I purchased a used F350 Dually a few months ago. It as a few problems but since the truck was cheap and I liked it I was hoping to get a few things fixed. I purchased a Chilton Truck manual but it's not helping with my problems. This truck was set up for 5th wheel tow. A Wahl water/fuel separator was added into the fuel lines. The problem is that the water fuel light comes on at about 40 mph. The truck runs fine with the light on and it goes back off once you get below 40 mph. The filter has been replaced twice. When the Wahl was added the fuel lines went all the way back to the mid section of the truck and back after the fuel transfer mech. Thinking there was a fuel pressure problem I removed those lines and rerouted the lines in the engine room. Previous I would run the truck with or without the Wahl and had the same results. With the lines rerouted I can at least pump the ball to have the filters full of fuel and lines pressurized. What seems to happen is that the fuel pump does not keep up with keeping either fuel filter full of fuel. Or is it the regulator cuasing the problem. Even if its just the standard fuel filter in the fuel line. I cannot find an air leak or suction anywhere. It does this with both rear and front fuel tank. Any suggestion? If I drive above 65mph it gets worse. I have to stop and pump the primer ball to repressure the lines. The second problem is on long drives the overdrive light starts to flash. The transmission goes into manual mode. If I stop turn the truck off and restart it's fine. I also do not have reverse lights. Could it be the switch and the transmission causing the problem with both no reverse lights and flashing overdrive light?

 

Looking at other threads I think SunHester may be right about a few things. Since my fuel filter is never full even after topping it off, I've got air coming in the lines. Going to check on options to fix. Tank and line mods?? Regulator?