I recently took over my son's 2001 Impreza. When filling the tank, after $4 or $5, the pump shuts off like the tank is full. After doing this 5 or 6 times, it will continue to fill. The gauge moves when gas is actually put in, so the prob is not the gas gauge. I was told it's some kind of sensor or air tube inside the tank that determines how much gas is in the tank. Any solution advice?