and you are good to start crankin.

 

[2004LB7]

Couldn't you just pull the FICM fuse? Or is it one of the ignition fuses?