Little background, I recently bought a 94 F150 project for my daughter. I knew from the ad that it has transmission issues but it looked pretty rust free, however when I got there they had neglected to say that id hadn't been started in several years. Well I'd already driven 250 miles so figured I it was worth the risk.

We got it home, hooked up a jump pack and was happy that it cranked over but wouldn't start. Sprayed some carb cleaner down the intake hose and it fired right up and ran. Then I put a fuel gauge on the rail and it had no pressure. Did some reading on troubleshooting and found that the switch was the likely issue. So I removed the switch and jumped the from the battery to the pump wires on the switch connector. I found that the front pump would not run at all, however the back pump would work and built 40lbs pressure. So with jump wiring the back pump and the engine fired up and idled, and ran well. I also went through the process of jumping pin 6 of the diagnostic plug and checked voltage and I am getting 12 volts. Removing the jumper from the diagnostic plug I would get would only get 12 volts at the switch for approx 1 second but after some reading I found that was normal. The other problem I noticed is after removing the jumper to the pump, I would lose fuel pressure pretty quickly. It would drop down to 20 within 30 seconds and then it would lose at a slower rate until it got back to 0. So after replacing the switch I found that it would not build enough pressure to start when just priming for 1 second.

I also tried to troubleshoot the fuel pressure loss by putting a small straight vice grip on the return line after stopping the pump but it still lost pressure. Then I tried on the supply line but it still lost pressure. I also checked the regulator and it was not leaking out of the vacuum port. However I'm not sure I was applying enough pressure to the hoses to keep it from flowing back but I was afraid to clamp the semi hard stock lines any tighter for fear of damaging them.

Tonight I just wanted to get it running so I could drive up and down the driveway to see what the transmission issues were. So I started by jumping the pump to get the pressure up to 40 lbs, then I removed the jump wire and plugged in a new switch hoping that it would keep the pressure and run. However after switching to the selector switch it would die within a few seconds. Tried this a couple of times almost convinced the new switch was bad too. Finally I moved the gauge up where i could see it from in the cab and found that after switching from the jumper to the switch I could see the pressure drop steadily and the engine would die. Now I'm trying to understand why I'm losing pressure while trying to run off of the switch but it keeps pressure while running off of the jumper.

Maybe I need to try with the switch plugged in and jump the #6 pin on the diag port.

 

Sounds to me like you have 2 bad fuel pumps. The front is dead altogether, and the rear runs but the check valve is bad.

 

Well it's likely that the back pump has a bad check valve, it could also the front pump with a bad check valve allowing fuel to return to the front tank. I haven't figured out a way to tell if it's front or back without possibly damaging the hose. I've watched video where they block of the return hose but in those videos they were clamping rubber hose and not this stiffer stuff we have.

I also don't understand why the back pump works off of a jumper wire but not the switch. Maybe it's not really getting 12V at the switch when it's running? I'm thinking I need to jump it to build pressure, then pull the jumper and then put a small jumper in place of the switch to jump the input pin to the pump feed pin and measure voltage when the truck is running. I can't think of any reason the switch wouldn't be getting 12v when the engine is running, I thought that the 12v at prime proved the ECU was good.. maybe not?

 

Right
Replace both pumps and the fuel filter

 

Wouldn't you try and diagnose the electrical issue of why the back pump won't run off of the switch before just replacing the back pump?

Seems like all I know now is one check valve is bad - either front or back. Back pump will pump fuel when running jumper from battery to switch, but back pump wont run or runs too slow to keep up when running from switch.

 

You say you are jumping from the battery to the switch. If you were trying to isolate the switch, you should be jumping over the switch. Are you sure you have power at the switch?

 

These are all common problems with these trucks when they sit for years without being ran.
Fuel pressure is maintained by the check valve in the Fuel Delivery Module (FDM) and the fuel pressure regulator. It doesn't matter if the fwd shuttle valve is stuck open regarding engine performance.
Your dropping fuel pressure is probably due to a leaking injector spraying fuel into your intake manifold.
I'd suggest draining your tanks of the old fuel, replacing both FDM, replace your fuel filter, and replace your fuel injectors.
Odds are your rear FDM is toast also and youll have fuel starvation problems when you try to accelerate.
Spectre, Carter, and Delphi seem to be the preferred brands. Make sure you check your fuel pump operation before you install them in the tank. A lot of member have gotten DOA fuel pumps from AutoZone and O'Reilly's.
Don't clamp your nylon fuel lines... They're 30 years old and they don't like it.

I got confused in your post which jumper wire you're talking about... The diagnostic port jumper wire or the fuel pump jumper wire.
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So did he ground to the fuel pump connector in bottom or picture above. When doing this I had 12V at cutoff switch and at fuel pump connector. When starting using back pump I was running wire directly from battery (inline fuse) to bottom left inlet on switch connector. When doing this it builds 40lbs of pressure and engine starts and idles. Then I was quickly pulling the jumper and plugging in the switch but it would lose pressure and die.

So either I don't have 12V coming to switch or ??

I think I'm going to try running ground to the fuel pump connector in the diag. connector and using the switch and see if it keeps pressure that way. That way I know wiring is good and switch is good. Also I don't think it's all going through fuel injectors, oil has stayed at the same level and doesn't have a fuel smell, but I do realize that could be an issue.

It makes sense to replace all those things in the long run but I'm just wanting to get it running long enough to assess the transmission issue and get an idea of what else might need fixed and make a plan from there. By assessing the transmission I mean just drive it around the section and see if it drives and shifts. All I know from previous owner was there were "transmission issues", however I found that speedometer and odometer were messed up, shift indicator in cluster had been taken out. In reading shift control is going through PSOM, so I just replaced cluster with a used cluster and odometer is working, I'm assuming speedo will work as well but unknown at this point. So at this point I don't know if transmission issues were related to PSOM or mechanical, etc.

 

This is how your force the fuel pump relay on using the diagnostic connector. With the diagnostic connector jumped like this, the fuel pump switch installed and in the 'rear' position, and the ignition in the 'on' position, is the rear fuel pump working all the time? You should be able to hear it.
If it's not working, look at the schematic I sent previously.
It could be corroded terminals on your inertia switch under the passenger side kick panel or a bad fuel pump relay under the hood.

 

Well this is embarrassing, I found the issue and the issue is me. I was doing just like your picture to check voltage to the switch. But before that to test the pump I was running a fused wire from the battery to the ouput wire on the switch connector.

switch connector
However, I went back out last night and did the jumper like your picture above with the switch in and the pump didn't prime. So I grabbed the old unplugged the new switch and plugged in the old switch and I heard the pump start. Then I noticed that the old switch was set to the front pump and not the back. That's when I realized when I was testing without the switch the loose connector was upside down and I was putting power to the number 6 pin but it was down in the bottom left (1 pin position). I had previously looked at the diagram online before going out to test and remembered the bottom pin was the back pump, and I was judging pump wire vs fuel gauge wire based on wire size so assumed bottom big wire was back pump having not noticed the connector was rotated.

Anyway with jumper in place and front tank selected I am getting pressure. Interestingly to me I was getting 40psi off the jumper to battery and only 30 now.

 

Great thread and just what i was looking for. One question, how does the prime work when the key is first turned on? Sometimes my pumps wont prime unless i toggle the tank selector switch a few times.

 

You should have 40 psi with the engine off, and 30 psi with the engine running because the fuel pressure regulator reduces the fuel pressure at idle.
If you only have 30 psi with the engine off, your rear fuel pump is failing or your fuel pressure regulator is stuck open.

 

The ECU has a timer in it that will run the fuel pump for 1-2 seconds when the key it put in the 'on' position. After, the 1-2 seconds, the timer turns off and the fuel pump won't run until the ECU sees the engine spinning.
I don't know why flipping the fuel tank switch will cause the pump to start running. The ECU doesn't know what position the switch is in.

 

Thank you. I’ll look at it this week and find out what the issue is. I have another EEC that i could try.