Fuel pump question

[JavaG66]

What are the chances of a fuel pump running constantly, but not pumping constantly? Fuel pressure drops out after truck has run about 10 minutes and dies.

 

[RonD]

You probably have 2 fuel pumps on an '86
Low pressure pump in the tank(from the old carb days)
And a High pressure pump in the frame rail under the drivers seat(added for fuel injection)

There is no fuel pressure sensor on Fords, the computer turns the pumps off and on based on RPM and engine load or speed(if VSS equipped).

The computer runs a Fuel Pump Relay to turn the pumps off and on, this is one of 3 relays located on the passenger fender.
The computer turns on the pumps for 2 seconds when key is first turned on, to prime the system, then pumps should stay off until engine starts and runs for a few seconds or longer.
If pumps are on all the time I would suspect a bad relay or wire at the relay.

There is a Fuel pressure regulator(FPR) on the fuel rail, these often fail when they get older, this would cause pressure to drop if it was stuck open.
The FPR is controlled by a vacuum line from the intake.
At idle vacuum is highest and fuel use is lowest so FPR is open.
When RPMs increase vacuum drops and FPR closes to allow fuel pressure to stay high.
If FPR was stuck open or partially open then idle would be fine but driving for a few minutes would cause fuel pressure to drop below where injectors could provide enough fuel to the engine.
Remove the vacuum line and check if there is any fuel in it, if there is or it smells like fuel replace FPR
Suck on the vacuum line(if no fuel present) see if you can tell if the FPR is opening and closing.

 

[JavaG66]

Thanks Ron,
I appreciate you posting all that information; it really helps troubleshooting.

By grounding the fuel pump relay at the STAR connector, I have been able to determine that one of my fuel pumps is bad, I'm just not sure which one.
With 'cool' conditions, I can build up a hefty pressure by grounding the relay.

With a 'warm' condition, the engine will eventually shut down. At this point grounding the fuel pump relay causes the pump on the rail to run, but pressure builds to only 10 psi. To me this sounds like the low pressure tank pump is providing the 10 psi, but I am confused because the rail pump is definitely running (but not pumping??). I can't tell for sure if the tank pump is running or not.

 

[RonD]

Pump in tank should be 7psi(approx.)
Frame rail pump should bring that up to 30psi-40psi

This is assuming Fuel filter isn't clogged up, that should be right in front of the frame rail pump, some were canister type that the cartridge could be changed, but some require a whole new unit, can get it apart to change cartridge.

Assuming FPR is working, with engine off, I would disconnect the frame rail pump's power(or often the ground is nearby and can be removed) and see what kind of pressure you get, also listen for the in tank pump.

There have been a few members that had clogged up screens in the older tanks, so both pumps were OK but they just could not get enough fuel to pump.

 

[JavaG66]

Short answer: In-tank pump is shot; high pressure pump may be wounded as well.

There is a connector right under the brake booster that feeds current from the fuel pump relay to both pumps. Ford molded both outfeed wires into one connector, so I made a lucky guess and cut the one that fed the low pressure pump. A spade & female were crimped back in place, and the wire was hooked back up. I installed a fuel gauge, cranked up the truck and figured that if I killed power to the rear pump the fuel pressure would drop like it does after having run 15 minutes and die. Wrong. No effect. Unplugged connector for both pumps and engine runs for at least 30-45 seconds before dying. I was amazed that it would run such a long time with residual pressure.
So I hooked up the low pressure pump, unplugged the high pressure pump and stuck a stethoscope to the fuel tank while the key was turned on for that 2 second prime. Silence. So judging by when I started having these problems, the truck has been running without a low pressure pump for a couple of months.
I still don't know what is happening in the high pressure pump to cause it to quit pumping after running 10-15 minutes; at that point the pump is running, but not pumping. So I may end up replacing it as well if having the low pressure pump back in service doesn't take enough of a load off to make it dependable again.

Again, thanks for helping me work through this; this one's been a challenge!

 

[RonD]

Does sound odd.
Are you sure the fuel filter is not the issue?

The electrical for the fuel system is like this:

Intake pump---------/----------inertia switch--------fuel pump relay-----EEC relay----battery
Frame rail pump----/

The inertia switch is in the passenger foot well it kick panel, it would cut power to fuel system in the event of a sudden stop(accident) or roll over.