93 F150 EFI/dual fuel tanks. I may have a front/rear selector switch problem or seemingly one pump is siphoning from the other tank. A few weeks ago I replace the rear fuel pump that was not pumping(truck would not get fuel to the fuel manifold). Just before it failed the "front" tank fuel gage stop registering.I filled the front tank up and the gas gage needle would not move. The rear would register although the pump went south. Today, while driving with in "front" tank switch selector mode, the truck began to stumble and I thought it was about empty and switched to the rear tank (which was about 3/4 full from a previous gas up, I had not used the rear pump/tank since then) and the fuel gage read empty and the truck stalled. I checked and found out the rear tank was empty!! Evidently the front tank/pump, while in the front switch mode must had siphoned the gas out of the rear tank? Am I looney or what. Is there a check valve in the line between the two tanks? I thought the fuel pumps contained the check valves. OR could somehow the electrical circuit within the front/rear selector switch become faulty and make the circuit to the rear tank pump ? I filled the rear tank up and the truck ran fine. I found the front tank was low by only about 2 gallons from the previous fill up! In the front tank/pump mode the truck would not start due to fuel, but once I filled the rear tank the truck would start when either the front or rear selector switch mode was pushed?Go figure..
I can't find a fuel or electrical schematic for the 93 F150 truck indicating dual pumps/tanks. Does anyone have info on a diagram? And most of all anyone can solve this phantom fuel problem. Thanks

 

Sounds to me like the tank valve is not switching, like it's always using the rear tank no matter where the switch is. Could either be a bad dash switch or a bad tank selector valve - usually it's a bad selector valve, located on the frame rail near the two tanks. Check that there is power at the wiring to the selector valve and that the power changes when the switch is changed. It's been a while, so I'm not sure of the number of wires or colors, maybe someone else here knows that.

Marty

 

73Custom,

sorry the 93 F-150 has no selector valve. Yust combined pump/sender modules in each gas tank, which feed the same fuel line to the filter.

my93,
to me it looks like the selector switch(SW) is shot and stays in the Rear Position. The wiring is given in Haynes, page 12-60 (please check by yourself). Here is the Selector SW wiring:
Pin 2 RED-YEL: 12V into SW, pin 1 RED: 12V to front pump (SW in Front pos.), pin 3 BRN-WHT: 12V to rear pump (SW in Rear pos.), pin 4 DK BLU-YEL: sender input from front tank, pin 6 YEL-LT BLU: sender input from rear tank, pin 5 YEL-WHT: sender signal to fuel gauge.
LoL Dieter

 

If one pump is pumping into the other one, then the check valves in the sending unit are bad. Some of the early 90's that had this set-up had a recall for this and Ford installed check-valves in the lines.

 

I just had this problem with my 91 F-150 4.9l. There was a recall and the fix was to replace the check valves on the front and rear tank pumps. These check valves go bad and then allow gas from the rear tank to come into the front tank or vice-versa when you are running the engine. My Ford dealer told me that the recall is good for 12 years or 120k miles which ever comes first. I had to buy the check valves at my dealer for $117. I removed the bed to change them and they fixed the problem. Hope this helps.

Ken Wilford
John 3:16

 

I'm seeming to have the same problems. On my '91 3/4 ton, when I switch to the rear tank, it dies (fuel starved). I have back-probed (using the Haynes diagram) and there is no juice to the rear tank from the switch. I cannot find a part number except for a switch listed for '92-97 from Motormite (MOT 49205, $12). Can't find one in stock. When and if... Hoping this is the only problem, it's cheap compared to the pump