Got a 1991 F150 302 Ford Truck with dual fuel tanks.
When I turn the key to start the trunk the fuel pump for whichever tank I'm on (front or back) comes on and will not go off and I can't start the truck as long as the fuel pump is priming/running. I replaced the fuel pump relay module, the ecc module, and fuel filter. I also check the fuel emergency cutoff switch. There is only two fuel pumps on my 1991, one on each tank. The fuel lines meet at the fuel filter and from the fuel filter run to each of the fuel injector rods. When I checked the valve at the fuel injector rod, gas sprays out, so I know gas is being pumped to this point. Anyone good a solution to my problem? Before having this problem, fuel pump would come-on for a couple of seconds then cut off and truck would start. Now can't start the truck because pump continues to prime. Help Please!

 

The LT BLUE/ORG wire running from the Fuel Pump Relay, Test Conn Pin #6 to the EEC (PCM) pin #22 is shorted to ground or you have a bad EEC Module (PCM Computer).

 

Thanks! Can you please expand on what you are saying. I'm a little confused!
Thanks. agian...

 

Disconnect the fuel pump relay and turn the key to "RUN" position and see if the pump comes on. If it does, then you have a ground somewhere. On the fuel pump relay connector there should be a LtBlu/Ong wire. Take an Ohm meter, put 1 lead on that wire and the other to a ground. Set meter to read resistance or ohms. If you get continuity, then the circuit is grounded. Remove the 60 pin connector from the computer and check for continuity again. If you still get a ground it's in the harness, if not your computer is grounding the circuit and needs to be replaced.

 

The LT BLUE/ORG wire is the wire that the EEC provides a ground on to close the Fuel Pump Relay and that turns on the fuel pumps.

If this wire was shorted to ground the fuel pumps would come on with the key (EEC Power Relay closes).

Normally the EEC closes the fuel pump relay for two seconds to provide fuel for starting and then opens the relay.

The EEC will not close the fuel pump relay again until it sees information from the Ignition System that the engine is cranking or running.

If the fuel pumps come on with the key and stay on without cranking the LT BLUE/ORG wire is at ground potential and that means that there is a problem inside the EEC or the wire is going to ground.

The LT BLUE/ORG wire was a TAN/LT GRN wire in older systems before 1990.

The EEC (Electronic Engine Control) was the Computer used to run the engine up until 1993 when its function was changed to also operate the Transmission and then the name was changed to PCM (Powertrain Control Module).

Before Fuel Injection the Computer was called the ECA (Electronic Control Assembly) and just controlled the Pollution devices and maybe the Carburetor.

All the above is taking into account that you have changed the Fuel Pump and EEC relays and it is Factory wired.

 

Thanks ALL! I'll try it and see what happens.

 

Mine done the same thing and it was vacume hose off the preasure regultor.I have a 1991 also.

 

With the engine not running there is not any vacuum on the hose and it would not make any difference if the hose was on or off.

If you have a broken fuel line or bad pressure regulator that would not allow fuel pressure to build up the EEC will still turn off the Fuel Pump Relay after two seconds to prevent a fire.
If the EEC does not then it has a problem.

You do get higher fuel pressure with the vacuum line off than on with the engine running. But this is the same pressure you get with the engine not running and pin 6 of the test connector shorted to ground. The more vacuum you have the less fuel pressure you have.

 

Sorry But my fuel pump run all the time and the truck wouldn't start pluged the vacume line up pump stopped running and truck started right up. I just done this last Tuesday.

 

However, mines got the 4.9 and only the one tank. It only happens every once every three or so days, not every time I drive the truck.

Anyway, heres what I have done so far, when the truck gets cranky the fuel pump runs and wont allow me to start. I pulled the FP relay and checked it with 12V, the coil pulled in like it should, contacts cycled. Then I checked the terminals in the fuse block where the relay plugs in at and it too had 12V, but in order for the coil to release inside the relay the 12V has to go away. According to the book the EEC has a timer in it that after a second or two opens up the ground and drops out the 12V to the fuel relay, dropping out the coil and interrupting the fuel pump circuit, killing the pump. I checked the fuel pressure @ the fuel rail and had 50PSI, that was witht he pump running(And truck not able to start).

After about 10 minutes of tinkering the truck decides to allow me to start, and the 12V going to FP relay drops to @10V thereby allowing the coil in the relay to drop out and allowed the truck to start. It really sounds like an EEC problem to me, but I dont want to just strat throwing parts at it until I find out exactly which one it is. What do you guys think?

Also, I have checked the EEC relay,and it worked, even swapped the FP and EEC relay, did the same thing.

I have two questions, on the timer in the EEC, is it a strait timer or does it look at other things before it opens up and cuts power to FP relay?
Also, I know the fuel pump in my truck os old and the check valve doesnt work right(As pressure drops off to almost nothing when it stops running) But when it is running I read ~50PSI at the schrader valve. Could this make a difference?

 

I've got a 90 parts truck and to get it running to drive it to the boneyard I had to cut off the relay and short the fuel pump to get it started. As soon as you turn the key on the pump is running and it fires right up.

 

In the middle of normal operation (cruising down the road), my truck acts as if someone turned the key off for a second then comes back on. During this momentary shutdown the tachometer drops to zero. All other electrical functions (radio, dashlights, ect.) are uneffected. I also have the problem with the fuel pumps staying on after I turned the key off and exited the vehicle. What in the world is wrong with my wheels?

 

I had the same problem just recently. The Ecm controls yhe fuel delievery system, the injector pulse and the ignition timing. Just about everything. I had to replace my ecm. Ifound that Auto zone was pretty inexpensive. The use a company Called Cardone to rebuild the ecm. You must match the computer code to the new one for it to work properly. ECM -to-go is also good but a little more expensive.
You can get the truck to run if you cover the intake while cranking or use some starter fluid but you can only idle. Under acceleration it dies out. The fuel pump stayed on the entire time the ignition was turned on. I also found that by using a Code Scanner I was unable to retrieve any codes. Classic case of bad ECM. My check engine light was on the entire time.
Depending on what went bad in the ecm you maynot be able to get the truck to run.

CHECK THE ECM BEFORE THROWING ANYMORE PARTS AT THE TRUCK

 

You have most likely a wiring or ignition switch problem.

Sounds like your EEC computer is OK. The Computer will not momentary shutdown the tachometer or turn off the Ignition system unless the SPOUT wire is going to ground. To check this pull out the SPOUT shorting bar (jumper) and run it that way.

Move all of the Ignition wiring while checking the wires with an ohmmeter.
Look close at the plugs of all the wiring.
Check for power at the Ignition coil while shacking the wiring, plug on the Ignition switch and the key.

 

My '91 Bronco does this periodically and I have not found out what it is yet but I have a bad battery (dead cell) and it seems to be more prevalent when the battery is weak. If I charge up the battery real good then it doesn't do it. Could low voltage be causing the fuel pump relay to stick or the ECM to not know the pump came on? When I turn off the key the pump stops running - I think if the relay is stuck it doesn't matter if the key is on or off.