Fuel pump issues, need help!

Discussion in 'Fox 5.0 Mustang Tech' started by RWR5.0, Nov 22, 2009.

  1. I have a 1992 Ford Mustang 5.0 five speed with intermittent starting issues. When I turn the car on I hear the fuel pump prime like normal, but then it won't shut off. other times it does not turn on at all. Once in a while the fuel pump will run for three seconds and shut off like normal and the car will start right up and run smooth. when I shut the car off and back on, the pump won't prime, and the car won't start.

    I have checked the inertia switch, I have hot spark, good fuel pressure, new fuel filter, and power to the injectors.

    I have tried to get the codes, but most of the time i can't get them to flash. I did get one reading while the car was running Codes:91,21,41,77, and 67.

    The fuel pump was getting 7v when i checked it, kind of a funny number i thought.

    I think it might be the engines computer, but this is because i can't think of anything else. anybody have any ideas?????????????

    Any way to check to check the computer???
  2. No way you can check it.

    Have you tried a new fuel pump relay, or ECM relay?
  3. Yeah, I checked the fuel pump relay, the eec relay, and the WOT relay. The car consistently starts when the pump primes correctly, but most of the time the pump just continues to run and the car won't start.

    My buddy helped me push a probe into the wire coming out of 22(light blue and orange) connection from the computer to the fuel relay. With the ignition on and the pump not running we grounded the probe to the body. This made the pump turn on anytime it was grounded for three tries in a row, then it just stayed on regardless of the weather it was grounded or not. I think that means a bad computer?

    I also messed with turning my distributor while the key was on and the car wasn't running. Turning the distributor made the fuel pump turn on and off a few times before it stayed on non stop. Not sure if that means anything either. We checked the wiring for resistance, and everything seemed good. When I wiggled wires with the pump running, it never seem to effect the system.
  4. Rotating the distributor and the pump priming is normal. When you do this, it sends a PIP signal to the ECM and this is what tells the ECM to fire injectors and run the pump. On startup the pump should prime, but wont start back until a PIP signal is recieved.

    This is good one. Ill have to admit. If you could possibly get the same symptom to show like you did before, where the pump wouldnt shut off. You could use a volt meter and probe a 12v wire and PIN 22 with the meter. If the meter registers 12v, then the ECM is holding the ground when its not supposed to. Then I would try and unhook the TFI connector and see if the pump stops. If it does, then its probably the PIP or TFI. If it doesnt, then its probably the ECM, a stuck Fuel Pump relay.

    You can also try and tap on the relay when the pump wont shut off. If it stops, then the relay is faulty. :nice: