I've got a crew cab '97 dually with the 7.3, and a 5 speed. I just got the truck a month or so ago (for $1,500- and it's loaded), and it's been flakey every day- I've had the filter assembly out of the truck 3 times to fix a fuel leak.

point is, this last time i let the truck sit for a few days before getting to the leak.. I had driven it with the leak for days, so yesterday i tear down the filter assembly like i had done twice before (and getting pretty good at it), then took it for a test run; seems the problem is fixed..

Then today I take it to get some breakfast, before heading out to do some work, and i realize the fuel gage reads 'full'.. since i know i almost ran the truck empty on both tanks, what's the deal? It reads full for both tanks, so I'm assuming it's an issue with the gage itself.. but why would it peg at full, even if the engine is off? i do have two tanks, so I'm sort of ruleing out an issue with one or both of the sending units.

Anyone ran into this before? it's a strange thing to just up and happen. it had been working flawlessly up until this morning.

 

Mine has been pretty weird also..bouncing from empty to full. You could trace the wiring harness down the length of the truck to make sure it's not grounding out along the way because of a wear spot?

 

no the problem is the sending unit in the tanks...

they wear out...

there is a little computer chip on the sending unit that the float arm touches to tell how much fuel is in there...

and after a while of rubbing it will rub the circuitry off the side of it and short in different places not knowing how much fuel u have...

 

my vote is for a bad fuel sender wire. remove the wire from the sending unit will give you a full reading. shorting the wire to ground will give you an empty reading. so it sounds like the fuel sender wire is broken or disconnected somewhere between the gauge and fuel tank selector valve.

 

thanks for the feedback

would a short in one tank affect both though? i switch from one tank to the next and the gage doesn't change- though if I drive for a while it will drop away from full. I burn up a quarter tank and it shows a loss of a quarter tank. This wouldn't be bad, if the tanks were full but they weren't. I'm certainly not dropping the tanks anytime soon; I just topped them off, and with winter setting in, it won't go far.

could it be the tank switch? either on the dash or the actual valve?

 

An open fuel level circuit will cause the gauge to read full, actually past full. There is no "computer chip" on the fuel sending units as mentioned before it's a simple analog potentiometer. If both tanks are exhibiting the same problem I suspect a problem in the fuel selector switch assembly.

Reference this EVTM page from my Ford EVTM manual



It is fairly close to the fuel system on your year of truck.

 

YEAY- a diagram!

thanks looks like i get to dig into the dashboard this winter.

I'd been leaning toward the switch, and this more or less confirms the problem is probably there. Only component that communicates with both tank senders. I've got to replace a bulb in the gauge cluster, and replace the headlight swith as well, so it's not all bad.. Seems their switch contractor back then sucked, cause the head light switch flakes too (fun when the headlights just shut down cruising down the road at night), and they'll go out sometimes switching from dim to bright- but I'm not going there until i get a new steering wheel to replace the worthless fat factory thing with the cruise buttns that don't work.

thinking about relocating the cruise buttons to aftermarket units dash mounted- anyone done this? or to the accessory buttons on a racing wheel? wondering what wires to tap into.

This thing sure has been a handyman special.

 

I changed the diagram to reflect what your diesel equipped truck should have for fuel selection, The previous one was for a gas truck. Your diesel is a little different.

 

my truck is a 5.8 f250 with dual tanks only my gauge reads full on rear tank and MT on the front all the time.

 

your rear tank sender is shorted out, and the front tank sender has a broken wire or open element in the sender. short to ground is full, open circuit is empty

 

I disagree with that statement tjc....Look at the values for the Full and Empty gauge reading in the diagram above. 145 Ohm is Full, Empty is 22.5 ohms. A short to ground is less than 22.5 ohms therefore the gauge will be pegged below the Empty mark. An open circuit tends to float at some value above ground which will make the gauge peg past full.

I had an open circuit issue recently on my '92 F350. My fuel gauge would slowly start to climb, then peg passed full.

 

i have not messed with anything newer than 91 on the fuel circuits, but every one i have ever worked on is empty = open circuit and full = shorted circuit.
i assume the fuel circuits would still be the same, with the sender grounding to give a full reading.
you may very well be rite on the newer trucks, they may work opposite of the older trucks.

and i just tested a fuel gauge for the 87-91 trucks. when you put power to the gauge like the sender wire is grounded out it reads full.