2011 Duramax cab and chassis has fuel transfer problems. back tank seems to have some suction on it any time that you open the fuel cap. Truck runs out of fuel with 1/2 tank showing on the gage and front tank actually has quite a bit of fuel in it. Wondering where to start with troubleshooting. Is fuel pick-up busted in front tank? Sensor in front or back broken? Vacuum or suction line problem? Transfer pump between tanks replaced. Electrical diagnosis instructions available? Any help appreciated.