The vented gas cap is designed to vent small amounts of air into the car gas tank line. The vented gas cap has a one-way release valve that is pressure-activated. When the pressure reaches a certain point (defined by so many pounds per square inch) the valve opens by a very small amount, relieving some of the pressure.

What are the symptoms of a bad gas cap?

• Cap does not tighten properly. One of the most common symptoms of an issue with the gas cap is a cap that does not tighten properly. …
• Fuel smell from the vehicle. …
• Check Engine Light comes on.

How do you test a gas cap?

To fix, expose the gas cap by opening the tank cover which is next to the compact box. If the gas cap appears tight, take it out and examine the seal that laps firmly to the cap. If the seal is broken, it won’t sit firmly on the gas cap, allowing gas to leak out. You should get a replacement gas cap seal.