My 1987 F-150 is doing some weird things. The rear fuel gauge doesn't work. The front fuel pump seems to be pumping fuel into the rear tank while also supplying fuel to the motor. When using the rear tank the truck runs fine. Upon switching back to my front tank i found that the fuel that had been there was gone. This is my thought. The check ball in the rear tank is bad allowing fuel to pump into it. At the same time the tank selector switch is broken only allowing the front tank to operate. Here's the kicker. The fuel gauge readings still switch when i operate the fuel tank selector switch. Anyone have any other suggestions?
When you flip the switch, can you hear the system changing pumps (even if you have to cycle the ignition key)?
With fuel in the front tank, does the truck run off the front tank?
It -sounds- like some-one has modified the front pump to operate in a way to pump fuel into the rear tank instead of delivering fuel to the system directly, I've never heard of an issue like this, unless someone has altered their system to allow the addition of a bed mounted aux. tank and then rigged it up to allow the transfer of fuel from one tank into the other without denying the operation of the front pump/tank.
This one has me scratching my head and thinking pretty hard, lol.
You might check with Ford. Years ago Ford had a recall on some valve/switch between the dual tanks. People were having problems with the fuel from one tank being pumped into the other, not to big a problem if the other tank was empty, but if the other tank was full, problems occured. Check this thread.