An EVAP leak is a fault in the evaporative emission control system (EVAP). The EVAP’s function is to keep gasoline fumes in the fuel tank from reaching the atmosphere. This keeps air pollution down and keeps the smell of fuel from reaching the inside of your vehicle.
Fuel vapors are emitted from vehicles any time there is gasoline in the tank. So if you have a leak, even if you are not driving the vehicle, those vapors are polluting the air 24 hours a day. Uncontrolled emissions like this account for about 20 percent of the pollution produced by vehicles. 3 symptoms that will let you know that there is an issue with the EVAP system:
Check Engine Light Illuminates. The Check Engine Light turning on while driving can happen for many reasons, one being an EVAP leak. A slight fuel odor may also be noticed, but it does not happen in all vehicles. If your Check Engine Light does come on, and you just filled up your gas tank, check the gas cap to see if it's loose.
Loose Gas Cap. The most common cause of an EVAP leak warning, or the Check Engine Light, is the gas cap not being closed properly. If the cap is not fully tightened or closed all the way, the Check Engine Light may come on. Before taking your vehicle to a mechanic, check your gas cap to make sure it is positioned on the filler correctly and closed tightly. It being open potentially leaves room for fuel to leak out of the car.
Leak in the Fuel System. If the gas cap is on correctly, there may be a more serious issue, such as a leak in the fuel system. Other issues could be a faulty vent o-ring seal, defective leak detection pump, defective purge valve, or another small leak somewhere in the EVAP system.
While it is safe to drive with an EVAP leak, you should get the leak repaired right away. EVAP problems can be challenging and the mechanic will have to use advanced troubleshooting techniques to determine where the leak is, as well as how severe it is.