You weren’t specific, but this is common to the 80’s – 90’s trucks with dual tanks. The problem most often stems from failure of the switching mechanism between tanks, or the in-tank pumps. Different years have different designs, which are troubleshot differently. Earlier trucks often have a frame-mounted switching valve, while later trucks use the in-tank pump/module itself to pump from the selected tank. Your tank-filling symptoms sound more like the later (after 1990) models, which means I would recommend replacing the pump/module in the tank that is filling up. A broken check valve in the module may be allowing fuel from the rear tank to enter the front tank, instead of directing its full pressure to the manifold fuel rail. This can cause the sputtering symptom, due to reduced fuel pressure. There is also the real possibility that if you are unable to (or just don’t) pump from the front tank, that the additional fuel coming from the rear tank will eventually fill the front tank to the point of overflow or leakage.
JS Auto Supply