Evaporative Emissions System: Description and Operation

PURPOSE
The Evaporative Emission (EVAP) System is designed to reduce Hydrocarbon (HC) emissions by preventing raw fuel vapors from escaping into the atmosphere by the use of a charcoal canister, Vacuum Switching Valve (VSV), a purge port at the intake manifold and a vented fuel filler cap.

OPERATION
The system holds fuel vapor collected from the fuel tank in the charcoal canister where it is stored until the engine warms up. The Engine Control Module then signals the VSV which opens and allows engine vacuum to draw fuel vapors into the combustion process through a check valve in the canister. When air is drawn through the canister, it opens another check valve in the canister that pulls fresh air through the system. This process in turn, pulls air into the fuel tank through the check valve in the fuel cap.