Hey everyone, lots of good info on the site so I'm hoping someone can shed a little light..

I've got a 96 F150 with a 4.9l and 5sp. Its been a great truck and I've got nearly 300k on it but.... I've had one recurring problem in the years I've owned it. It has been intermittent and a real pain to track down. Sometimes, it will lose a lot of it's power while driving, and at times, stall completely. It sounds like it's starving for fuel. I've checked the TPS and MAF and they check out OK. I've replace the fuel tank selector switch as well. One thing I noticed tonight (its been giving me trouble the last two days) while trying to track it down... when I turn the key to the on position, the fuel pump cycled on and then off like normal, then after a few seconds, it came back on and stayed on. I'd never noticed it before, so it was just right place, right time I think. It isn't normal for the fuel pump to do that, correct? And if it isn't, what does that indicate? I'm figuring the fuel is bleeding back into the tank somehow and that could cause my loss of power. Is it possible that the valve that reroutes fuel from front to back is malfunctioning and sending fuel either back to the tank, or to the other tank? I've chased my tail on this thing so long, I might be just grasping at straws.

 

Just a bump .. I have seen bad fuel pump relays stick and keep the pump running . They can also drop voltage , not supplying proper voltage to the pump . Just a thought !

 

Thanks for the reply.. I forgot to mention that I had already switched out relays as well. I've gone over this thing so many times with what I thought would be the "obvious" its hard to keep up with it all

I just found another thread with a similar problem and the PCM was suspected to be the culprit. Does that sound about right?

 

Classic symptom of a bad PCM (computer).

 

Thanks, I'll start looking in that direction then.. There is one other item that I'm not sure about though. Up on the passenger's side firewall, where the Haynes manual says the MAP sensor would be is a some sort of sensor but I can't find anything in the manual that looks like it, and my truck has a MAF sensor on it, so that would mean it wouldn't have a MAP sensor, correct? So if it isn't a MAP sensor, what is it? It has 3 vacuum lines coming off of it and a two wire connector.

 

MAF equipped trucks do not have a MAP sensor. The valve you are referring to is the Vapor Management Valve (VMV). It's part of the evaporative emission system. Prior to 1996 Ford used a simple Canister Purge (CANP) solenoid located near the throttle body then attached to the charcoal canister. The operation is basically the same; it's used to vent fuel vapors from the tank to the intake.

 

awesome, thanks for clearing that up!

 

It does not sound like you have a fuel problem.
More than likely it is a cap problem inside the PCM Computer as pointed out above.
If the fuel pump times out with the key on and then restarts over and over again then the computer is reinitializing. But I think you are saying that the pump comes back on and stays on. If so then it is not being reinitialized by a shorted wire some place.

 

Yes, that's correct. I have never noticed it doing it before, so maybe this is a new problem, or maybe I just happened to be in the right place to hear it. Either way, after about 10 seconds or so, while I had the key on doing the MAF checks it the front fuel pump came on and stayed on till I turned the key off. Turned the key off and tried again two more times and it did the same thing. Third time I turned the key on, it cycled like it should, and it ran fine in to work this morning.

 

will the problem occur at idle is it happening at the moment. Did you ever do a fuel pressure check.

 

Oh great...

The 95 and older half tons use an EEC-IV controller and constantly running fuel pumps are apparently a pretty good indicator that one or more electrolytic capacitors inside the controller have failed.

A 96 half ton uses the newer EEC-V controller, and I wasn't sure what the "classic" symptoms would be when those caps failed (all caps will eventually fail) so I'll be watching to see! If it turns out to be leaking caps, I'm just gonna crack mine open and replace them whether they need it or not. Don't like failures that can be avoided by simple maintenance!