Q: Will I Lose Gas If My Gas Cap Is Loose?

If your gas cap is loose, you will not lose fuel in its liquid form. You will, however, lose fuel in its gaseous state which is called a vapor.

Gasoline absorbs heat and, if left in an open container, will convert from a liquid into a vapor. The fuel vapor escapes into the atmosphere where it mixes with combustion gases in the ozone layer that surrounds our planet and becomes what we know as "smog."

Fuel systems on cars are designed so that all of the fuel vapor is contained within a closed system and cannot escape into the atmosphere. This is done to reduce the amount of smog in the air.

The gas cap is a vital component of your car’s fuel system. It contains a dual-action relief valve that will relieve excess pressure in your fuel tank. It also allows atmospheric pressure into your fuel tank if an excessive vacuum develops.

Vehicles built since 1996 have an emissions monitor test that checks to see if your fuel cap is doing its job. If the leak test fails, the Powertrain Control Module (PCM) will illuminate the Check Engine light and set a diagnostic trouble code for an emissions leak. The most common cause of this is the driver failing to properly reinstall the fuel cap after filling their gas tank. Another cause is a broken rubber seal on the gas cap itself.

If you have a Check Engine Light illuminated, get in touch with a qualified technician. We’ll do our best to diagnose the cause and offer you options for the most reasonable course of repair.