If the fuel system is in-tact and the in-tank pump craps the bed, thr IP will still pull fuel and run the truck.
Once the fuel system is taken apart and air gets in there, you need the in-tank pump to bleed air from the fuel system.
Kind of a pandoras box.
Originally the engine just would not start one day. I had noticed a leak from the 12pt plug on the side of the IP had gotten much worse, but I was in the middle of moving. The truck ran for about 2 weeks after the move, then would not start.
So, I worked on the leak, and fixed it, and it still would not start, so then I scanned this forum for help, and found this thread, and a few others on the same topic. I had noticed that the oil level had been slowly rising since I bought it 1.5 years earlier, so I followed the instructions here and ordered a set of o-rings, dismantled, cleaned and resealed the hydraulic head.
Once all together, and leak free, I purged the fuel system, confirmed ample fuel flow to the injectors, then tightened up the hard pipes, still no start. All I get is a small amount of black smoke coming from the exhaust pipe, but no sustained firing.
On this thread you recommended changing the fuel filters, which seemed reasonable. I had ordered a complete set of filters for my deuce, so I changed all of the fuel filters. Still no fire, but smoke.
Then I drained the oil, changed the two oil filters, and put fresh oil into the sump. Still no fire, but smoke.
Now, it seems reasonable that the fuel tank and screen for the lift pump, and/or the fuel and return lines are plugged, since it has become clear to me that the truck was never serviced since it left the military, and I have run it on waste vegetable oil since I bought it, and I can see crud in the fuel tank from vegetable oil, and I see no fuel flow along the return line, but I have verified ample fuel flow along the fuel line. So, I plan to work on that when I can get to it.
If scrubbing out the fuel tank does not work, then I plan to pull one injector for inspection. If it is excessively coked or excessively warn, then I plan to send them off for a rebuild, and/or buy a new set. Since I make my own diesel fuel having a spare set of injectors, and rebuilding them myself has become an annual service. So, I plan to learn how to rebuild them myself.
I also plan to put one injector on a hard pipe that is rotated out of the way, and crank the engine to see if the IP can make pop pressure, because the HH might just be too warn to do so.