6.2 Diesel Compression Check

Howdy,

Just did a full compression test on one of my M1008's today. This ones my main driver every few days. The truck has 90k miles right now and gets maybe 6k miles a year. My compression results were all good but this being my first time doing a compression test on it, I had a few questions. I have an 100 psi variance in my numbers. Top being 420psi and bottom being 320psi. I would think the numbers would be somewhat more uniform. Is this normal? Also should mention I used a good brand name tester and all glow plugs were removed to eliminate cross cylinder pressure if there was a gasket failure.

Here is the results.

cyl #1 420psi
cyl #2 320psi
cyl #3 340psi
cyl #4 340psi
cyl #5 320psi
cyl #6 350psi
cyl #7 360psi
cyl #8 400psi

The truck is running great and starts 1st or 2nd crank every time in any weather from -30c to +30c without being plugged in. Let me know what you guys think of these results. Thanks

should not have that much variation. your numbers have about a 25% variance.
plugs in or out should not make a difference. leakage could be valves or rings, numbers that low could be both.
you should not have more than 10% variance. anything lower than 370 is a tired motor. I had between 390 to 420 on mine so your high numbers look good.

My motor was around 60K when testing it.

Maybe I should do a retest. Just reading through the 6.2 shop manual now and it says the motor should be hot. I know it was warm but it was only after a 5 min drive. I wouldn't consider that hot. Also says the lowest reading should not be less than 80% of the highest cylinder and no cylinder reading should be less than 380 psi. Also says cranking speed must be 180 RPM minimum. I know the 4th and 2nd cylinder the starter was starting to slow down from cranking on it so much beforehand. Also says to allow 6 "puffs" of each cyl. I did ten good seconds of cranking on each so I probably had more "puffs". Should've read the manual before I guess. Time for a recheck tomorrow. Thanks for the insight.

blueblaze said:

Maybe I should do a retest. .... Also says cranking speed must be 180 RPM minimum. I know the 4th and 2nd cylinder the starter was starting to slow down from cranking on it so much beforehand.

Click to expand...

It is important to have uniform cranking speed though compression test.

My guess is the motor is fine, it is good to learn how to do a compression test, and be proactive about maintenance.

But in my experience, it is better not to over think it if the truck is running well.

I figured the motor was fine. But I wanted a baseline so I could measure wear in the engine as life goes on. That's why I questioned the big variance. I should have gone over the gm manual first.

It may not matter. But I have NEVER checked the compression on any motor that is running fine and I have no issues with. Why would I? I Never checked the compression of any CUCV I owned. I have some that have over 200K on them and get the snot pounded out of them on a daily basis. They do not idle as smooth as they should I think that may be compression related. But just checking compression and then worrying about or guessing how long the engine is going to last IMHO is totally NOT needed. If you are having problems ? Maybe. But I would say run it and maintain it. It will serve you well. I have had a few submerged in water and had no issues after draining them and running them. I suspect it has not added life to it and may have damaged it. But it still runs and runs good. Have a great day.

When the tools that are used to evaluate the condition of a 6.2 cost more than a rebuilt 6.2... Run it till it breaks then replace it.

This is true haha. But I have a few 6.2s and 6.5s so the tools were a no brainer. Still can't believe how cheap these motors are. Yet the price of the whole diesel truck itself just seems to be going up and up. I couldn't give away a running 6.2 right now for 100 bucks where I live, yet it seems a running rust free 4wd 6.2 truck is about 6500. In my local I've only seen 3 cucvs come up for sale in the last couple years and not one of them was less than 8 grand. And all of them sold. Yet if a cucv has a 350 swap it's a hard sell. go figure.