Evaporative Emissions System: Description and Operation

An evaporative emission control system is used to prevent emission of fuel vapor. The vapor generated in the fuel tank while driving or the engine at a stop passes through a tank pressure control valve and enters the EVAP canister where the charcoal absorbs and stores the fuel vapor. The EVAP canister purge valve is controlled by ECM according to signals from various sensors. Only when the following conditions are satisfied, ECM opens vacuum passage of EVAP canister purge valve.
1. When engine is normal operating temperature.
2. When engine speed is higher than specified.
3. When throttle valve opens wider than idle position (idle switch OFF).
4. When engine is running within specified load. As a result, fuel vapor in the canister is sucked into intake manifold.
In this state, the canister is purged or cleaned by air drawn through the filter at the bottom of the canister. The tank pressure control 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, it opens the valve to let the vapor flow into the EVAP canister. On the other hand, when the pressure in the fuel tank becomes negative and reaches its specified value, it opens the valve to let the air flow into the fuel tank.