Abstract: A fuel cap retainer and gas tank cap for a fuel tank filler tube is disclosed in which the fuel tank filler tube is sealed if the gas tank cap is not properly installed on the fuel cap retainer and when properly installed permits vapor pressure equalization in the fuel cap retainer. A baffle plate extends across the fuel tank filler tube near a distal end of the fuel tank filler tube. The baffle plate has a refueling opening into which a fuel filler nozzle is adapted to be received and a valve opening. A flap valve is spring biased into a closed position sealing the refueling opening. A movable valve element is associated with the valve opening and biased to a closed position in which the valve element seals the valve opening. The movable valve element is shiftable to an open position in which the valve element permits fluid flow through the opening. The gas tank cap has a structural element for holding the movable valve element in the open position only when the gas tank cap is properly installed.