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91' F150 351 / Fuel pump relay issue


Old 09-02-2015, 11:06 PM
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Default 91' F150 351 / Fuel pump relay issue

Hi all,

I'm in need of some help. I've just purchased a 1991 F150 with the 351 windsor. The seller has told me about an issue with the fuel pump relay getting hot and the fuel pump then cutting out.

Test driving the truck confirmed there is some issue, it would begin to cough and splutter and i could not accelerate. I would have to turn the truck off then on to reset the issue and it would run for another 10mins before happening again. After turning the truck off and on the time between the truck playing up would reduce each time.

One time it would not restart and I could not hear the fuel pump priming the system (bled the fuel rail to confirm it wasn't at pressure). After "wigling" the relay a few times it started to work again.

There is a fare amount of dodgy wiring all over the place with this truck. I'm wondering if anyone has the wiring diagram of the fuel pump system and or knows what size/type of relay controls the fuel pump? (whats there at the moment looks aftermarket and damaged from what appears to be overheating).

fyi - I've purchased original shop manuals on ebay but wont see them for a couple weeks. I was hoping to get stuck into troubleshooting this weekend.

Thanks in advance for any help!!
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Old 09-05-2015, 04:02 PM
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It should be a usual black square relay in the power distribution box. Try swapping in the one from air conditioning.
If another relay is also getting hot, I would think it's been rewired with too much power going through.
A lot of them are set up to have 12 volts for the 2 second prime then 6.5 volts for constant run. I just remember reading that somewhere, but you could see how it might overheat a relay.
You can also turn the power distribution box over and remove the bottom to see if there are any wiring problems underneath there.
That's assuming your truck has a power distribution box - beside the air cleaner box ?
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