Oil level in HPOP is topped off, crank case oil level is good.

Phil, why would you think that it wouldn't be orings? A stumble like he said on a free rev is a very telltale symptom that injector orings are bad. Plus it has 400k on it.

It's not bad to change them, but with those miles it's likely the injectors need attention too. If it were me and money was real tight (like I assume it is if you're starting a business), I'd change them and HOPE for the best. If you do them, you'd be under the valve covers so it would be a good idea to compression test it so you have an idea what you're working with as far as a healthy motor. Glow plugs get removed to do a compression test, so you'd probably need a new set of those too. Orings and glow plugs are about 20 bucks a cylinder together. So, around 160 bucks or so for oem parts (which you want).

Remember I'm saying this still thinking you probably have some worn injetors too..

If you pull the valve covers you can start it (won't make a mess) and watch for oil spitting out of the spouts for the injectors and see if its fairly even amongst all of them. There is also a measurement you can take on the injector, but I can't remember off hand how to do it. Someone may pop in and fill me in there.

The measurement you are thinking of Darin is under the armature plates. Remove the four screws from the coil, and remove the coil. Remove the spacer, and there is a square plate with a screw in the middle. Don't try to remove that screw! See if you can get a .003 feeler gauge under that square plate. If not, try a .002. Should be no less than .002, if that won't go, the injectors are worn.

Thanks Bill!

Thanks you guys! I'll try to check into that as soon as possible.

1 more question;
What symptoms can a bad PCM have?

A dead truck is about the only symptom.

I was thinking that he said the fuel filter wasn't black and there wasn't any rubbery pieces in the bowl.

It still may be injector o-rings, but when I had this problem (wasn't as severe) it didn't get resolved until I got the injectors rebuilt. I tried just doing orings, but it didn't help much.

With 400k miles the HPOP could also be getting worn out. It could be short on HPO, but I would expect a CEL though.

Yeah, there's no CEL present and it does work.

How do you check to see if the HPOP is going out?

I think you need to get a scanner and check the duty cycle of the IPR. I'm not sure what the numbers should be, but someone here does.

Bad injector o-rings can also cause the duty cycle to be higher than normal though.