What is a gas cap and where is it located?

There was once a time when the main reason that someone would need to buy a replacement gas cap was because they were in such a rush to get to their destination that they left it sitting on the fuel pump after filling up the gas tank. After all, most people viewed fuel caps as a cover to the gas tank to prevent fuel theft and protect the supply from getting contaminated with dust, dirt, etc. and not much more than that so buying a new one could wait. Nowadays, in addition to providing those same functions, the fuel cap is a crucial part of the emissions system of modern cars and trucks.

Beginning in the 1970s, automakers were required to include an evaporative emissions control (EVAP) system in every new vehicle. This system is designed to prevent fuel vapors from entering the atmosphere by containing and reintroducing them back into the engine to be burned. This is a sealed system, and a correct fitting and sealing gas cap is needed to prevent any leak of pressure.

How do I know the gas cap needs to be replaced?

Today, a lot of gas caps have a tether or leash to prevent an accidental abandonment at the pump, but not having a gas cap is a definite indicator that it needs to be replaced. In fact, the check engine/service light will usually turn on if there's an issue with your EVAP system. The gas cap might especially be the culprit if the light pops on after you've finished pumping gas. If your gas cap is not holding a certain amount of vacuum pressure, it's going to need to be replaced. There are a few signs to look out for.

If the seal is damaged in anyway, the cap will most likely do a poor job of maintaining pressure and remaining in place. The gasket on the gas cap can also break from being dry and brittle. And, in very rare cases, there may be an entirely different gas cap that was not specifically made for or intended to be on your vehicle.

There are a few common OBDII fault codes related to EVAP leaks you can look out for:

• - P0440 – Large EVAP leak
• - P0442 – Small EVAP leak
• - P0443 – EVAP Purge Solenoid Control Circuit
• - P0449 – EVAP Vent Solenoid Control Circuit

A properly functioning fuel tank cap not only helps protect you, those around you, and the environment from harmful gas fumes, but it also helps prevent the wasting of fuel. Your automotives fuel supply may evaporate faster with a defective gas cap, resulting in having to refuel more frequently.

Different types of fuel caps are available, providing drivers today with options. There are locking gas caps, and these types are locked into place with a key to add an extra layer of security and protection for those motorists who desire it. There are also non-locking gas tank caps available, which are twisted and snapped into place.

Can I replace the gas cap myself?

The gas cap is an easy replacement and can be done by any do-it-yourselfer. It's usually as simple as removing the bolt that holds the gas cap tether. Then you can remove the cap entirely. Then place the bolt to the new tether, tighten the bolt to the vehicle, and twist the gas cap into place until it locks.