Evaporative Emissions System: Description and Operation

Evaporative Emission Control System Schematic:

SYSTEM DESCRIPTION

The Evaporative Emission Control System (EECS) used on this vehicle is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon storage canister. This retains the fuel vapors when the vehicle is not operating. When the engine is running in closed loop, the canister purge solenoid allows fuel vapor to be purged from the carbon canister and consumed in the normal combustion process.

Vapor Canister:

VAPOR CANISTER

Gasoline vapors from the fuel tank flow into the tube labled "TANK". These vapors are absorbed into the carbon. The canister is purged when the engine is running above idle speed and in closed loop. Ambient air is allowed into the canister through the fresh air port. The air mixes with the vapor, and is drawn into the intake manifold.



EVAPORATIVE CONTROL

The ECM operates a pulse width modulated solenoid valve, which controls vacuum to the purge passage in the charcoal canister. Normally the solenoid is in the "OFF" positon, and is controlled by the ECM, which blocks vacuum to the canister purge passage. The ECM regulates the purging of the canister. Purging does not take place in open loop or at idle modes.
The ECM turns "ON" the solenoid valve and allows purging when the engine is warm and in closed loop, and after the engine has been running a specified time.