flareside_thunder

Don't just jump right on the "bad fuel pump bandwagon" test **** first. Someone could bumped into ya in the parkinglot and it woulda kicked over the inertia switch. like already said...it should be behind the passenger side kickpanel and looks like this:


Could also just be a bad fuel pump relay so that'd be worth checking into.

mr7confused

the manual actually says that little switch can be triggered by potholes and things, pretty sensitive so you never know.

fordtrucknut

no it has about a 3-5 sec run time with key on engine off.

and

fordtrucknut

no the pump has about a 3-5 sec run time when the key is ccled to the on engine off .

and as far as the safety switch, both of my trucks are farm/ off road trucks and i've never once set the safety switch off even when i was playing in the woods and rolled her on her side it still never activated.

ppcasm

Just thought I would add this to the discussion. I've recently had to do a bunch of work to one of these trucks and so far what I've learned about the fuel system (at least on the 1990 f150 I worked on with this same engine) is that the inertia switch, which "trips" in case of a jolt (usually an accident, but not always) is located under the dash, usually to the upper left of the brake pedal. If it's tripped it'll be in a popped out position, to reset it you press it in and it should click. Depending on your truck the fuel pump could be the inline kind, or the in tank kind, either way these pumps connect directly through a relay (usually on the driver side fender wall under the hood) and the ecu is able to switch this relay on and off. What is generally supposed to happen is that when you turn the key to the on position and not starting the pump comes on for 1-3 seconds to prime the engine, and then the ecu shuts the fuel pump off through the relay (I should probably note here that it appears on 90s and above that this relay is the first thing that comes on and it actually looks to provide the ecu with power, I was able to test the relay circuits on the fuel pump by looking at the check engine light with the key in the on position, if the check engine light comes on when the key is sitting in the on position and not started then that means the ecu is booted, and very likely means that the fuel pump relay is in fact working.) It's my understanding that at this point the ecu polls engine speed at which point if the engine speed goes over 400rpm (basically quite a bit above idle, hell, it might even kick while attempting starting) then the ecu flips the fuel pump to a continuous on state until the key is either turned off, or until the rpms drop below 400rpm (such as in an engine stall.) I believe the entire thing is to save fuel pump time in the chance that you just sit there with your key in the on position so that it doesn't cause any unneeded pump wear by pumping fuel needlessly (which ends up right back in the tank). Then of course the fuel goes through the fuel filter to the fuel rail. At the fuel rail what you generally have (at least on the one I worked on) is a fuel pressure regulator that sits on the fuel rail, the one I have is vacuum controlled, but basically it's a valve that controls fuel rail pressure based on engine load and a few other things, any "over" pressure and it bypasses and bleeds back into the gas tank. Just thought I would put this down here since maybe if anyone browses it then it might give them an idea of what to look for when they have any fuel problems with these vehicles.

ooseven

I have a 1992 f150 4x4 5 speed manual 300 straight 6. And I'm having problems with my fuel system.
I bought a new computer, new fuel pump, new ignition switch. And the only way it will start and stay running is to jump wire the tank switch and to keep the key turned on to starting position. The kill switch is good and not tripped. We have a book for it and it says the fuel relay switch is in the fuse box under the hood. But the one that the auto shop gave me does not go there. It is the same black square box relay but it has side walls that extends down as where the ones in the fuse box has none. We checked behind the air filter unit, is not there. Under the dash is not there. I heard it may be above the radio. Does any body have a clue to where it might be??

blount

Check your fuel filter on the driver side under the truck. I had the same problem