I recently got a 94 f150 5.0 and it was running rough, so I changed the TPS and cleaned the IAC also removed a plug and play the previous owner installed and filled both tanks. Then fuel started pouring out of the rear tank neck while it was running. I changed the rear tank fuel vapor filter and the lines upto where it meets the front tank, it ran great for a while then I filled up both tanks again and when the front tank got down to about 1/4 fuel started coming up out of the rear tank neck again ?? Anybody else ever have this issue before, first time for me.

 

There is a problem with the shuttle valve in the in tank fuel pump in the REAR tank. The entire fuel pump sending unit/module needs to be replaced to correct this.

Make sure the gas cap for the rear tank is not allowing fuel vapor to exit.