Fuel pump won't stop pumping, please help.

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Nov 2, 2009 | 09:16 PM

low-blow00

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Fuel pump won't stop pumping, please help.

1991 F-350 crew cab 4x4 xlt lariat.
My fuel pumps won't turn off when I just turn the key to on.
I have replaced the fuel pump, sending unit, fuel pump relay, fuel pressure regulator, fuel filter. I have tried a few tests. I plugged of the end of the pressure line after the fuel filter to make absolute pressure against the pumps. They didn't kick off. Now I don't know where they are supposed to get told that the fuel system is pressured up. But there doesn't seem to be anywhere else. The truck won't start.
Please if you know of anything I am missing, please help.
Thank you Logan

Nov 2, 2009 | 09:25 PM

Conanski

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There are only 2 things that can cause this, a shorted fuel pump relay and a faulty PCM(Computer or EEC).

Nov 2, 2009 | 09:40 PM

SparkyShaft

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If you have an ohm meter, try an ring out the wires coming from your pump relay. By unplugging the plug for the pump at the tank an ring each one to ground an some hunting an it just might be your pickup in your dizzy. But first remove the relay, all it would take is a ground somewhere shorted like Conan said, That wire nicked anywhere an accidently touching bare metal it will cause it to stay on cause it works off an ground switch with your dizzy an computer in your distributor, if you notice if you loosen your dizzzy nut an turn dizzy while truck is on not running obviously, it will turn on the pumps. It is what runs the relay on a ground switch. Which when running would keep them running constantly.

Nov 3, 2009 | 07:24 AM

low-blow00

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Thanks guys

Thanks I think that I narrowed it down to the EEC. I traced back the wires to the pump from the relay. They seem to be alright.
So do you guys know if the EEC from a 1994 7.5L will work or am I restricted to just the 1991?
Thanks Logan

Nov 3, 2009 | 09:06 AM

Conanski

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Computer has to match the engine/trans combo, so if both those trucks have the same tranny it will work.

Nov 3, 2009 | 05:44 PM

brett12396f150

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I talked with a ford mechanic about switching computers like that, mines a 96 and it was a 97 truck. He said he wouldnt even go a year different. Just something to think about, i mean hes been there since the 70s lol.

Nov 3, 2009 | 05:56 PM

Conanski

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There are a lot of subtle differences between computers.. even between models from the same year but often these differences aren't enough to cause a noticable problem. If the OP's truck has an E4OD then he needs another computer for that exact same powertrain combo but any year will work. If the truck has a manual or C6 then a computer form another truck with either trans would work since these boxes are not computer controlled.