Bad Fuel Pump: Any suggestions before buying a new one?

Fuel Pump Troubleshooting for 91-93 Mustangs

Clue – listen for the fuel pump to prime when you first turn the ignition switch on.
It should run for 5-20 seconds and shut off. To trick the fuel pump into running,
find the ECC test connector and jump the connector in the lower RH corner to
ground.

If the fuse links are OK, you will have power to the pump. Check fuel pressure –
remove the cap from the Schrader valve behind the alternator and depress the
core. Fuel should squirt out, catch it in a rag. A tire pressure gauge can also be
used if you have one - look for 37-40 PSI. Beware of fire hazard when you do this.


No fuel pressure, possible failed items in order of their probability:
A.) Tripped inertia switch – press reset button on the inertia switch. The hatch
cars hide it under the plastic trim covering the driver's side taillight. Use the
voltmeter or test light to make sure you have power to both sides of the switch

B.) Fuel pump Relay:
On 91 cars, it is located under the driver seat.
On 92 and later model cars it is located below the Mass Air Flow meter.

C.) Clogged fuel filter
D.) Failed fuel pump
E.) Blown fuse link in wiring harness.
F.) Fuel pressure regulator failed. Remove vacuum line from regulator and inspect
for fuel escaping while pump is running.

Theory of operation:
Read this section through several times. If you understand the theory of operation,
this will be much easier to troubleshoot. Refer to the diagram below frequently.

Diagram of the fuel pump wiring for 91-93 cars.



The electrical circuit for the fuel pump has two paths, a control path and a power
path.

Remember that the computer does not source any power to actuators, relays
or injectors, but provides the ground necessary to complete the circuit. That
means one side of the circuit will always be hot, and the other side will go to
ground or below 1 volt as the computer switches on that circuit.

The control path consists of the computer, and the fuel pump relay coil. It turns
the fuel pump relay on or off under computer control. The switched power (red
wire) from the ECC relay goes to the relay coil and then from the relay coil to the
computer (light blue\orange wire). The computer provides the ground path to
complete the circuit. This ground causes the relay coil to energize and close the
contacts for the power path. Keep in mind that you can have voltage to all the
right places, but the computer must provide a ground. If there is no ground, the
relay will not close the power contacts.

The power path picks up from a fuse link near the starter relay. Fuse links are like
fuses, except they are pieces of wire and are made right into the wiring harness.
The feed wire from the fuse link (pink/black wire) goes to the fuel pump relay
contacts. When the contacts close because the relay energizes, the power flows
through the pink/black wire to the contacts and through the dark green\yellow
wire to the inertia switch. The other side of the inertia switch with the
brown\pink wire joins the pink/black wire that connects to the fuel pump. The fuel
pump has a black wire that supplies the ground to complete the circuit.


Power path:
Power feed: Look for 12 volts at the pink/black wire (power source for fuel pump relay).
No voltage or low voltage, bad fuse link, bad wiring, or connections. Remember that on 92
or later models the fuel pump relay is located under the Mass Air meter. Watch out for the
WOT A/C control relay on these cars, as it is located in the same place and can easily be
mistaken for the fuel pump relay.

Relay: Turn on the key and jumper the ECC test connector as previously described. Look
for 12 volts at the dark green\yellow wire (relay controlled power for the fuel pump). No
voltage there means that the relay has failed, or there is a broken wire in the relay control circuit.

Inertia switch:
The location for the inertia switch is under the plastic for the driver's side taillight.
There should be a round plastic pop out cover over it, remove it to access the switch button.
With the test connection jumpered and ignition switch in The Run position as described above, check
the brown/pink wire. It should have 12 volts. No 12 volts there, either the inertia switch is open
or has no power to it. Check both sides of the inertia switch: there should be power on the dark
green\yellow (inertia switch input) and brown/pink wire (inertia switch output). Power on the
dark green\yellow wire and not on the brown/pink wire means the inertia switch is open.
Press on the red plunger to reset it to the closed position. Sometimes the inertia switch will
be intermittent or will not pass full power. Be sure that there is 12 volts on both sides of the
switch with the pump running and that the voltage drop measured across the switch is less
than .75 volts.

Pump wiring: Anytime the ignition switch is in the Run position and the test
point is jumpered to ground, there should be at least 12 volts present on the
black/pink wire. With power off, check the pump ground: you should see less
than 1 ohm between the black wire and chassis ground.

Make sure that the power is off the circuit before making any resistance checks.
If the circuit is powered up, your resistance measurements will be inaccurate.



Control path:
Relay: The red wire for the fuel pump relay coil gets its power feed from the ECC relay.
No 12 volts here, and the ECC relay has failed or there is bad wiring or bad connections
coming from it. The ECC relay is located on top of the computer, which is under the passenger’s
side kick panel. It is not easy to get to, you must have small hands or pull the passenger side
dash speaker out to access it.

Relay: The light blue/orange wire provides a ground path for the relay power. With the test
connector jumpered according to the previous instructions, there should be less than .75 volts.
Use a test lamp with one side connected to battery power and the other side to the light blue/orange
wire on the fuel pump relay. The test light should glow brightly. No glow and you have a broken
wire or bad connection between the test connector and the relay. To test the wiring from the
computer, remove the passenger side kick panel and disconnect the computer connector.
It has a 10 MM bolt that holds it in place. Remove the test jumper from the ECC test connector.
With the test lamp connected to power, jumper pin 22 to ground and the test lamp should glow.
No glow and the wiring between the computer and the fuel pump relay is bad.

Computer: If you got this far and everything else checked out good, the computer is suspect.
Remove the test jumper from the ECC test connector located under the hood.
Probe computer pin 22 with a safety pin and ground it to chassis. Make sure the computer
and everything else is connected. Turn the ignition switch to the Run position and observe
the fuel pressure. The pump should run at full pressure.
If it doesn't, the wiring between pin 22 on the computer and the fuel pump relay is bad.
If it does run at full pressure, the computer may have failed.

Keep in mind that the computer only runs the fuel pump for about 2-3 seconds when you turn
the key to the Run position. This can sometimes fool you into thinking the computer has died.
Connect one lead of the test light to power and the other lead to computer pin 22 with a safety pin.
With the ignition switch Off, jumper the computer into self test mode like you are going to dump
the codes. Turn the ignition switch to the Run position. The light will flicker when the computer
does the self test routine. A flickering light is a good computer. No flickering light is a bad computer.
Remove the test jumper from the ECC test connector located under the hood.

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)
for help on 88-95 wiring Mustang FAQ - Engine Information

Fuel pump runs continuously:
The fuel pump relay contacts are stuck together or the light blue/orange wire
has shorted to ground. Remove the fuel pump relay from its socket. Then disconnect the computer and use
an ohmmeter to check out the resistance between the light blue/orange wire and ground. You should see
more than 10 K Ohms (10,000 ohms) or an infinite open circuit. Be sure that the test connector isn’t
jumpered to ground.
If the wiring checks out good, then the computer is the likely culprit.

Prior to replacing the computer, check the computer power ground. The computer has its own
dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to
it's proximity to the battery, it may become corroded by acid fumes from the battery. It is a
black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire. You'll find it up
next to the starter solenoid where the wire goes into the wiring harness

The above post pretty much covers it all. I was going to suggest the inertia switch or the FP relay. It would be a shame to go to all the trouble to replace the pump if that wasn't it. In general the FP on these cars gets louder and louder before they puff out. So the noise is usually the tip off if it's going bad.

The ford pumps are made by walbro, as are almost all the performance pumps.
Just get yourself a walbro 155lph pump, you save money by going unbranded.

If you plan on a supercharger get a gss340 255lph, but don't waste your time with it if you don't need it.

Personally I would get the 255lph hp pump since we have return type systems so no fuel is ever wasted going smaller can only hurt i the future.

Wow. Seems unlikely the filter could be 100% blocked to 0 lbs. of pressure, and I've never heard of a FP regulator that would fail in that way either. Don't take this wrong, but is it possible you are out of gas, like maybe someone siphoned you dry with gas prices like they are ? Is the car on an extreme incline ? If the pump is turning you should be getting fuel it seems. I would be tempted to take the pressure gauge off and just see if it would finally pump some fuel out of the fuel rail into a can or something. Make sure there are no sparks near the can obviously.

Here are some useful tips...

I have done the tank removal three times, and the main issues are getting the car up on jack stands and getting the gas out of the tank. DO NOT try to do this job without jack stands. Becoming a pancake is not part of the repair process.

Some 5 gallon paint pails lined with garbage bags are good to hold the gas. The garbage bags provide a clean liner for the pails and keep the loose trash out of the gas so you can reuse it. A piece of 1/2" garden hose stuck down the filler neck will siphon all but a gallon or so of the gas.

Remove the filler neck bolts and put them in a zip bag. Disconnect the supply & return lines by removing the plastic clips from the metal tubing. If you damage the clips, you can get new ones form the auto part store for just a few dollars. I have used tie-wraps, but that is not the best choice. Then you remove the two 9/16" nuts that hold the T bolts to the straps. Put the nuts in the zip bag with the filler bolts. Pull the plastic shield down and away from the tank. Once the tank drops a little bit you can disconnect the wiring for the pump & fuel quantity sender.

The pump assembly comes out by removing a large metal ring that unscrews from the tank. You are supposed to use a brass punch to tap on the ring so that you don't make sparks. Look closely at the rubber O ring gasket when you remove the sender. The pump assembly requires some twisting and turning to get it out the hole.

Look very closely at the electrical wiring. The stock fuel pump wiring can overheat and melt the insulation. Mine had some really crummy plastic tubing slid over the quick disconnects. If the wires ever got together, there would be sparks inside the fuel tank and no more Mustang. I eliminated the splice in the middle of the wiring and went straight from the pump to the feed through connectors for the wiring. It required some soldering and crimping of new tabs on the wires, but it made a neater job.

Inspect the pump mount to metal tubing bracket. Mine broke and I couldn't get it to solder back together. I drilled a small hole for a machine screw & self locking nut to hold the clamp and bracket together.

The pump is easy to get off the mount but is somewhat difficult to get back it the tank without damaging the sock filter or tearing it on the tank baffle. When you install the metal ring that holds the pump in place, watch out for the gasket O ring. Some RTV may be helpful if the ring is not in excellent condition.

The tank to filler pipe seal is a large rubber grommet. Inspect it for hardening, tears and damage. At $20 from the Ford dealer, it might be a good idea to replace it.

I used a floor jack to help lift the tank back in place. You may find that it is the only time you really can make good use of a helper.

Too bad it doesn't sound like something simple, guess you will have to drop the tank and see what's going on. Could the tank have gotten contaminated with something ? 10,000 miles seems like an early death for a fuel pump, mine went 170K before I decided I was pressing my luck.

Jrichker has a lot of good tips there. I would also suggest trying to do this job in daylight out in the open, or at least use flashlights and/or florescent lights if you have to work in the dark. I hate getting soaked with gasoline knowing all it would take is one hot light bulb breaking and I'd go up like a Roman candle. It happened to a guy around here a few years ago, severly burned him and burned up his house. Keep a fire extinguisher handy.

Also plan ahead to get the doughnut for the filler neck. Otherwise you loose a lot of the liquid gold on the ground when you fill it up. 1982-97 Mustang Fuel Filler Neck Grommet by 50resto at LRS - Same Day Shipping!

Do as jricker suggests and just get the grommet, it's for sure shot.

Yikes indeed. I'm guessing the steel core was all that was preventing a leak ! Pretty scary to think about gas spewing out on a hot exhaust ! That might have been enough to cook (and vaporize) your gas and cause the bucking you spoke of earlier.

See Gas Tank Sealer Kit For Cars - Fuel Tank Sealer - Auto Fuel Tanks - Eastwood for a fuel tank cleaner and sealer kit. Cost is about $50.