My rear tank is over filling my front tank! I just bought this truck and am unfamiliar with the fuel system . I can’t even figure out how to drain the tank! Because of the overfilling I can’t drive it because it’s on empty
any help would be amazing
dan

 

Hi Dan,
If everything is working other than the overfilling of the front tank, you should be able to switch to the front tank and use the vehicle.

Now , if you want to correct the fuel from going to the front tank when the rear tank is selected,you need to replace the entire front tank fuel pump module ,which includes the sending unit and check valves. .

 

Thanks for your quick response! I guess I didn’t mention that I can switch and run from the front tank except it will only run for maybe 10 minutes

 

Do you think it is trying to tell you something?

 

If it is , I haven’t got it🤣

 

It is telling you it is malfunctioning and needs to be replaced.

 

I know this might be very late to the game, but the fact is the 89 had 2 types of swap over fuel valves, one is electrical and one is mechanical (aka no DC electricity).
See photos below (credits to https://www.cargurus.com/Cars/Discussion-c5246_ds720925)

Both of these swap over units are located just past the front tank on the inside of the main truck bed frame and is quite easy to reach and remove. (It is located just about where the bed frame meets the cab on the drivers side, on the inside of the channel frame and you can locate it by looking for the 2-3-4 bolt head that hold it in place by knelling down and taking a peek). However! Remove all possible gas from both tanks before removing this valve as the fuel lines will drain all gas from "BOTH" tanks in a siffling effect.
You can also remove the gas this way or get a hose and siffle the gas out from the inlet (where you put the gas in).
Either way, you best have 4 five gallon "empty" gas cans handy as I think the front tank holds 18+gallons (Back tank holds 17+).

Now the important stuff!
If I am not mistaken "only on the mechanical version" there is a small reserve tank at the base of the valve itself which acts not only as a small "half quart of gas" reserve tank to help remove any air locks when you swap tanks.
But it also helps continues to supply fuel to a running vehicle should the other tank be ran until empty (sucking air) as to keep (1) you from having to manually purge the entire fuel line past the valve ,(2) in case of an air lock/gap and (3) keep the inline high volume fuel pressure pump primed as you never want that pump to run dry..$$$$
While this is not the issue or problem in the above it is important to note!

Also to note: this truck uses 3 electric fuel pumps. The 2 low pressure pumps are located within the gas tanks (1 in each tank) as to prime the fuel lines up to the 3rd pump which is an Inline High-Pressure/volume pump to above 90psi that sits about 2 feet away in front of the swap over valvles (basically right under your foot). on the dirvers side and under the vehicle.

The mechanical swap over valve operates and actuates from the in-tank low pressure fuel pump (in the back tank), which when energized, pressurizes the supply fuel line up to the mechanical valve.
Once the pressurized fuel reaches the mechanical sawp over valve it forces the valve up or down which in turn shuts off the fuel flow to/from the other tank. Thus re-porting the fuel flow to the high pressure fuel pump (90psi) before entering the fuel rail. at the fuel rail, the fuel pressure regulator, opens and allows the return of excess fuel back to the tank that is energized.

In some instances where the mechanical (or electric) valve fails, the return and or supply can stick half way open/closed allowing the fuel to fill the other tank that is not energized thus causing the fuel from one tank to over fill the other tank.

The fact that your truck runs for only 10 minutes once swapped over to the other tank says 2-3 things,

1. the swap over valve is not working properly (sticking half way open), or the low pressure fuel pump in the tank is about to go bad as its not creating enough low pressure to actuate the valve fully. (Replace the gas pump in the tank).
2. when you switch to the other tank, the low pressure fuel pump on the other tank may or may not be running/working, or likewise creating enough pressue to revers the swapover valve just as above (#1) and/or the mechanical switch is stuck half way aka going bad. (Replace the pump in the tank)
3. One or both the above are at fault, but you are running on the reserve (about a half quart of gas) located within the swap over valve itself ONLY IF it is a mechanical valve unit.

The fact that your Other tank fill tells you 2 other things (if possible).

1. The swap over valve is sticking open or half open/closed on the return line allowing fuel to fill the tank or both tanks.
2. The swap over valve is sticking open or half open/closed on the supply line thus allowing fuel to back flow on the supply side to the the other tank.

Since the truck is now over 32 years old its about time for fuel pump failure to say the least, Both front tank and back low pressure fuel pump probably needs to be replaced as it is not working properly and are not creating enough pressure to cause the swap over valve to actuate.
However!!!! It could be the swap over valve itself. but its a good Idea to replace all at this point.
All together it should cost around $300-$400 in parts plus labor unless you do it yourself.


Above is the Mechanical version of the swap over valve which still cost about $200-$300 as of 2020.



Below is the electrical version, which you can see the wire plug in beneath the unit.

Electrical version of the selector valve.





Lastly... I cannot find the part numbers for these units as I believe they have been discontinued by Ford..... but I find them all over ebay and other places, just be sure they are new after markets (not refurbished) and not some cleaned up used part from some junk yard.

The proper search string is :

1989 Ford F-150 - Fuel Tank Selector Valve

Dropping the gast tanks is really easy as well.lol