I have an '87 F150 4.9L with dual tanks. The PO did a crazy rewiring job on the fuel system. I'll try to explain it, but it's pretty screwy. The front tank is bad. They replaced the rear pump and tank and had a bunch of electrical problems. to sum it up, the rear pump works only if the switch is set to the front tank. They also managed to get the fuel gauge to stop working, but that's a whole other problem. The truck was leaking bad, so I climbed under and realized both lines to the front tank are not connected. I'm assuming the line the gas is coming out of is the return line and because of the goofy set up, the fuel is being sent to the wrong tank. How do I get this straightened out so the return fuel is going to the correct tank? I don't wanna just reconnect the lines, because then the gas will be going to the tank that leaks. I need to get his fixed FAST and I'm not sure what to do. If anyone even had a diagram of how the dual tank system works, that would help me figure this out.

 

I'll try to explain how it works. Fuel lines have no valves, valves are located in the top part of fuel pump assembly. When fuel pump gets power, gas presses on "fuel straight" valve, and that valve opens "fuel back" valve to that tank. "Fuel back" line (and another one) just separates into both tanks (you may see it on frame near to front tank front part), so if one of tank's lines are not connected, fuel will go directly to the ground. That valves located in fuel pump assembly are normally closed, so if they are fine in your front tank - just reconnect them and everything should be fine. If valves in front pump are stuck (they mostly get stuck particularry opened) - your front tank will get filled. That valves can not be repaired, it can be done only by puel pump assembly replacement.

 

Okay, so hopefully if I reconnect the lines, the fuel will go to the rear tank, and not the front? Also, on the pump, how can I tell which is the return? And, hopefully the last question, if this doesn't work and fuel is still going into the front tank, closing off the line completely should fix the problem at least temporarily, right?

 

Hampster described how the system works on '90-'96 trucks. '87-'89 trucks are different. There's a selector valve on the frame that controls where the fuel goes. I'm not exactly sure how it works but I think it works similiarly to the 2 separate valves on the later models because there are no wires to the selector valve. I would check that valve, it has four fuel lines to it and also a fuel reservoir screwed to the bottom.

The return line is the smaller line.

 

I'm sorry I forgot about fuel selector valve existance on pre-90. It works basically the same way. If you have a "spare" fuel pump (some old one you will never use again) - you may try to hook it up to selector valve, incerconnect output and fuel back line on the "engine side" and clean it with some heavy duty fuel system cleaner - it helped a bit to make one of my pump's valves work better (still not good enough, just better). It took me for about 40 minutes of pumping chemicals through it to see changes.

 

Crap, I hooked it all back up last night. I guess that means my fuel is going to the wrong tank.

 

Okay, so since I only use one tank, can this valve just be bypassed?

 

There are single tank trucks, so you would have to get the junction block from one of those. Just bypassing it won't work because of the reservoir on it, it's needed.

 

I'm thinking about rerouting the line to the rear tank. Eventually I need to fix this whole fuel system the right way, but for now I just need to get it back on the road.

 

I don't see why capping the lines wouldn't work. I'd do that.