Evaporative Emissions System: Description and Operation

Evaporative Emission Control (EVP) System:

The evaporative emission control system is used to prevent emission of fuel vapors. The vapors generated in the fuel tank while driving or when the engine is at a stop enters the charcoal canister where the charcoal absorbs and stores the fuel vapor. Only when the following conditions are all satisfied is throttle body vacuum applied to the canister purge control valve to open the valve.

^ Engine is running.
^ Engine coolant temperature is high (engine is warmed up).
^ Throttle valve opens larger than idle position.

With vacuum applied to the purge control valve, fuel vapor in the canister is drawn into the intake manifold through the purge control valve and purge line. In this state, the canister is purged or cleaned by air drawn through the filter at the bottom of the canister. When engine coolant temperature is low, the vacuum signal line is open to atmosphere through the Bi-metal Vacuum Switching Valve (BVSV).

When coolant temperature is low, throttle body vacuum is not applied to the purge control valve, and the valve is closed. In this state, the canister is not purged.
A fuel cap check valve is provided to keep the pressure in the fuel tank constant. When the pressure in the fuel tank becomes positive and reaches its specified value, the purge control valve opens to let the vapor flow into the charcoal canister. On the other hand, when the pressure in the fuel tank becomes negative and reaches its specified value, the fuel cap check valve opens to let air flow into the fuel tank.