Why is there a hissing sound when opening the fuel tank cap?

[Gloria]

Opening the fuel tank cap produces a hissing sound due to the pressure difference between the inside and outside of the fuel tank. The carbon canister in the car's fuel tank is designed to balance the internal and external air pressure. The function of the carbon canister is to maintain this pressure balance. If the carbon canister malfunctions, the fuel tank cap cannot release the pressure, leading to an imbalance between the internal and external pressure. As a result, a hissing sound occurs when opening the fuel tank cap. This sound is more common in summer because high temperatures cause the gasoline to expand. Since the vehicle's space is fixed, the sealed fuel vapor creates pressure inside the tank, which becomes higher than the external pressure, resulting in the sound when opening the fuel tank cap.