Specifically, the gas cap seals your car or truck’s gas tank against escaping gasoline vapors. You know that clicking sound the gas cap makes when you turn it clockwise to close the tank?

There could also be a leak present where fittings enter and exit the fuel tank itself, like the fuel filler neck. Besides ...

Gas tank caps: They play an important part in your car's fuel system, even setting off a check engine light on the dashboard ...