Start easiest first.
Make sure tank has fuel. And more than 10 gallons as I have ran out multiple times, and after driving hundreds of these trucks and running out at quarter tank...know it is more common than not, (Wish I didn’t have to say that)
If no go, Check fuel solenoid, (driver side of engine, above the fuel lever in side of injector pump) make sure it sucks all the way in and STAYS sucked in after starting the engine, and does not slowly push back out, hitting the fuel lever into the closed position. A sure sign of a bad one is it slowly moving, or getting HOT quickly. Also check the manually operated kill lever on bottom of dash just to the right under steering wheel, as it has a bicycle cable that pulls the fuel lever to the off position when pulled from inside the cab, and when pushed back in can often just kink up the cable under the hood instead of pushing the lever, or not quite fully open the fuel lever which could limit the fuel flow.
If no go, Next is push primer bottom on lift pump, (driver side engine, lower middle area of engine) and check for prime. If button pushes easily, and does not get harder after a few dozen pushes, you have lost prime and not in a easy couple minute fix.
If you can’t get solid prime, Next requires tools, as filters and water separators are next, and make sure to fill them up if possible with clean fuel, and re-prime using the lift pump primer button.
If no go, Next I would pressurize the fuel tank with a fuel cap with a valve stem installed, or a grocery bag with some rags or something soft inside and a blow gun with the rag-bag pushed as tight as possible around the blow gun tip inside the fuel inlet and try to create a decent seal for 20-30 seconds (3-4 psi is PLENTY, or for sure stop if your tank expands with a loud boink sound. This is much easier with two people as you want to inspect for leaks in the fuel lines while it’s under pressure. So many times it is the fuel lines under the cab are cracked, but not very visible until you see dripping or a wet spot start to appear on the hose, or around a hose clamp connection. Fairly easy to replace though.
If you still can’t get consistent prime, Next I would open the bleed screw on the injector line right above the water separator filter (driver side, up high above the lift pump, and to the rear of engine a bit), and allow the pressured tank to push any air out, as you also have someone push the primer button like a madman, then close the bleeder, and finish off priming with the lift pump button until hard, or you hear pressure building inside of injector pump. (It hisses slightly when fuel starts to flow into it after it has lost prime)
If no go still, next I would remove the fuel line from the top of the tank and pull out the copper pickup tube and inspect for hairline cracks near the top of the copper tube just under the treads that screw into the top of the tank. (Have delt with this many times, and is the main cause of phantom losses of prime when everything else is fine)
Next is really inspect all the injector lines, fuel lines, filters, etc again with pressureized fuel tank again, with a clear chemical grade temporary clear fuel line to visibly watch fuel flow, and to witness any bubbles, and where they come from. This is where lift pump issues will reveal themselves, or any air bubble entrances into the fuel system can be seen.
If you don’t fix it by now, it may be Injector Pump itself, which is a whole new level of **** it.
