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I see that you have not received an answer yet. I had a dual tank Diesel which had a switch under the dash to change tank feeds. Both tanks reported fuel amounts to the same fuel gauge as each tank was switched on.
If this is the system you have (using gasoline) then simply turning the dash control should isolate the rear tank. If yours is different, you need to look underneath the truck at the fuel line routing.
For gasoline engines there was an idea to meet EPA Emissions standards by closing the fuel loop. This meant that unused fuel that was brought to the engine was returned back to the fuel tank. Some things were different for Trucks by weight Class and because of the EPA rules, trucks keep their carburetors while cars went to fuel injection. The "light" Trucks changed to fuel injection before "heavy" Trucks.
Most of the fittings to fuel components can have a taper machined to match fuel components only. There can be different thread seats, even Metric for replacement parts. Your best bet to match components is disconnect the problem part, take it with you, and then get a cap plug to block the flow. This is providing that the rear tank does not need the loop anymore.
Having worked for Ford for 25 years and having seen this very same problem (fuel flows between tank and overflows) many times there is only one cause of the fuel flooding the other tank, the fuel switching valve on the frame rail under the truck is detective, this valve contains special silicone rubber poppet valves that open when the front or wear tank fuel pump is selected, it then seals when the tanks electric pump is shut off at the selector switch, your poppet valves are leaking, all to a common problem. Back in 2000 when I retired this switching valve was no longer sold by Ford parts, it is obsolete. I suggest you check with Ford to see if it is available from some other source or call a local parts store like NAPA or Carquest and inquire is anyone in the aftermarket has them. The last choice is a used one, but of course it could have the same problem as yours as age and deterioration of the rubber valves is what makes them leak.
It should not cause no problems but instead of removing the tank leave it in the truck take out all the fuel in the tank and jus discontinue use of it. This will save you alot of time and trouble and i also suggest that if anyone besides you drives this vehicle then i would put a locking gas cap on it so it cant be accessed at anytime except by you.
You probably have a defective fuel-tank selector valve. Its located on the left frame rail, halfway down. While the vehicle is running, tank"A" is pumping fuel to the engine, while tank "B" is getting the unneeded fuel that is returning from the engine. Thats why it is overflowing.
The leaking from one tank to another was covered by a recall from Ford. Not sure if they still foot the bill for installation of the required check valves. These are still available from the dealership. The rear one is the hardest to install. The check valve can be installed without pulling the rear tank. As for your front sender I would clean all of the connections to the sender and make sure it has a decent ground. If it still only reads at the 1/4 tank level you will have to replace the sender to rectify the problem.
A FORD EQUIPED WITH DUAL TANKS THIS YEAR MODEL HAS CHECK VALVES IN THE FUEL PUMPS .THE FUEL RUNS FROM THE REAR TANK WHEN IT IS POWERED ON TO A TEE AND INTO THE SAME LINE AS THE FRONT PUMP . THE PROBLEM IS AT THE BOTTOM OF THE FRONT FUEL PUMP THERE IS A CHECK VALVE ON THE FOOT AND IT IS NOT WORKING.WHILE THE TRUCK IS RUNNING WITH THE REAR FUEL PUMP ON IT IS PUMPING FUEL INTO THE FRONT TANK . IT WILL RUN OUT OF THE NECK WHEN IT IS FULL.THE FIX IS SIMPLE REPLACE THE FRONT FUEL PUMP. THEY DONT MAKE A CHECK VALVE REPLACEMENT I CHECKED I HAD THE SAME PROBLEM AS YOU.
your truck is a 4.9L, make sure you have power to the front tank fuel pump. is the power to the pump is ok and you can hear the pump run for a second or two when you key the vehicle on... replace the fuel tank selector valve
Please Help ME by Remembering to RATE ME Thank you Vrey Much !!!
This vehicle does not use an external fuel tank switching valve. A supply check valve in the fuel pump assembly opens when the outlet pressure from the energized fuel pump exceeds the opposing check valve spring force.
R E C A L L 1991
THE DUAL FUEL TANK SELECTOR VALVE MAY MALFUNCTION AND CAUSE AN OVERFILL CONDITION IN ONE OF THE FUEL TANKS, RESULTING IN AN OVERFLOW OF FUEL PAST THE FILLER CAP.
THE OVERFLOW OF FUEL PAST THE FILLER CAP CAUSES A FUEL LEAKAGE, WHICH CAN RESULT IN A FIRE WHEN EXPOSED TO A SOURCE OF IGNITION.
REPLACE THE DUAL FUEL TANK SELECTOR VALVE.
SYSTEM: FUEL TANK, AUXILIARY.
VANS AND LIGHT TRUCKS WITH DUAL FUEL TANKS PRODUCED IN THE FOLLOWING PERIODS: F150, F250, AND F350 LIGHT TRUCKS BUILT BETWEEN FEBRUARY 28, 1989 AND JULY, 1989; ECONOLINE BUILT BETWEEN FEBRUARY 28, 1989 AND FEBRUARY 11, 1991.
NOTE: FORD HAS AGREED TO RECALL THESE VEHICLES BUT PARTS ARE NOT YET AVAILABLE.PARTS WILL BE AVAILABLE IN DECEMBER 1991. YOU WILL RECEIVE A LETTER FROM FORD ADVISING YOU WHEN TO SCHEDULE AN APPOINTMENT WITH YOUR DEALER TO HAVE THE RECALL WORK PERFORMED. IN THE MEAN TIME, YOU CAN TAKE THE FOLLOWING PRECAUTIONS:FILL EACH TANK TO ONLY 1/3 FULL. IF YOUR VEHICLE LEAKS FUEL OR IF YOU SMELL FUEL, STOP USING THE VEHICLE AND CALL FORD AT (313) 446-4654 AND ADVISE THEM OF YOUR PROBLEM. REFER TO FORD SAFETY RECALL NUMBER 91S39. AN OPERATOR WILL TAKE YOUR NAME AND TELEPHONE NUMBER. A FORD REPRESENTATIVE WILL CALL YOU BACK