friends. thanks in advance for the help


I replaced both fuel pumps in my 1991 F150 5.8 after experiencing a cross flow issue. the system work perfect up to that point.


the front pump is now inop. I can hot wire it and it makes noise, ill hook it all up and the truck wont run. the polarity is correct. its not making pressure. I changed pumps out thinking I had a defective pump...same deal. will run off of the rear tank all day.


front...nothing, ever if hot wired.


there is 6-7 volts at the plug and the gas gage is pegged to full when plugged in


..the stupid thing should run if hot wired...but it's not.


this is a western truck with no rust and all wiring seems good.....


help !!!

 

How are you hot wiring it?
6-7 volts sounds like zero volts to me.
Use a loading test light for the fuel circuit and not a VOM.
How do you know the polarity is correct? Could be running backwards.

 

 

What else have you checked? there is a lot more to the fuel system then just the pumps. Have you checked the tank selector switch? thats the most commonly bad part, the second is a faulty relay. have the relay tested or replace it.

 

ChRoNo16,
Both the the front and rear tanks use the same relay. He has ran a wire from the battery POS post to the to the pump at the tank and that would by pass the selector switch.

jspriver,
Most of the time when someone uses a meter and reports that they have 6-7 volts it means the voltage from the fuel pump relay is zero and the voltage will not light a loading test light. It also means they are using a DVM.
The 6-7 volts with a high impedance DVM is coming from the PCM FPM pin #8 and not the fuel pump relay. That is why we say to use a loading test light in the fuel system.

When the front tank pegs the fuel gauge you either have a full tank of fuel or the FDM has lost its ground. The front fuel pump and the front gauge sending unit use the same ground. This ground wire splices into the ground wire in the harness that is used for the rear tank also along with the tail lamps. It grounds to the body at the left (drivers side) radiator support or in some older years under the dash on the drivers side.
The fuel tank itself is not a ground point and may not be grounded and if grounded at all it would be grounded from the fuel hanger that holds the FDM.



/

 

thanks for the reply's


I fiddled with some things today..


with everything hooked up, the pump will prime but the truck wont run off the tank. is that the relay or tank switch that has died ?


joe

 

If they run for one second and then die each time the key is turned to the run position then the tank switch and the relay should be OK. The same circuit is used during the running of the engine.

 

if it runs for a second then doesnt check the inertia switch. I do believe it is either tripped or bad.

 

The truck runs great on the rear tank. The front primes but won't supply the fuel to run the truck. The gage is pegged past full too. It's a new pump. I have another new one that I can swap in but if I keep swapping I'll run out of wire to butt connect !

 

Do the pumps run OK when check them on the work bench?

 

Here's how I have it set up. It is all hooked up as if it were tucked up under the bed. Again, the truck runs fine on the rear tank. When I switch to the front tank I can hear the pump prime the fuel gauge pegs past full and the truck dies. If I run a straight 12 V to the pump it doesn't seem like it's putting out any pressure. So I am not sure if I have a defective pump... this is a brand new pump. The other one I replaced it with was also brand new. I'm going to beat this problem if it kills me

 

for those of you who are interested, I had not one but two defective fuel pumps. Third pump I put in solved my problem

thank you Again for all your replies and help

 

That crap is the worst, I cant tell you how many bad parts I've gotten in a row before, on multiple occasions from multiple brands on multiple vehicles. Anymore, it's worth it to foot the bill for OEM parts to not beat your head against the wall and have confidence that it will last a while. Glad you figured this out.