Cars Dies And Fuel Pump Wont Prime


I want to measure mine. It doesn't look that tall.
10 Year Member
Jul 6, 2006
Recently my car has decided that after 25-30 minutes it will shut the fuel pump off. When this happens it won't prime when i turn the key or if it does it will immediately die again. I have already replaced the fuel filter and relay with no luck. Before I replace the pump which is only a year old is there anything else I need to check?
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StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
Dublin GA
Fuel Pump Troubleshooting for 87-90 Mustangs

Revised 15-Sep-2011 to update check for relay, socket & wiring condition

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 upper LH corner to


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 power relay – located under the driver’s seat in most Mustangs built before 92. It is located under the MAF on 92 and 93 cars. Be careful not to confuse it with the A/C WOT cutoff relay which is in the same area. See the diagram to help identify the fuel pump relay wiring colors. Be sure to closely check the condition of the relay, wiring & socket for corrosion and damage.
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.

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

The control path consists of the inertia switch, 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 inertia switch
(red/black wire) then from the inertia switch to the relay coil and then from the
relay coil to the computer (tan/ Lt green 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 (orange/ light blue wire) goes to the fuel pump
relay contacts. When the contacts close because the relay energizes, the power
flows through the contacts to the fuel pump (light pink/black wire). Notice that
pin 19 on the computer is the monitor to make sure the pump has power.
The fuel pump has a black wire that supplies the ground to complete the circuit.

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.

Now that you have the theory of how it works, it’s time to go digging.

All voltage reading are made with one voltmeter lead connected to the metal
car body unless otherwise specified

Check for 12 volts at the red wire on the inertia switch. No 12 volts at the inertia switch,
the ignition switch is turned off or faulty or there is no power to the ECC (computer )
power relay. To be sure look for good 12 volts on the red wire on any fuel injector:
good 12 volts mean the ECC relay is working. No 12 volts and the ECC wiring is at
Look for 12 volts on the red/green wire on the ignition coil: no 12 volts and the
ignition switch is faulty, or the fuse link in the ignition power wire has blown. No 12 volts
here and the ECC relay won’t close and provide power to the inertia switch.
Check the Red/black wire on the inertia switch, 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 Red wire and Red/Black wire. Power on the
Red wire and not on the Red/Black wire means the inertia switch is open. Push the button
on the side of it to reset it, and then recheck. Good 12 volts on one side and not on the other
means the inertia switch has failed

Look for 12 volts at the Orange/Lt. Blue wire (power source for fuel pump relay).
No voltage or low voltage, bad fuse link, bad wiring, bad ignition switch or ignition
switch wiring or connections. There is a mystery connector somewhere under the
driver’s side kick panel, between the fuel pump relay and the fuse link.

Turn on the key and jumper the fuel pump test connector to ground as previously
described. Look for 12 volts at the Light Pink/Black 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.

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.

The yellow wire is the fuel tank sender to the fuel quantity gage. The two black wires are grounds.
One ground is for the fuel tank sender and the other is the fuel pump. The ground for the fuel pump
may be larger gauge wire that the fuel tank sender ground wire.

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.

You should see less than 1 Ohm between the black wire(s) and ground. To get some idea of what
a good reading is, short the two meter leads together and observe the reading. It should only be
slightly higher when you measure the black wire to ground resistance.

The Tan/Lt Green wire provides a ground path for the relay power. With the test
connector jumpered to ground, there should be less than .75 volts. Use a test
lamp with one side connected to battery power and the other side to the
Tan/Lt Green wire. 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.
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.

Fuel pump runs continuously: The fuel pump relay contacts are stuck together or the Tan/Lt Green 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 Tan/Lt Green 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

If all of the checks have worked OK to this point, then the computer is bad. The
computers are very reliable and not prone to failure unless there has been
significant electrical trauma to the car. Things like lightning strikes and putting
the battery in backwards or connecting jumper cables backwards are about the
only thing that kills the computer.

See the following website for some help from Tmoss (diagram designer) &
Stang&2Birds (website host)


I want to measure mine. It doesn't look that tall.
10 Year Member
Jul 6, 2006
The car has been on the back burner for a while but i finally got around to working on it. Fuel pump was bad. It would prime but i was getting insufficient psi at the rail. A buddy donated a fuel pump he had decided not to use. Car fired up and ran great. On a trip to the store it cut of again. Fired it back up and drive it home. Following the above list again the computer ground was indeed loose. I had Moved it earlier in an attempt to clean the wiring up and it just clearly was not in a good location.moved it back to its stock location and i must say idle seems to have improved and it has cleared up the ground issue i was having with my dash lights.


I want to measure mine. It doesn't look that tall.
10 Year Member
Jul 6, 2006
Well it appears i was premature in celebrating. It happened again on the way to work.I'm almost positive it's ignition related as it backfires loudly when i start the car again. My gut tells me it's the tfi.


StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
Dublin GA
Backfiring out the intake is either a valve stuck open or a lean mixture or spark plug wire(s) connected to the wrong cylinder(s). Check compression on all cylinders and then look for vacuum hoses loose, cracked, or misconnected. Check the line for the vapor recirculation system – it is easy to knock loose and not see it when you connect the air pump plumbing. If the vacuum line for the EGR valve and the air pump are cross connected, some very strange things can happen. Check the mass air flow electrical connection and see that it is tight, the same goes for the fuel injection wiring harness connectors up on top of the manifold near the firewall.

Sticking valves: If a intake valve is bent, has a bad spring or is misadjusted, the engine will sometimes backfire through the intake. Use a vacuum gauge connected to any convenient spot on the intake manifold. Run the engine at 1000 RPM & look for 18-21 inches of vacuum with a steady needle. A problem intake valve will make the vacuum gauge needle sweep 5-10 inches.

Lean fuel mixture breaks out into several sub categories:
A.). Vacuum leaks
B.) Air entering the intake without passing through the MAF on Mass Air cars (89-95 models).
C.) Failure of the MAF, BAP/MAP (Baro or Manifold Air Pressure, same sensor, different name), ACT (air charge temp), or ECT (engine coolant temp). These should set a code in the computer.
D.) O2 sensor problems: one or both O2 sensors with low output or bad O2 sensor heater ground. This should set codes 41/91. The O2 sensor heater ground is an Orange wire in the engine mounted fuel injector harness. Ground it to the back of the head or intake manifold.
E.) Leaking exhaust gases from EGR valve at WOT or EGR opening when it should not be open.
F.) Poor fuel delivery due to bad fuel pump, clogged filter or bad fuel pump wiring. Look for low pressure or fluctuating pressure. Standard injector pressure is 39 PSI at idle, with the vacuum line disconnected from the regulator and capped.
G.) Clogged fuel injectors.- see the cylinder balance test below
H.) Fuel injector wiring problems causing injector not to deliver rated flow (dirty or stuck shut injectors).
I.) Computer problems: (computer problems are not common like sensor problems)
J.). ROM has bad data in fuel or timing table. This should also set a code in the computer.
K.) Failure of one or more of the computer's driver transistors for the fuel injectors. No code set on this one. Use a noid test light to test the injector wiring & injector drivers,
L.) MAF calibration off or mismatched to injectors.
M.) ACT or ECT bad. Sometimes the sensors will be off calibration, but not bad enough to set a code. If they falsely read too high a temp, the engine will back off fuel delivery.

The HO firing order is 1-3-7-2-6-5-4-8.
Non HO firing order is 1-5-4-2-6-3-7-8

Cylinder balance test:
See the procedure below to dump the codes and place the computer into diagnostic mode.

Warm the car's engine up to normal operating temperature. Use a jumper wire or paper clip to put the computer into test mode. Start the engine and let it go through the normal diagnostic tests, then quickly press the throttle to the floor. The engine RPM should exceed 2500 RPM's for a brief second. The engine RPM's will increase to about 1450-1600 RPM and hold steady. The engine will shut off power to each injector, one at a time. When it has sequenced through all 8 injectors, it will flash 9 for everything OK, or the number of the failing cylinder such as 2 for cylinder #2. Quickly pressing the throttle again up to 2500 RPM’s will cause the test to re-run with smaller qualifying figures. Do it a third time, and if the same cylinder shows up, the cylinder is weak and isn’t putting out power like it should. See the Chilton’s Shop manual for the complete test procedure

Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on.

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 26-July-2011. Added need to make sure the clutch is pressed when dumping codes.

Codes may be present even if the check engine light hasn’t come on, so be sure to check for them.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

Post the codes you get and I will post 86-93 model 5.0 Mustang specific code definitions and fixes. I do not have a complete listing for 94-95 model 5.0 Mustangs at this time.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. On a manual transmission car, be sure to press the clutch to the floor.
Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.

If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.

The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and driveablity problems

Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, and clutch (if present) is pressed to the floor, and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Alternate methods:
For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

Or for a nicer scanner see Equus - Digital Ford Code Reader (3145) – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30.


I want to measure mine. It doesn't look that tall.
10 Year Member
Jul 6, 2006
Backfire was out the exhaust. almost like fuel was still being pumped after the spark cut off. I had a spare distributor so i swapped the tfi modules which only made another issue. Car won't run with the spout connected. Didn't have this problem with the last module. I looked for the code 18 but it want there. Runs strong and won't die with the spout out.


StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
Dublin GA
At this time, do you have a code 18 present?

Running with the SPOUT out locks out the computer from advancing the timing.


StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
Dublin GA
At this point, I would pull the TFI module off the distributor and take it to the Auto Parts store and have them test it for you.


Founding Member
Apr 21, 2002
Pensacola FL
Outside of the diagnostics, that should be done anyway... there are some clues.

Running for a while, then cutting out is often a bad TFI or coil.
When beginning to fail, these parts will run fine until they reach operating temp, then have issues.
The engine will usually not run again until the temp of the part has dropped. How much drop needed depends on how close to total failure the part is...


I want to measure mine. It doesn't look that tall.
10 Year Member
Jul 6, 2006
The answer is in the distributor i think. Taking both of then to the store and getting the modules tested. Not a fan of this as they will likely test fine while cold. The coil is less than a year old. Not ruling it out as bad but I'm more inclined to believe there is a bad module or pip.


Founding Member
Apr 21, 2002
Pensacola FL
Yeah, I forgot about the PIP.
We just changed my son's, and it was acting up as it got hotter.

But it didn't kill the car... Just ran badly at certain RPMs.
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