Why would the gas go from the back to front tank on a 89 F150 after changed the back tank fuel pump?

Same with mine.....You might have crossed the pump wires....Go and recheck it....sounds like one lead might have gotten switched I do not think this would be a wiring problem. Unless you wired the flow solenoid wrong directing the fuel to the front tank instead of the fuel rail. Or you may have crossed the hoses on the direction the fuel returns. Allowing the rear to flow into the front.Does the truck run when you try and start it with the back tank on only ? I would think if it does your return hose is on the front tank instead of the rear. If your fuel pump was ok and you happened to replace it. I would check this out; and if it is, replace the directional solenoid. PLEASE note Im trying from old memories as to how these things work. About a wiring problem check the pump first, then check the solinoid flow divider

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