TTurner

So I will start with saying i don't know if there is anything left for me to meter/diagnose etc on this truck. Ive searched to see if someone else has had similar problems and have not achieved much luck. So with nothing else here I go....

Symptoms:

1988 Ford F-150 5.0 w/ C6 Tranny 2wd (EGR and SMOG Delete, problem was present before the delete.)

After sitting over night truck starts completely fine, after a couple minutes you can watch fuel pressure bleed down to nothing than the truck won't start.

Yes the fuel pump runs when you turn the key on, no the low pressure pumps do not seem to be working. However both low pressure pumps are not working and they were both replaced about 2 years ago with very little mileage put on them. I find it hard to believe they would both go out at the same time.

The relays are good to both the EEC and the fuel pumps.

New:
Fuel Pumps (minus the older in tank pumps)
Fuel Filter
Pressure Regulator
Relays
Fuel Selector Valve
Injectors

I am at a loss..... it is 100% a fuel pressure issue ignition is completely fine with adequate fuel pressure but I can not seem to figure out what the fuel pressure issue is.

Is it possible that i could need new in tank pumps again already??? I did not use Motorcraft Pumps, they are Airtex brand.

If you have any more questions to help narrow down I.E. what i have or haven't checked please let me know and i will let you know.

Any and all help would be very appreciated. Thanks for your time.

vjsimone

Sounds like you High Pressure pump is on it's way out after struggling so long sucking the fuel out of the tanks with the non working pumps.

Do your fuel gages work?

There is a common ground wire for the tank pumps, possibility the the ground wire for the tank pumps are no longer connected.

You have a fuel selector valve ?

TTurner

The high pressure pump is brand new and seems to run fine in the morning.


Yes the fuel gauges work, 2 years ago I replaced everything in the tanks the entire sending unit w/ new filter and pumps.


Where is the common ground wire??? That may be the problem I am looking for.


Yes I have a brand new Motorcraft OEM Fuel Selector Valve installed, the previous one was leaking from the reservoir.

vjsimone

There is a ground wire splice in the harness near the front tank sending unit, the two pump grounds are connected there. Also the gage ground is spliced there as well, since your gages are working, this may not be your issue, but worth checking out.

vjsimone

Have you measured for voltage at your Fuel Tank Selector Switch connector for the in-tank pumps?

You are positive the in-tank pumps are not running?
Have you checked for fuel flow to the Selector valve?

When you lose pressure, is your HP Pump still running?
You might want to measure/monitor voltages on the wire that powers the pumps to see if it is dropping out.

TTurner

Yes I have measured voltage at the switch and voltage seems good.


Yes, I cannot hear them running and also can not feel any fuel moving through the lines. There is no fuel reaching the selector valve.


When I loose fuel pressure the HP Pump will cycle like it does when you first turn the ignition on.


I have measured voltage... when you turn the key on you get 12V to the pump once the pump stops the initial cycle, voltage drops.


Hopefully that answers your question.

vjsimone

You disconnected the 2 lines to the selector valve an there is no fuel flow from either line from the tanks ??

vjsimone

So, disconnect the selector switch connector.
Monitor the voltage on the Red/White wire (pin2?).
Start the engine.
Do you lose the power to the wire, or lose the fuel pressure first ?

Eric Fullerton

Have you tried to hot wire the pump relay? That would verify everything downstream of the relay (or not) and point to a signal to the relay problem. I put a permanent one on a switch on my 87, just to fight vapor lock. Came in handy the other day, truck would start then stall a couple times, then drive normally. I flipped the switch with the engine off, and the high pressure pump started squealing like a pig caught under a gate. The replacement pump should arrive tomorrow.

TTurner

I will check the power on the selector switch tomorrow and get back with you.


To answer the other question yes I pulled the lines at the fuel selector valve that were coming from the tank and had no fuel flow.