I rarely work on anything other than 60's trucks but I decided to bring one of our trucks home from work and see if I could figure our why we are not getting fuel from the rear tank. The truck is a 1994 F450. From what I have discovered on the board the truck has in tank fuel pumps and the dash switch turns the pumps on and off and switches the guage.

Please let me know if I am correct here!!

And...what should I do next?

 

I just bought a 1994 F-250 with a similar problem, but my front tank reads half full. When you flip the switch from the rear tank to the front, the truck stutters and stalls and will not restart till you switch to the other tank. When you turn the key to on do you hear a whine for about 1-2 seconds? That should be the fuel pump "charging" the fuel line. I haven't fixed mine yet so I am not sure what the cause is. I HAVE researched the fuel pump issue and there are TWO fuel pumps on these dual tank critters...

 

Same with the truck I brought home from work except the tanks are reversed. I know the rear tank on our truck is full as I tried to add gas on the way home. It is full and reads full. Front tank guage works fine too.


I hear nothing. I am going to get below the truck and have someone turn the key tomorow.


I also tried swapping the relays under the hood. I'm not sure which is which so I just swapped ones that had the same part number. This did no good.

 

sounds like the tank selector switch, or selector motor is bad.

 

With the ignition on but the engine not running it does switch from tank to tank according to the gas guage.

Where is the selector motor?

 

under the truck, driver side frame rail, about even with the back of the cab area. what you have to do is see if it is switching tanks. see if you can feel the valve switch when someone in the cab flips the switch.
the selector valve is a common failure on these trucks, i would suspect that before i suspect the fuel pump. the fuel pump will only come on if the selector valve for that tank is open. this prevents the pump from turning on, overheating, and burning up.

 

All I see under the truck on the frame rail is some sort of a canister that looks sort of like a fuel filter. It is mounted in a clip that is attached to the rail. From that the hose goes up int the engine area. There is also a second hose that looks as if it comes from the back tank and follows this hope up into the engine. There is a third hose that comes from the back and crosses the frame and heads up into the engine area on the other side. I have yet to investigate where in the engine these go. Any orher place the selector valve could be?

 

i am not up on the 94 trucks, my newest old body style was a 92. do you have a pair of lines coming off each tank? there will be a supply line and a return line from each tank going into the selector valve, then a supply line and a return line going to the engine.
if you only have one set of lines, it is possible someone previously removed the selector valve and eliminated one of the tanks from use.
the fuel gauge sending units are selected by the switch in the cab, not the selector valve.

 

I'll save you guys some trouble. Trucks built after 1989 do not have that switching valve, the fuel system is real simple just a delivery module inside each tank and the send and return lines are Tee'd together at the tanks.

The Fuel Delivery Module is more complicated and it is where all the problems occur with these later trucks, it contains the pump and a pair of check valves to prevent crossflow between tanks, and the fuel level float is mounted to the outside of the resovoir canister. If you don't get any fuel from a tank there are really only two possibilities, no power to the pump or the pump is dead. The fuel pump relay in the engine bay provides power to both tanks via the selector switch and an inertia switch(crash shutoff), so if one tank runs you can eliminate these parts as possible causes.
This is what an FDM looks like....


And this is what is inside it...

 

good info Paul .only my 88 and 89 trucks had dual tanks. the newer ones had single tanks.
am i correct in assuming the fuel pump is controlled by the same in cab switch?
so switching from front to rear tanks also powers the front or rear fuel pumps, and the powered down pump has a one way valve to keep fuel from the pressure and return lines going into the powered off tank?

 

Thanks for the info!!!

So it sounds like my next step is check for power at the tank. Is power continuous at the tank when power is switched to that tank?

If there is power it sounds like I get to drop the tank unless there is room between the bed and the tank to get to the pump. It may be easier to remove a couple boards from the deck. Thoughts anyone?

How I long for mechanical fuel pumps!

 

the 89 at the body shop still has the original pump in it with over 300k miles, my 2000 is on it's third pump with only 12k miles on it.
it is a crap shoot as to how long a pump will last.

 

The truck with the bad pump has 364K on the clock. No one wants to drive it far as they have to stop for gas. You know 10MPG or so.

Time to siphon out a full tank I fear.

 

Great info and thanks for the pics. Dup I have changed 2 in-tank fuel pumps in diff vehicles in my life and had to drop the tank each time. If I were you I would remove bed boards. I will switch out my truck bed in the near future and will wait till then to do my repairs. Thanks guys, I have a better idea where to start now.