Evaporative Emissions System: Description and Operation

EVAP System

The evaporative emissions system is used to prevent emission of fuel vapor into the atmosphere. On most models, the vapor generated in the fuel tank and carburetor bowl vent (if equipped), enter the storage canister where the charcoal absorbs and stores the vapor. During certain conditions, vacuum is supplied to the storage canister and the fuel vapor is drawn into the intake manifold to be burned in the combustion chambers. During this process, the storage canister is purged by air drawn through the filter located on the canister. On some models the fuel vapors are routed to the positive crankcase ventilation system and stored in the crankcase until the engine is started. Most systems use a sealed pressure/vacuum relief fuel filler cap.

Figure 1:

The vapor pressure sensor and VSV for vapor pressure sensor are used to detect abnormalities in the evaporative emission control system.
The ECM decides whether there is an abnormality in the evaporative emission control system based on the vapor pressure sensor signal.