When idling about 1000 rpm, there are almost no air bubbles coming out of the injection pump. When cranking, air does leave the injection pump. Also, when idling there is a pool of bubbles in the line from the IP to return line. They just float and stay in the line.

Sounds good.

Did you check for any restriction in the return line to the tank?

I haven't. I was planning on disconnecting the return and supply lines to a can of diesel to help tell me that. Going to try to pick up a lift pump and secondary filter for preventative measures.

Where you put the secondary filter matters-filters should be post LP,for pre LP you should only use a screen and nylon preferably.

Yeah, the secondary filter is after the lift pump (still mounted to the intake plenum). Only managed to get the filter, but it hasn't been changed in a while so hopefully it helps some. Quick question, is there any fuel pressure before the lift pump or only after? Curious. And thank you again for the help you're providing.

Only pressure after the LP.

If any pressure or vacuum is in the fuel tank you need a new AC delco fuel cap that is properly vented for diesel use.

Ran the test with the sending (from right after the primary filter) and return line as well as installed the new filter. Still running the same. Gonna try to get a lift pump tomorrow and see if that helps (NAPA has one for only $20).

Just letting you know it might be a few days before I can install the lift pump (if I manage to grab one today/tomorrow) as it's currently snowing and my little car cannot make it to NAPA (sometimes I hate slicks).

Well, the new lift pump helped fill up the lines better. But I still cannot get the truck to idle. Still having the same issue. Idles for a minute with my foot on the throttle but after that it will only fire up for a second and then it dies no matter what I do with the throttle (not touching, 1/4 way, 1/2 way, varying, etc..).

Is the LP moving fuel?
Wonder if the sock in the tank might be plugged?