There is a roll over valve in the tank to prevent fuel from spilling if you ever crashed and rolled your car. Maybe it is stuck, but it is probably debris as mentioned especially if you don't have a gas cap.
Member of the Dropped Valve Seat Club.
Replaced #4 Piston, Rings, Rod+Bearings, AERA Re-Manufactured Cylinder Head and a light Hone. Then CLEANED the hell out of everything.