Some specs: 1986 F-150 Supercab..302 CID..AOD Trans..Dual factory installed tanks..16,000 orig miles.
My rear tank is transferring it's fuel to the front tank. If there is more fuel in the rear tank, it will fill up the front to overflowing out of the fill neck, no matter whether the switch is set front or rear. The selector switch is functioning properly. EX: both tanks are half full. The next day the front tank is 3/4 full and the rear tank shows 1/4 full. Eventually, the rear tank will register empty and the front tank will be full or overflowing. Truck runs just fine and I can switch between front and rear tanks. Can someone please help me. Thanks.
Roland
rhptat2@yahoo.com

 

The tank switching valve is stuck and/or one or both tank pumps are not working.

The valve is located on the drivers side frame rail and looks like this:

http://i608.photobucket.com/albums/t...ordfuelres.jpg

The valve switches both the fuel feed lines from the tanks, and the EFI fuel returns from the engine back to the tanks. The valve works on fuel pressure provided by the selected in tank pump. (The dash switch only switches power to the tank pumps and the senders for the corresponding fuel gauge).

There is a fuel filter in the bottom bowl part of the valve body which can be replaced, (but won't help your problem). The bottom section screws off.

You can open up the top section and try to clean the valves out and see if you can get both valves moving. Don't try to dissemble the valves. You will likely not get them back together. You can blow air through the inlets and press the valves with your finger to see if they are switching properly. Use carb cleaner spray to clean stuff.

Here is a photo of the insides. Note the upper diaphragm is removed.

http://i608.photobucket.com/albums/t...s/FUELRES2.gif

I ended up replacing my valve with an aftermarket electric motorized switching valve but that was quite an involved project. Quite a bit cheaper than a new OEM switching valve which is a dealer only item.
http://i608.photobucket.com/albums/t...TankSelect.jpg

While the selector is disconnected run the tank fuel lines into a container and see if the tank pumps are working. Have someone switch the ignition on and off repeatedly (not start). note that the pumps only run for a few seconds if the engine does not start. Then switch to the other tank and do the same.

 

Thanks much...I will look into those methods. I'll keep you posted on the progress. Roland

 

At todays gas prices, just remove the rear tank and you remove the problem, + most of the not so needed xtras. The back tank is only 14 gallons, not enough to worry about. Just keep a gerry can for long trips into the wild.

 

I like the duel tanks because with one tank it's a pain to have to fill up every 150 miles on a long trip.

Another option would be to get the aftermarket 45 gallon rear tank and abandon the front tank. With this tank though you have to move the spare tire somewhere else. 4x4 also has a skid plate which could not be used.

 

[QUOTE=klricks;1367160]The tank switching valve is stuck and/or one or both tank pumps are not working.

The valve is located on the drivers side frame rail and looks like this:

http://i608.photobucket.com/albums/t...ordfuelres.jpg

Hi, do you if these valves were still used in 1991, I can`t find it on my frame rail......

I ended up replacing my valve with an aftermarket electric motorized switching valve but that was quite an involved project. Quite a bit cheaper than a new OEM switching valve which is a dealer only item.
http://i608.photobucket.com/albums/t...TankSelect.jpg

I got this electric thing from ROCKAUTO, can you give me directions how to connect it, in case my 1991 model uses it, since I ain`t got no clue...

TIA,

oletimemuscle

 

I will try to dig up what I did.
What problems are you having?

You will need to go and find out exactly what kind of switching valve you have now.
At some point the troublesome non electric valve was discontinued and replaced with something similar to the electric motorized valve. I thought it was used only a couple of years?
Note that you need the 6 port version for any EFI engine. Rockauto shows the 3 port and 6 port. 3 ports are for carb engines. I thought all were EFI by 91?
They also dropped the frame rail high pressure pump and went to a single high pressure pump in each tank.

The wiring at the dash tank select switch is also different.

You will need to find a wiring diagram for your '91 fuel system.

 

Wow, lots of homework....
Well, thanks for your quick reply, mine is a EFI and I have a shop manual, so the wiring diagram is no prob.
Mine is pumping fuel from one tank to the other and vice versa,no matter what position the switch is in, already changed the switch but same thing.
Checked with ROCK and found this selector valve, electric and 6 port, not knowing it is not a simple bolt on.. In the meantime I had time to do some more research and reading and found out that LMC does offer this with another name ( fuel reservoir assy ) and this looks like a simple exchange job...in case I find this reservoir...
Sounds like a lazy solution, but since I`ll have to replace rear leaf & helper springs and shocks, all bushings out rear and front trailing arm bushings incl. front coil spring rubber insulators, check my P/S pressure switch and remove both tanks to replace fuel pumps I guess in this case I`m allowed to go the path of least resistance....
So before you invest any more time & effort to help me let me check my frame rail tonight,I`ll keep you informed of what I`ve found, it might take until tomorrow due to the little time diff between our two countries...
Again, TIA,

oletimemuscle

 

Wow, lots of homework....
Well, thanks for your quick reply, mine is a EFI and I have a shop manual, so the wiring diagram is no prob.
Mine is pumping fuel from one tank to the other and vice versa,no matter what position the switch is in, already changed the switch but same thing.
Checked with ROCK and found this selector valve, electric and 6 port, not knowing it is not a simple bolt on.. In the meantime I had time to do some more research and reading and found out that LMC does offer this with another name ( fuel reservoir assy ) and this looks like a simple exchange job...in case I find this reservoir...
Sounds like a lazy solution, but since I`ll have to replace rear leaf & helper springs and shocks, all bushings out rear and front trailing arm bushings incl. front coil spring rubber insulators, check my P/S pressure switch and remove both tanks to replace fuel pumps I guess in this case I`m allowed to go the path of least resistance....
So before you invest any more time & effort to help me let me check my frame rail tonight,I`ll keep you informed of what I`ve found, it might take until tomorrow due to the little time diff between our two countries...
Again, TIA,

oletimemuscle

 

The valve on '86 is on the frame near the driver door. You should be able to easily see and follow the fuel lines from the tanks. They will lead you to the valve on your '91 wherever it is. Also there should be a fuel system diagram in your shop manual if it's the FORD manual.
If you have 4x4 then you may need to remove the skid plate in order to see the lines and valve.