checked vent valve normally open. tried powering up vent valve solenoid it closed. had power on both wires at solenoid tried running with power disconnected at solenoid figuring valve would stay open still had vacuum at gas cap when i removed if valve is open how did tank develop vacuum. hate to throw parts at cause they aint cheap either any more suggestions on pinpointing the problem would be appreciated thanks p.s. could purge valve malfunction cause the tank to build vacuum even if vent valve is open thanks again