Correct way to do a Compression Test?

Goad

Member
Feb 29, 2020
73
3
8
Las Vegas, NV
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

Well-known member
Dec 15, 2010
3,456
244
63
Norcal
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

Well-known member
Jun 23, 2009
4,428
78
48
TX of course
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

don't know sh!t about IFS
Staff member
Oct 21, 2009
18,206
440
83
Phoenix Az
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
 

JoshH

Daggum farm truck
Staff member
Vendor/Sponsor
Feb 14, 2007
13,205
155
63
Texas!!!
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).
 
  • Like
Reactions: 2004LB7

Goad

Member
Feb 29, 2020
73
3
8
Las Vegas, NV
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

don't know sh!t about IFS
Staff member
Oct 21, 2009
18,206
440
83
Phoenix Az
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.
 

Users Who Are Viewing This Thread (Users: 0, Guests: 1)