Donno

I have a '90 V8 F150 MT with dual tanks. The previous owner did some fuel tank work (pulled partway down and patched a hole), and now I have these symptoms:

1. Dash switch between F and R tanks works, and gauge works properly for each
2. Truck runs fine on either tank
3. BUT, return fuel goes to the opposite tank. F returns to R and vice versa. A problem right now because of new leak(s) which I intend to repair.

It looks like the person "simply" switched two lines. However, I spoke with the person and they didn't think they had.

I see a part called a "fuel tank selector valve," with appears to be 6-port (3 supply, 3 return) for my truck:
http://www.autozone.com/selectedZip,.../selectZip.htm

From what I can tell, this works on pressure from the pump delivering pressure in the selected tank, and switches which supply and return lines are active. Could the valve fail in a way that acts the way observed, or is it likely the PO switched the two outlets? The "easy" fix seems to be swapping the return ports off this valve, but...

Thanks in advance for any input.
Don

aliens8mycow

I'm not sure exactly what year they changed, but I don't think your 1990 would have that valve. It might, though. You could get under the truck and take a look along the driver's side frame rail and see. About where the door is, you'll see the fuel filter, and the valve should be toward the rear from there, but not further back than the filler neck on the front tank.

There were multiple recalls for these trucks due to the check valves in the fuel lines failing. This causes the issue that you're having, allowing fuel to return to either tank instead of just the one selected.

If it's like my 1991, the lines will run back from the filter, and you'll see a 'Y' in the lines. One side goes to the front tank, and the other side runs on back to the rear. If that's the case, there's no way to switch the lines between tanks (not enough length to do it).

The only way to stop the cross-filling is to change the fuel pumps, or remove the lines from one tank and cap them so that the fuel has no choice but to run to/from the other tank.

Donno

You are right. I checked the rail, and it isn't there.

So it appears the internal pump check valves went bad. Now, I don't plan to use the rear tank, so I think I would do the capping you suggest. I have never worked with the fuel line material I found underneath. It appears to be a fairly thin plastic material, and at the "y"s you mentioned it looks like it is perhaps attached via a chemical or heat method (not detachable). Do you have a suggestion for cutting/capping these lines?

Thank you for your help! Don

aliens8mycow

Back at the rear tank, you should have some flexible rubber hose running from the plastic fuel lines to the tank. This would be the place to cap. You can either pull the rubber hose from the hard line, which leaves the complete length to work with if you decide to hook things up again later, or your can cut the rubber lines in half and cap them.

Either way, you'll need (3) 3/8" hose barb fittings from the plumbing section at your local hardware store. They need a threaded end and the barb end. Get caps to fit the threaded end, fill it all up with RTV sealant, and put the caps on. Pick up a few small hose clamps too, so you can clamp the barb fitting where it meets the hose. If you're pulling the hoses loose from the lines to reuse later, you'll need to pick up some 3.8" ID hose from the auto parts store too. You only need 6 or 8 inches for each line. You can wire tie the hoses to a frame rail when you're done.

Now... You get the parts, you get prepared, you're gonna do this... Drain the rear tank before doing anything. Either run the front one low and siphon the gas from the rear into the front, or siphon it out into a few containers. Leave the filler cap off while you do this, disconnect the negative battery cable, and be prepared to get some gas in your face.

Do you plan to remove the whole tank? If you do, you can leave the wiring connector wire tied to the frame, and just disconnect the filler tube, and you're two nuts away from dropping the whole tank out of your way.