i have replaced the relay and the high pressure pump; truck will run fine for 5-10 minutes and cut off, each time i connect a pressure guage to schrader valve and shows no pressure. after a few minutes, i turn ignition on , pressure goes to 40 , engine starts , runs 5-10 minutes ,same problem all over again. i can hear the in-tank pump load up when i turn the key before starting engine.... any ideas? ...thanks.

 

Which year? Some years had two pumps: low pressure pump in the tank, and a high pressure pump in line. If you have two pumps, maybe one of them isn't working.

 

88 model, has two pumps, i have already replaced the high pressure pump, and you can hear the whirring sound at the filler cap for the in tank pump when you turn the ignition on. i was just wondering if there was anything else to check before dropping the tank to replace in tank pump....i guess it could still be the problem even though you can here the whirring sound?...i also replaced the module...thanks.

 

A couple of suggestions:
1) One of the leads to the EEC-IV self-test connector grounds the fuel pump relay. You might try grounding that lead with the key on and see if the fuel pressure behaves itself.
2) I don't remember the specifics, but I remember reading an article at www.ford-trucks.com about the inertia switch causing problems. It gets old and doesn't stay closed. If the fuel pumps are losing power on startup, that might be the problem.
3) For a dumb suggestion, the gas tank's full, right?