Why is there gas inside the fuel tank?
3 Answers
The reason for gas presence in the fuel tank is the pressure difference between the interior and exterior of the tank. The internal pressure of the fuel tank is higher than the external atmospheric pressure, causing air and some gasoline inside to flow under pressure, creating a jetting effect. When this airflow passes rapidly through narrow spaces, the vehicle emits what is actually a mixture of air and fuel vapor. Additional information about fuel tanks is as follows: 1. There is a charcoal canister inside the vehicle, located between the fuel tank and engine, which functions to balance internal and external pressure. If a jetting phenomenon occurs when opening the fuel tank cap, it may indicate an issue with the charcoal canister that should be promptly inspected. 2. The fuel tank jetting phenomenon is closely related to seasonal changes, especially during summer when rising temperatures cause gasoline inside to expand and partially vaporize, increasing internal pressure above external levels. Therefore, jetting when opening the fuel tank cap is quite common in summer. 3. After filling the tank completely at a gas station, reopening the fuel tank cap may also produce jetting. This occurs because the saturated gasoline in the tank causes increased pressure.
That day I got startled when I heard a hissing sound from the fuel tank while refueling. Later I specifically asked the 4S店 technician and learned that having air in the fuel tank is actually quite common. Gasoline itself is volatile, especially during hot summer days when fuel evaporates faster, causing these fuel vapors to accumulate in the upper part of the tank and create positive pressure. Modern cars are equipped with a charcoal canister system specifically designed to collect these fuel vapors, which are then sent into the engine cylinders to be burned when the engine starts. If you frequently hear noticeable airflow sounds, pay attention to whether the fuel tank cap gasket is aging and leaking air, or if the charcoal canister vent is clogged. Once, my car kept triggering the fuel pump to shut off during refueling, and it turned out to be due to a clogged charcoal canister.
The mechanic at the repair shop told me that fuel tank vapor generation is completely normal, as the volatile formulation of gasoline naturally produces gaseous molecules. The key is to ensure the proper functioning of the entire vapor recovery system, including the charcoal canister, fuel tank cap, and piping. Sometimes, if the fuel tank cap gasket is deformed or not tightened properly, it can lead to a particularly strong gasoline smell. What's more concerning is prolonged high-pressure conditions, which may increase the workload of the fuel pump and, in severe cases, even affect the accuracy of the fuel gauge. During my last car maintenance, the mechanic specifically warned against forcibly opening the fuel tank cap when there's excessive vapor pressure, to avoid the risk of gasoline splashing out.
