How Can I Tell if a Gas Cap Is Bad?

Written by kristin jennifer
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How Can I Tell if a Gas Cap Is Bad?
Newer cars have a internal flapper that prevents gas from escaping if the gas tank is improperly installed by the owner. (gas tank image by Mat Hayward from

Gas caps protect oil tanks from dirt and debris that can clog engines. Clogged engines can lower vehicle fuel efficiency and negatively impact the environment. For older cars, gas caps keep gas vapours from evaporating, creating a pressure in the fuel tank that forces fuel vapours into the canister for intake into the combustion engine to be burnt. Inspecting a gas cap is easy for anyone to do. If any repairs are warranted a trip to the auto parts store or a local mechanic can help you replace your gas cap.

Skill level:


  1. 1

    Stand near your gas tank. Identify whether you smell gas. The smell of gas will be notable if the gas cap is cracked, malfunctioning or improperly tightened.

  2. 2

    Take off the gas cap. Note whether you hear a hissing sound when the cap was removed. The hiss is the result of pressure being released in the gas tank. If so, the cap was properly installed and is functioning fine.

  3. 3

    Inspect the cap for cracks or damaged threading. Cracked or damaged gas caps should be replaced immediately.

  4. 4

    Insert the gas cap. The cap should indicate which direction to turn and for how many clicks. Most gas caps say to turn right for one click. Do not over turn. If you turn and do not hear any clicks, your gas cap needs replaced.

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