I have a 1988 f150 with the 4.9 i6 and fuel leaks between the tanks, i can fill the rear tank and it sends most of the fuel into the front tank and vise versa..... any ideas on what to look at?

The seal in the selector valve is bad on the front side.
Replace the valve or rebuild it.
Disconnect the battery ground cable before doing any work around gasoline.

There are fuel lines inside the gas tank. Over time they can rust causing them to break or leak. You have one of two options, first you can pull the tank and replace the line. The second option is to keep the gas tank fuel level above the leak. And the fact that this happens mainly when going up a hill, suggests the leak is lower in the tank. All the gas shifts to the rear of the tank and exposes the leak. NOTE: The leak is probably just above the level you usually keep the tank filled to. So if you keep it at 1/4 that's where you'll find it etc....

Ok sounds like a electrical problem being an 89 its fuel injected eather 5.0 or 5.8 each tank has its own fuel pump and sending unit a dashboard switch, relay, fuse and switching motor. check your fuel line from the front tank it could be clogged. check for 12v pwr. to the front pump the sending unit will require 4V but not much. as soon as the fuel rail pressureizes the ECM shuts the pump off using the relay. Usually the tank switch goes bad first.

If the gauge works on the rear tank but not the front, the problem is most likely the sending unit in the front tank. You will have to drop the tank to change it. I guess what I would do is, set your trip meter to zero when you fill the front tank. Then, run that tank to the point where the truck is starting to run out of gas and then switch to rear tank. Keep track of how far you can go on the front tank. Then when you refill the front tank reset the trip meter and switch tanks about 20 miles before you think it's going to run out. It's not a good idea to run those pumps dry. Just make sure you have gas in the rear tank. If you do decide to change the sending unit, run that front tank as empty as you can, also, I would change the pump too. In 92, I think the pump or the sending unit can be replaced seperately. The worst thing about changing those in tank fuel pumps is getting the gas out of the tank. No trip meter? Just write down the mileage from the odometer. Good luck.

It might already be full (maybe the gauge isn't working). Is the inlet hose to the tank from the filler kinked up? I hope this helps you.

I believe this is same truck as Ranger

Pump is in tank

Removal
NOTE: On some vehicles, fuel gauge sending unit is not available separately. On those vehicles, to replace sending unit, replace entire fuel pump .

1. Remove fuel tank as described in this section.

Removal and Installation
NOTE: Make sure fuel tank is properly supported during removal and installation procedures.

1. Drain fuel from fuel tank .

1. Loosen fill pipe clamp.

1. Remove bolts securing shield and bracket assembly to frame (5005) and remove assembly from vehicle.

1. Remove fuel tank filler pipe from vehicle.

1. Support fuel tank .

1. Remove bolts and nuts from rear fuel tank support strap and remove rear fuel tank support strap .

1. Remove bolt and nut from front fuel tank support strap and remove front fuel tank support strap .

1. Lower fuel tank .

1. Remove fuel feed hose at sender push connector.

1. Remove fuel return hose at sender unit push connector.

1. Disconnect sender and fuel pump electrical connector.

1. Remove fuel vapor hose from fuel vapor valve .

1. Lower fuel tank from vehicle

1. Remove fuel tank to bench. Remove any dirt that has accumulated around the fuel pump attaching flange so that it will not enter the fuel tank during fuel pump removal.

1. Turn the fuel tank sending unit locking retainer ring (9A307) counterclockwise with the Fuel Tank Lock Ring Wrench Tool T90T-9275-A or T86T-9275-A and remove fuel tank sending unit locking retainer ring .

1. Remove the fuel pump and sender assembly.

1. Remove fuel pump mounting gasket (9417) and discard.

1. If necessary, disconnect spade connector and remove two screws and fuel gauge sending unit.

You probably have a defective fuel-tank selector valve. Its located on the left frame rail, halfway down. While the vehicle is running, tank"A" is pumping fuel to the engine, while tank "B" is getting the unneeded fuel that is returning from the engine. Thats why it is overflowing.

I have to say i'm only guessing, but it sounds like the valve that switches the flow of gas from one tank to the other must be sticking part way open, or bypassing or something, and anything your engine doesn't require, is being returned to the front tank, instead of the rear.

yea fords with two tanks are known for getting stuck if you dont switch the tank constantly. you have to get to the valve that switches the two tanks, and free it up. it definitley isnt anything but the valve.

This is a common problem. On the driver's side frame rail is a large black plastic assembly that the fuel lines connect to. This a fuel switching valve what usually happens is the valve goes bad and the fuel gets pumped from one tank to the other