The Evaporative Emission Control System (EVAP) keeps fuel vapors from leaking into the atmosphere by collecting and storing the vapors in a canister filled with carbon pellets. The carbon pellets collected by the EVAP system and temporarily stored in the canister are released back into the combustion process when the engine is turned on. The valve controls the airflow through the canister. Part of the valve is connected to the hose and has a filter at the end.
Sometimes called the solenoid, the valve has two parts. The purge solenoid is generally closed and controls the flow of vapor to a car's engine. The valve only opens during testing, which allows a vacuum to enter the system. The vent solenoid maintains airflow into the canister. A control module determines when the valve should close during tests.
The car's engine computer controls the valve and tests the system for leaks. When the valve detects a leak, it triggers the "check engine" light to alert the driver to an issue. If the "check engine" light comes on, it's usually a sign that the EVAP system is in need of maintenance.
Included in all contemporary cars and trucks, the EVAP system is a large component of emission control standards.
Gasoline fumes contain a high level of hydrocarbon gases, which are harmful to the atmosphere. Hydrocarbon gas molecules are highly reactive; they damage the ozone layer by bonding with its molecules. The ozone layer protects the Earth from harmful ultraviolet rays from the sun. Its depletion results in increased surface temperature on the planet and a higher risk of UV-related illnesses among its inhabitants.