Evaporative Emissions System: Description and Operation

System Diagram:

The evaporative control system prevents fuel vapors generated in the fuel tank from escaping into the atmosphere.

Fuel vapors from the fuel tank flows through the fuel tank pressure control valve and vapor pipe/hose to be stored temporarily in the evaporative emission (EVAP) canister.

When the vehicle is in operation, fuel vapors stored in the EVAP canister flow through the evaporative emission (EVAP) purge solenoid, purge port and intake manifold plenum to the combustion chamber. When the engine coolant temperature is low or when the intake air quantity is small (when the engine is at idle, for example), the PCM brings the EVAP purge solenoid into the OFF state to shut off the fuel vapor flow to the intake manifold plenum. This ensures the driveability when the engine is cold or running under low load and also stabilizes the emission level. An evaporative emission (EVAP) ventilation solenoid is provided between the EVAP canister and atmosphere to monitor for OBD-II EVAP leaks. This solenoid is normally OFF. However, it turns ON when monitoring the OBD-II EVAP leaks and shuts off the atmosphere flow to the EVAP canister. Furthermore, a fuel vent valve is provided to the fuel filler tube to prevent over filling the fuel tank.