What Causes the Presence of Air When Unscrewing the Fuel Tank Cap?

[MacHope]

The reasons for the presence of air when unscrewing the fuel tank cap are as follows: 1. Clogged carbon canister vent pipe. When the carbon canister is clogged, abnormal fuel tank pressure may occur. It is recommended to regularly inspect and clean accumulated activated carbon or dirt, which is caused by gasoline evaporation. 2. Negative pressure inside the fuel tank. While driving, the carbon canister solenoid valve remains open to allow continuous adsorption of evaporated gasoline vapor. If a hissing sound is heard when unscrewing the fuel tank cap, it indicates negative pressure inside the tank, especially common after long-distance driving. 3. Gasoline expansion due to high temperatures. In summer, higher temperatures cause gasoline to expand, creating pressure inside the tank that often exceeds external atmospheric pressure. This results in a hissing sound when opening the cap, though this issue is generally not serious.