my 86 f150 with a 90 5.0 motor in it will not run. i did a swap and changed the 86 harness onto the 90 motor. the wire for power to the fuel pump was cut, so when i put it back together, the pump pumps, and i can seperate the wires, get in the truck, and itll start and run till the gas is gone, but when i hold the wires together the pump does not shut off when it builds pressure like it should. what could be the problem? i also dont have a belt on the front of the truck yet, still finishing her.. but it runs, but it only runs if the fuel pump is constantly running. PLEASE HELP!

 

The pumps do not shut off with the engine cranking or running.
The fuel pressure has nothing to do with the pumps running or not running.

 

i thought that when you turn the key on you hear the pump pumping fuel, then itll stop, then you start the truck and it pumps again or runs off some gravity feed **** or something. So is it okay if i just connected those wires? it would mean that the pump always pumps when the key is on and running too..

 

without the pump doing what it is supposed to do (pump), then how do you expect the truck to run? that initial noise you hear is the pump coming on and ensuring that there is fuel pressure, and it continues to run after that. if you dont beleive me when its running disconnect the fuel pump harness and see how long it runs. not too long....

 

No... it does not. The pump primes for 2-3 seconds when the key is turned to "Run", but then shuts off until after the key is turned back to "run" from the "start" position. If the engine fires up, the pump will then continue to run, but if it does not, you will hear another prime, and it will again shut off.

As far as your problem, sounds like something in the prime circuit isnt hooked up proplerly. Ford changed alot when they went to the 87-91 body styles when it comes to wiring, your gonna need to get the correct schematics and trace some things out.

 

alright thanks man, you were more help than the two guys sounding like smartasses. i know the pump has to pump, but it DOES shut off when it builds pressure, and i know it runs after the truck starts... but as i was saying if i put power to the pump it doesnt shut off after a few seconds like it should.