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1986 F250 460 Carb 4x4 auto on crank I get fuel pressure ( disconnected at carb and ran a line outside of engine bay to the ground) and good flow.
So when I crank and start the engine it will start and run for approximately 45 seconds then it dies out with lack of fuel. I can restart after several cranks and will run for another 45 seconds I can do this over and over again.
What i have done is i have messed with the fuel tank selector valve as it was stuck not allowing fuel from either tank. It is now completely open for the front tank with both the source and return ports for the front tank open to the fuel lines that go to the carb. I have check the in carb fuel filter amd cleaned and I can blow thru it with a little force. There are no other fuel filters on the truck whatsoever. The only fuel pumps are the in tank pumps for both front and rear. The rear tank is not operating at all and was not when I purchased the truck. The front tank was operating normally amd the truck drove great. I wanted to check and see if the truck would die out on the rear amd switched it to the rear. It died and then I switched it back to the front. And that's what started the entire issue.
Last edited by Rmccaig86; May 8, 2020 at 05:46 PM.
Reason: Updating for others
Your fuel pumps have a direct 12v connection when cranking, but afterward receive their voltage from a fuel pump relay and a oil pressure switch activates the relay. The diagram below shows the circuit.
Ok so multi part question
1. Will the fuel relay cause the pump to stop running after I let go of the ignition switch and the engine is running?
2. I am reading oil pressure on the dummy gauge on the dash, does that mean the oil pressure switch side of the fuel pump operation is working correctly or can I read oil pressure on the dash and that side of the switch cause the fuel pump to shut off when I let go of the ignition switch?
3. Can the fuel selector valve malfunctioning cause either of the pumps to not function when I let go of the ignition switch upon engine starting?
When I pulled the selector valve off and switched it back to the front tank I did so by jumping pins with a spare battery and getting the solenoid to switch back to the front side. Would this cause a malfunction in pump operation.
Ok so multi part question
1. Will the fuel relay cause the pump to stop running after I let go of the ignition switch and the engine is running?
Yes. Power to the pump system goes through the circuit to "Z". "Z" is shown in the other diagram. When the engine is cranking, power goes straight to "Z" from the PK/BK wire from fusible link T. Fusible link T is located on the starter solenoid.
When you let go of the ignition switch and leave it run, power from fusible link T goes dead. "Z" now get's it's power from fuel pump cut-off relay and the inertia switch. Speaking of the inertia switch that can be something you can check, it has a reset button on it. The picture below shows the location of the inertia switch.
2. I am reading oil pressure on the dummy gauge on the dash, does that mean the oil pressure switch side of the fuel pump operation is working correctly or can I read oil pressure on the dash and that side of the switch cause the fuel pump to shut off when I let go of the ignition switch?
.
The dash gauge operates on a oil pressure sending unit. The fuel pumps operate on a oil pressure switch. You have both plumbed into your engine block. Two different circuits, two different devices.
3. Can the fuel selector valve malfunctioning cause either of the pumps to not function when I let go of the ignition switch upon engine starting?
When I pulled the selector valve off and switched it back to the front tank I did so by jumping pins with a spare battery and getting the solenoid to switch back to the front side. Would this cause a malfunction in pump operation.
No. The fuel selector valve on the frame only has two jobs. To switch the liquid lines, and to switch the sending units for the dash gauge.
Here’s another look at the in tank fuel pump power supply. Dave is correct, the fuel pump gets power when the starter is engaged from one circuit, and then gets its power via another circuit when the engine is running.
The tank selector relay determines which fuel tank gets power for both the start and run circuits. So the fact you have power to the pump with the starter engaged suggests that part of the wiring is okay and the problem is in the run circuit.
Ok sorry for the delay on a response. So I purchased a new fuel relay ( the elusive green and black relay) and replaced it. Figured it was $14.00 bucks so even if it wasn't it it didn't matter to much. So ....it wasn't the relay. So next I'm going to dig into the run circuit. I know it's not the switch on the dash other wise it would not be showing me the front tank at all ( I assume). So here is something that go me thing thinking. Is it possible that the fuel selector valve is causing the fuel to backflow. I ask because when I pulled the valve off to get it reopened on the front tank. I opened both source and backflow portions of the valve. So is the source line supposed to be the only open valve and then when the run circuit is off the backflow opens and the source closes. It kinda makes sense in my head that may be it. Cause would it not just create a loop for the fuel. The reason I'm believing is has something to do with the valve is because the truck was running great before I tried to start messing with getting the rear tank functional. I flipped the switch to the rear and now here we are.
You said when you crank it, it runs. So that would not be a valve or plumbing issue. What is the difference between cranking and running? The oil pressure switch and relay section that feeds power to the pump. The pumps get power directly from the starter solenoid during cranking, but afterward get power from the oil pressure switch/relay circuit.
Ok so to clarify the fuel pressure switch has two wire coming from it (both red with yellow stripe) now the strip is different on each wire however they are still red with yellow. I trace the wires all the way to the fender so I could easily reach them and I did key out engine off continuity test and I received a tone for continuity. I all did a key on engine running test amd received the same result.
Here are my questions
1. Will there be continuity with the engine off? Or is the pressure switch bad?
2. If it was bad and in the closed position would the engine not run the way it should normally?
3. I'm sorta still thinking it's the switch on the dash, Thoughts...?
Also this is the pressure switch I tested. Switch is center photo
Ok so after some more testing I found that when they key is in the ON/RUN position the switch has no continuity and if i jump the two wires i can hear the relay operate.
Now I started the truck and jumped the wires together and it still sputted out.
I took the fuel selector switch out of the dash pulled it apart and verified that all the contacts were good.
So I know the Dash switch is working.
I assume the oil pressure switch is functional.
I know the relay is functional.
I assume the in-tank fuel pump is working other wise I wouldn't recieve the initial start.
Here is another question. If I place key in ON/RUN position and jump the oil pressure switch wires with the engine not running in theory the fuel pump should run with the engine off correct?
