Driving the '64 Spitfire home from a weekend in Benson and Bisbee, Arizona the other week, we filled the gas tank in St. David. Several times after that the gas cap would open. Not a big problem, my wife or I could turn around a bit and push it closed. It's aggravating but at this time no damage done. It doesn't open once the fuel level is below three quarters of the tank. I'm wondering why the cap opens on its own.