I'm trying to iron out a problem. This F350 dual tank has a fuel pump mounted in the frame rail under the driver. When I bought it, I had to replace it to drive it home. One the way home it kept stalling and I had to stop and jiggle the wiring under there. It might be that it's just overheating and stopping until it cools, but it happened once when I hit a bump. I'm pretty sure it's wiring, but have a question.

When you turn the key on, the pump comes on and never stops. Is this the normal mode, or is it supposed to cycle on and off?

 

Someone correct me if I'm wrong.

The '87-'89 trucks all have three fuel pumps. One on the frame (high pressure) and one in each tank (low pressure). The in tank pumps provide enough pressure to open the fuel tank selector valve and the high pressure pump pulls the fuel the rest of the way providing the fuel pressure required at the fuel rail. IMHO, the high pressure pump would have to run constantly to provide constant pressure as needed. I believe it's normal.

Tim

 

The high pressure pump and in-tank pump selected by the tank selector switch on the dash run constantly with the engine running. The fuel pumps should turn on for about two seconds when the key is first turned to run, but will turn off if engine rotation is not detected within those two seconds. I would trace the fuel pump wiring and check the relay to make sure there are no problems in those areas. The fuel pump relay is activated by the engine computer, so either the control wire is shorted to ground, the relay is stuck in the on position, or the computer has failed the fuel pump control circuit and defaulted 'on.'

 

I 2nd that

 

Thanks all. I'll look it over to see if it's shorted anywhere. Might have to take it to someone. I'm very short of time now. The frame pump stays on all the time that the key is on, unless it's not running at all when I need it to run.