What Causes the Suction Sound When Opening the Fuel Tank Cap?

[McIris]

Opening the fuel tank cap and hearing a suction sound is due to the internal pressure of the fuel tank being lower than the external atmospheric pressure. The fuel in the tank continuously evaporates, causing the internal pressure to rise. To mitigate the effects of these vapors and prevent direct emission of fuel vapors into the atmosphere, which would cause pollution, modern vehicles are designed with a pathway connecting the fuel tank to the engine's intake manifold, known as the EVAP (Evaporative Emission Control System). The EVAP system includes a charcoal canister and a control valve. The charcoal canister absorbs fuel vapors, and the control valve opens under certain conditions. When the valve opens, the negative pressure from the intake manifold draws the fuel vapors into the manifold, where they are burned by the engine. The negative pressure inside the fuel tank is generated during the opening of the control valve, fuel pump operation, and as the fuel level decreases.