Is it normal to hear a hissing sound when opening the fuel tank cap?

[LeRose]

It is normal to hear a hissing sound when opening the fuel tank cap because, after unscrewing the cap, some gas enters the tank, causing a pressure difference between the inside and outside, which results in the hissing sound. Below are the relevant explanations: 1. There is a significant amount of gasoline vapor inside the tank, leading to relatively high pressure, which is released the moment the cap is opened. Although the pressure inside the tank is high, it does not become infinitely large mainly because, while driving, the gasoline vapor pushes open the carbon canister valve at the top, enters the carbon canister through the pipeline, and is absorbed by it. 2. Once the gasoline vapor reaches a certain concentration, it stops evaporating further, naturally reducing the pressure. However, if you hear a thumping sound from the fuel tank while driving, it might be due to a blocked vent valve on the fuel tank cap, which can be resolved by clearing it. It is important to note that if the fuel pump nozzle frequently shuts off prematurely or if the pressure is excessively high, it is advisable to have it checked at a 4S shop, as it could be caused by damage to the carbon canister, its pipelines, or the carbon canister solenoid valve.