1994 F150 no fuel!

Hey everyone, I need some help with my truck. It has an intermittent no start problem that I've narrowed down a bit to a fuel issue. 1994 f150 5.0 dual tanks, no codes, newer wires/cap/rotor/coil. In August the front tank would not start at work so I ran on the rear tank for about 2 months fine, then the rear stopped as well.

Banging on the rear tank actually got it to start again. Could have been coincidence (Assumed fuel pumps, but both?) Got it home and fuel pressure tested. Honestly the tests were all over the place low/not there. Did a couple random things like swapped back in the stock coil and found the pcv valve hose slipped off. Started, ran for 3-5 min and just shut down. I NOTICED AT THIS TIME THE PUMPS WOULD BOTH PRIME AND STAY RUNNING WHILE TRUCK WOULDN'T START (I think this is weird and points to a computer issue).

Decided to put in new rear pump yet all of a sudden things seemed back to normal, started ran for another 2-3 months until again it would crank and not start. Jumped it and ran for a minute before dying out. Unplugged battery and after reinstalling It ran and now it's home not starting.

I've been able to get spark and spraying some throttle body cleaner got the truck to run for a few seconds. I have zero fuel at the rail and cannot get the pumps to prime now. Fuel pump relay seems to click and I've begun testing the circuit at the pumps, inertia switch, and fuel tank selector. I believe I should have 12V all around with the key on right?

Thank you for reading this far I'd appreciate any help or advice!

I don't have the diagram for you at the moment but you maybe someone can post it for me (I'm o a cell phone) well need to take a look at the fuel selector valve pigtail and possibly the selector switch

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I have everything ready for another test, but i did test yesterday. I seem to be getting an initial voltage of 12 on the turn of the key. But immediately drops out. I should be seeing a steady 12 correct? Fuel pumps will not turn on at the moment, zero reaction on the fuel pressure gauge.

Here you go.

Thank you pole71. I need help understanding my results. Everywhere I probe I get an initial 12v jump with turn of key (brothers helping) then immediately the circuit will drop back down to lower readings (9.8V-2.6V i.e.). Is the initial 12 V supposed to hold. I've followed this from the pumps--selector switch--inertia switch--to power distribution box. I am still getting the initial 12V jump during key on/brother cranking but very quickly losing it to read 9.8V. Honestly that is the DKGRN/YLLW wire to pin 8 that says FPM.

This waveform is of a blower motor, not a fuel pump, but a motor is a motor. Anytime I get an intermittent blower/fuel pump problem, the first thing I do is look at a current waveform. This first wave is of the NEW blower motor. This is how a motor, be it a pump or a blower, should look.



This is a wave of the bad motor. When the wave looks like this, I replace the blower/pump, whether it is working at that moment, or not. When, not if, the motor stops in a bad spot, it cant get started. Banging on the tank jostles the pump, just enough, to get out of the "dead" spot.



Having Dual tanks decreases the probability of this being the problem.
You may, or may not, have access to a scope. I just thought I would throw this out there.

Jasonpsapp17 wrote: Is the initial 12 V supposed to hold.

Yes. If you are testing on the power supply side of the circuit, it should hold 12v. You have a voltage drop.

Thanks I appreciate the knowledge. What's happening now is making me think the PCM has a problem. I actually went ahead and pulled the computer/bought replacement capacitors. Mine look okay but I'm wondering if they could be bad. 194K miles on the vehicle.

www.ford-trucks.com/forums/1435937-1995-...troubleshooting.html

This thread sounds like me almost exactly. It's just at the moment the fuel pumps won't prime. In the past the constant running of pumps has happened.

Earlier the truck just randomly started and ran for 15min before i turned it off. Wouldn't start again. Only on throttle body cleaner it'll run again for a few seconds. Definitely not getting fuel somehow

This is a good video on voltage drops. The example is for a starter, the same applies to ANY electrical load.

Jasonpsapp17 wrote: In the past the constant running of pumps has happened.

Earlier the truck just randomly started and ran for 15min before i turned it off. Wouldn't start again. Only on throttle body cleaner it'll run again for a few seconds. Definitely not getting fuel somehow

:blink: I can't explain that, at this point. But, if your dropping voltage at the fuel pump, I don't think THAT problem is with the PCM. Power for the fuel pump comes from 20 amp FUSE O, which is in the engine compartment relay box. What voltage do you read at THAT point when energizing the fuel pump? If you get a strong, solid battery voltage, move down the line and check the wire going INTO the inertia switch (DK GRN/YEL), then the wire going OUT OF the inertia switch(PNK/BLK). Keep moving down the circuit, working your way towards the fuel pumps.

Jasonpsapp17 wrote: In the past the constant running of pumps has happened.

This makes me suspect the Fuel Pump Relay.

I have swapped the horn relay with the fuel pump relay and produced the same crank no start. I haven't found a steady 12V anywhere really, the GRN/YLW wire from PCM to the power distribution box I get that initial 12v and immediately will drop down to 8-9V. I'm wondering where else to check 'before' the PCM. I even checked inside the cab where the PCM is located, same voltage inside like the harness outside

If FUSE O is showing a voltage drop, then you need to work your way back closer to the battery..(my next stop would be before and after the fusible links)...All the way back to the Battery cable connection, if necessary. SOMEWHERE is a dividing point between voltage drop and no voltage drop.

Okay fuse O is dropping on engine crank. 12ish to about 10-11. Normal or is that typically going to happen. Which path am I following back to the battery? I don't see on the most recent diagram you sent.

Jasonpsapp17 wrote: Okay fuse O is dropping on engine crank. 12ish to about 10-11. Normal or is that typically going to happen.

It is normal for system voltage to drop a little while Cranking. I would say you do not have a voltage drop at Fuse O. What about WITHOUT cranking? Key on, fuel pump priming stage. Compare voltage at these points....(Fuse O, and terminals 87 and 30 of the relay)

Okay truck started yesterday and ran, was able to turn off and on a number of times.

There is a gurgling/buzzing near the fuel pressure regulator and vacuum lines that Is present engine running and upon shutdown. Trying to trace it but cannot pinpoint (sounds like it's from the fpr). Someone once said it was an evap leak.

I used some propane to find a vacuum leak but no change. Honestly here's what I did but I do not think I changed anything.

1. I moved some of the relays around (done before/no difference).
2. Added a little water to the battery (figured I've been cranking+ no start, but hey who knows).
3. Had checked voltage on the circuit at the RED/BLUE+ORNG/GRAN+yellw few minutes before start. Circuit actually held 9.47 this time (did not drop to basically 0V) and the fuel pumps came on but zero pressure readings

It seems at times the truck likes to start and then it won't. The electronics store guy said fuel pump relay when I said pumps constantly running in the past. I'm assuming the relays are good but could how can I check those terminals exactly? I'll be looking into it but I really appreciate your help and patience!