Mechanics

F150 FUEL PUMPS

1994 Ford F-150

Engine Mechanical problem
1994 Ford F150 6 cyl Two Wheel Drive Automatic 80,567 miles - EEC IV

I have a 1994 Ford F150 XL With a problem I see several people are having but no solution. This is my Fathers truck, owned since new and it sits in the garage quite a bit.

Truck will not start.
It has dual tanks.
With ignition on, engine not running, you can hear the fuel pump running continously. It normally shuts off shortly after turning the key on. (Builds pressure then stops until start or pressure drops)
I switch from front tank to rear, not running with ignition on, both pumps do the same thing, each run continously but vehicle will not start.
While looking under the hood with key on, engine off, the fuel pump did stop. I turn key to start and truck ran fine until I shut it off. Then same thing. Pump runs continously, either front or rear, will not start.

I have replaced fuel pressure regulator thinking it was always open. That did not fix the problem.

I replaced fuel filter. No go.

I have disconneted powertrain Control Relay, key on, pumps stop running, won't start.

I have disconnected Fuel Pump Relay, And PCR, Replaced, No start. (Saw this on a post)

Lastly, CEL is on, jump diagnostic plug, and CEL stays on, does not flash code.

I have been able to get started working the throttle, no power from engine, undrivable.

I have reviewed more postings and found others with the same problem. One suggestion was to change both pumps. &Quot; one pump relies on other to work properly". As I understand it, I have two tanks, each with thier own pump. Pressure lines join at fuel line " manifold" inside left frame rail, then into a single fuel filter. I assume a check valve(s) inside the fuel pump keeps fuel from going through when the other pump is on, same for the return fuel lines. Can you explain how one of these fuel pumps (check valves.)Fails that niether pump will build pressure?

I'm stumped, please help.

William
Avatar
Will12463
September 18, 2010.



Hi will12463,

Thank you for the donation.

Fuel pump has in-built one way valve that prevents fuel returning to the fuel tank from the pump that is not working.

Check the fuel pressure with ignition switch turned on and off several times to ensure it is withn specs.

The CEL not blinking any codes indicates either it is faulty or has wiring faults, more likely the former.

When ignition is turned on, the fuel pump would prime for 2 to 3 seconds and turn off, not because pressure has built but rather it is timed to cut off by the PCM. The fuel pump is not designed to cut off when power supply to it is available as the fuel pressure regulator would control the pressure and allow excessive pressure to return to tank. For in-tank fuelpressure regultaor it would flow out of the pressure relieve hole into tank, for those equipeed with return hose from the pressure regulator, the return hose would allow that.

As the fuel pump is not cutting off, it indicates the PCM is not performing correctly. When this occurs you are not able to start, confirming the PCM is suspect.

You are most likely looking at a PCM that is intermittently failing.

KHLow2008
Sep 18, 2010.
Great responce and quick too!
So, the pumps are controled by the PCM and not some type of pressure switch. Truly that should have been obvious.

YOU: " The CEL not blinking any codes indicates either it* is faulty or has wiring faults, more likely the former.&Quot; ME: Is it* the PCM?

So the fact that the pumps are running when they shouldn't be, is a PCM issue, possibly why CEL just stays on during diognostics, (no voltage pulse from meter pin). It is all making so much sense!

There are problems with the 1990-1993 F150s. Fuel being transfered from on tank to another, fuel pump regulator failing closed, breaking check valves, causing fuel to be transfered into the tank not being used, overflow, creating safety and enviroment issues, bla, bla, bla.

THIS IS NOT THE SAME ISSUE! I've read folks replacing relays, would fix it for a day, a month. Replacing both fuel pumps, do it again in a couple months.

INTERMITTENT PCM FAILURE! That's it! Because the pumps are running when they shouldn't be! PCM is the only thing that will turn the pump on! CEL is on but can't get diognostics to work.

You the Man!

Thanks, I'll let you know if we're right, but how could we be wrong! (Famous last words)

Tiny
Will12463
Sep 18, 2010.
" YOU: " The CEL not blinking any codes indicates either it* is faulty or has wiring faults, more likely the former.&Quot; ME: Is it* the PCM? &Quot; Yes, I meant the PCM.

" Thanks, I'll let you know if we're right, but how could we be wrong! (Famous last words), " That is the kind of attitude that makes the world go round in a cheery mood.

Remove the PCM cover and check for signs of stains on the printed circuit board that are most likely caused by leaking capacitors. They are the usual cause of intermittent PCM failure.

KHLow2008
Sep 19, 2010.
So there is a conclusion to my initial question and advise, I want to report the results. I did remove the PCM. Looked inside the PCM and yes, capacitors did show signs of leaking. I was not ready to take on fixing the board, especially with all the clear coating on entire board. I have seen folks do this to install new flash memory, speed improvements. I wanted to get this done! So I ordered a new reman from Autozone, $244 the cheapest I found with a warrenty.

Removing the PCM was a pain! Jack up the front left wheel, remove wheel, remove all body screws holding the plastic inner field, move inner fender to gain access to the PCM on the left firewall. Remove bolt in PCM to take off wiring harness, remove two nuts and rubber seal. Pull out PCM.

Installed new PCM and all is good. If I have additional problems, I'll update, If this is the last you hear from me, than all was good.

Thanks CarPros,
VERY GOOD ADVISE! If I followed my internet research, I would have continued down the wrong path. Only spent $100 more than I should have with what I thought was the obvious solution

You folks are great, worth every bit of my donation!
William

Tiny
Will12463
Sep 29, 2010.
Good to know you have positive results.

Thank you for using 2carpros and have a great day.

KHLow2008
Sep 30, 2010.