Why does gas escape when opening the car fuel tank cap?

[DeBrielle]

When opening the car fuel tank cap, the escaping gas is gasoline vapor. Due to gasoline's high volatility and constant sloshing during driving, it evaporates faster, creating pressure inside the tank. This phenomenon mostly occurs in summer and is normal. During routine refueling, try not to fill the tank completely to avoid this situation. A faulty carbon canister can also cause gas to escape when opening the fuel tank cap, especially in summer when temperature spikes prevent the carbon canister from promptly balancing internal and external tank pressure.

[DiIvan]

When the car fuel tank cap is opened and gas comes out, it is mainly due to the pressure buildup inside the tank from fuel vaporization under high temperatures. The fuel tank is a sealed system. When the car is driven or parked under sunlight, the rising temperature causes some liquid fuel to turn into gas, increasing internal pressure. The moment the cap is opened, the pressure difference causes the gas to rush out suddenly, similar to opening a shaken can of soda. This vapor primarily consists of unburned gasoline components, hence the gasoline smell. While this is normal, it can be slightly dangerous, especially if the cap is opened immediately after driving. Modern vehicles are equipped with an Evaporative Emission Control (EVAP) system to recycle these vapors and prevent pollution, but the system may not perfectly handle all pressure fluctuations. To minimize gas release, it's best to park in a shaded area for a few minutes before opening the tank cap when refueling. Remember, this phenomenon is common and nothing to worry about, but always ensure the engine is off and avoid smoking or open flames near the fuel tank to prevent accidents.

[VanAutumn]

Every time I open the fuel tank cap when refueling, there's always a rush of gas coming out, which is a bit concerning. But actually, it's due to the buildup of fuel vapor in the tank. When the car is running or parked in a hot place, gasoline evaporates due to heat, creating gas pressure. The cap is sealed, so the pressure is trapped inside. When you suddenly open the cap, the pressure is released instantly, causing the gas to escape. This isn't a major issue, but safety precautions are important: the escaping gas is flammable and can easily ignite, so it's recommended to turn off the engine before refueling, slowly twist the cap to release the pressure, and never smoke or use a phone nearby. If it's a new car model, there might be an issue with the vapor system that needs checking, but generally, regular maintenance of the fuel tank cap and pipes is sufficient. Parking in shady spots whenever possible can reduce this phenomenon, protecting both yourself and prolonging the vehicle's lifespan.

[Alexis]

The hissing sound when opening the fuel tank cap is caused by releasing pressure from fuel vapors. When the car is started or parked in high temperatures, gasoline in the tank evaporates into gas, building up pressure; when you unscrew the cap, this gas naturally escapes. I've been driving for years, and this is common, especially noticeable in summer. Don't panic, it's usually harmless. But if there's an unusually large amount of gas or a strong odor every time, it might indicate a poor seal or system leak, and it's advisable to have it checked. Normally, letting the car cool down a bit before refueling helps.