I just bought a 1986 ford f250. My uncle hacked up the diesel fuel routing. Now I'm having trouble getting fuel. I want to re route everything correctly, the way it was from the factory. He claimed that the selector valve was malfunctioning, sending all return fuel to the rear tank. He then installed some cheap valve that has given him nothing but problems, yet he is stubborn enough to think that is not the problem. I installed a new mechanical fuel pump before my cousin lent me a mighty vac. With the mighty vac we found out that fuel wasn't making it past the cheap valve. Now i was hoping someone has a diagram, a pic or even an idea about how the fuel lines are routed. also i read online that the fuel tank selector valve can be rebuilt with a few new o-rings and drilling the old rivets. If not does anyone know how to differentiate between the fuel selector valves at the auto parts store. I'm pretty sure that the supply line is 3/8'' and the return line is 5/16''. I have six feet of both. I switch between tanks right next to my ac controls, i think the selector valve is electrical, there are some wires hanging near(2). and the selector valve that is mounted to the frame doesn't have a reservoir. I've read places that my 1986 diesel f250 has a dual function reservoir. I got under and looked but could only find what looks to be a fuel tank selector valve. I'm pretty sure that the fuel is reaching the cheap valve because when I move the valve it leaks fuel.

 

The factory valve can definitely stick and return fuel to only the rear tank, while pulling from the front tank. It's a slight design flaw that is why Ford got rid of the second tank in future generations.

As far as routing, why not first just bypass the entire valve, hook things up directly to one tank or another. You can then test if things are working properly. Then, you can change your bypass lines to hook to the other tank directly. Verify that that tank is working properly.

One you are sure that it's /nothing/ but the selector valve, you can then hook up a selector valve and test that.

You really want to make sure of the tanks by themselves, because:
1. The pickup cone can crumble and break, meaning that you can only empty the tank down to 1/4 tank before running out
2. Crud in the tank can get sucked against the fuel pick-up screen, preventing being able to suck enough fuel(so the engine barely runs)
3. You can have air leaks(rusted pinholes) in the fuel pick up assembly on the outside of the tank, leading to sucking air with your fuel(rare, but one person here had that problem).

 

How would i go about testing the selector valve? Thanks.

 

I'd probably take it out, get the wiring diagram for it and make sure it changes position when power is applied to the "switch" terminals. IIRC, you apply 12V one way for tank A, and the other way for tank B.
Put a short piece of tube from the Tank A input into a bowl of fuel, and use your suction on the output/fuel supply line. Verify that when the switch is one way, you get fuel out of the output side. Verify that when the switch is on Tank B, you get air out(because the tank B input is not connected in this test). Do the opposite for Tank B tests, then flip the whole thing over and do your return side the same way.

You'll also then need to check before you put it back in that you are getting power to the correct wires under the truck, and it's switching when you push the switch on the dash.

 

Thanks Macrobb