Fuel pump wont shut off??

Discussion in 'Fox 5.0 Mustang Tech' started by evan9r, Apr 11, 2008.


  1. evan9r

    evan9r New Member

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    Seems like most everyone has a problem with the fuel pump going out. Well mine will not shut off when the switch is turned on initially, the pump usually cycles on then off but not now for some reason. I thought the MSD box was bad but I sent it off and they said it was good. Also I dont have a spark except when the ignition is cut off. Is this computer related or what? I have an aftermarket fuel pump if that matters, and I also replaced the fuel pump relay.
  2. bill302

    bill302 New Member

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    check your fuel pump relay...prolly bad. I could help you more if i knew the year model..


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  3. velocidad

    velocidad New Member

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    Fuel pump

    I'm the original owner on a 92 Mustang Gt, I also have a problem with the fuel pump, I turned on the igntion, the pump will not cycle by the time the tow truck was on route I tried it again and pump cycled! Like it was said, could it just be my fuel pump relay??? Also Its the original fuel pump!

    Thanks!:flag:

    1992 Mustang Gt 219,xxx miles no engine mods
  4. bill302

    bill302 New Member

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    more likely its a bed fuel pump relay...
  5. enyawix

    enyawix Member

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    I will make a howto on how to test a 4 and 5 pin relay when I get home about 11:30 CST
  6. evan9r

    evan9r New Member

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    Its a 90 model. I have replaced the relay but no go.
  7. velocidad

    velocidad New Member

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    Fuel pump

    BILL302, thanks for the response and I will do that first. If its the fuel pump what do you all think I should install as for as brand, type, etc......

    Thanks! :flag:

    1992 Mustang Gt 219,xxx miles no engine mods
  8. HISSIN50

    HISSIN50 "How long does it take to get help in here? SN Certified Technician

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    When the pump is running-on, check the relay coil's ground wire (lt green IIRC on older aero foxes; not sure on yours). If it's shows continuity to ground, there's an EEC (ground strap) issue or a shorted wire (or jumped FP terminal at the STC).

    If the relay coil is not excited but the common and N.O. terminals are both hot, the relay is having issues. Replace it.
  9. evan9r

    evan9r New Member

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    Can you please elaborate on the first part? I think this might be my problem. Ive checked the wire and the relay and the relay seems to be good-its brand new. The wire is grounded which means maybe my computer is out? or distributor?

    Also my spark issue may be unrelated I dont know. But is the orange wire coming out of the 6AL supposed to be hot all the time? The one that goes to the positive side of the coil. Or only when turned on?
  10. HISSIN50

    HISSIN50 "How long does it take to get help in here? SN Certified Technician

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    With the key on and during the prime-out (or while the engine idles), you should have 3 hot wires and one ground. With the key on but after a prime out, you should have two hot wires.

    If your relay's negative coil wire shows constant continuity to ground, chances are the wire has a short (though I suppose the EEC could be the issue. Normally the ground strap in the EEC takes a dump though and the pump won't prime at all). To remove the EEC aspect, you'd have to disconnect the EEC electrical connector and ohm out the circuit. Do go over the self test connector because if that FP input was grounded (for some reason), that will permanently ground this wire.

    The wire to the coil should only be hot when the key is on (or while cranking). Confirm what yours is doing. How do you know you only had spark as you shut the key off?

    I will also note one commonality between spark and the fuel pump. The PIP. It tells the EEC to send spark (and injector pulsing), and when the PIP is active, the EEC keeps the fuel pump running.
  11. enyawix

    enyawix Member

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    :(

    No point in testing. I would put the pump on a 60amp switch until I can fix it correctly.
  12. OonDeanisS

    OonDeanisS Founding Member

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    i have an 86 with a 351w in it, and my fuel pump wont turn on. it has a MSD box and coil, but im not sure where the power should come from. THe coil?
  13. enyawix

    enyawix Member

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    my fuel pump is on a relay and the relay is on the ign switch
  14. evan9r

    evan9r New Member

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    Well I found part of the problem...my computer is fried..so Im looking for a A9L
  15. HISSIN50

    HISSIN50 "How long does it take to get help in here? SN Certified Technician

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    What part of the EEC fried? How was this found out?
    It's relatively tough to screw up an EEC.
  16. evan9r

    evan9r New Member

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    When the car quit running there was a strong stench like burnt electronics. I figured i t was the MSD box since the smell came from below my seat where my MSD box is. I started wondering about the ECM and just pulled it out and opened it up and the circuit board has several things fried on it. Not sure what happened but its burnt.
  17. HISSIN50

    HISSIN50 "How long does it take to get help in here? SN Certified Technician

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    Check over your wiring so you dont burn up another EEC. They don't generally spontaneously combust.
  18. evan9r

    evan9r New Member

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    I will do that. Ive only had the car a year so everything that is done to it was done by someone else. The MSD wiring was pretty bad, smaller wires to bigger wires, etc. I completely re did it so maybe that will help. The only thing that was different at the time of meltdown compared to all the other times was I had hooked up the MSD retard ignition box so it would retard ignition on startup. After this happened I took the box out completely.
  19. evan9r

    evan9r New Member

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    Well I got a A9S computer and plugged it in and the fuel pump still stayed on with just the key on. I do have spark and the car will run; very poorly, will not idle. I have check or replaced about every relay there is.

    The wire that got fried on the computer is pin # 57 which is a red wire called the power relay wire and it goes everywhere..to alot of different sensors and relays.

    I dont seem to have fuel pressure either...at least no fuel comes out when I press the check valve. Its a paxton regulator, could this be bad?
  20. evan9r

    evan9r New Member

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    Does the fuel pump build up pressure and then shut off? Or does it just cycle on then off no matter the pressure?

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