Fuel Vapor Rollover Valve
WARNING: This page is about a different car, the 1997 Mazda Millenia. However, it is still accessible from the selected car via links, so may be relevant.
Vehicle is equipped with 2 fuel vapor valves, built into top of fuel tank. With valve in upright position, air should flow through valve. With valve turned upside down, air should not flow through valve. If fuel vapor valve does not operate as specified, replace fuel tank.