Evaporative Emissions System: Description and Operation

Evaporative Emissions Control (EVAP) 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, the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.

Vapor Canister With Encapsulated Purge Solenoid:

VAPOR CANISTER

Gasoline vapors from the fuel tank flow into the tube labled "TANK". Any liquid fuel goes into a reservoir at the bottom of the canister to protect the carbon bed above. These vapors are absorbed into the carbon. The canister is purged when the engine is running above idle speed, and commanded by the ECM. Ambient air is allowed into the canister through the air tube. 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. Under cold or idle conditions, the solenoid is turned "OFF" by the ECM, which blocks vacuum to the canister purge passage. The ECM uses an input from the oxygen sensor to regulate the purging of the canister.
The ECM turns "ON" the solenoid valve and allows purging when all of the following have been established:

1. Engine is warm.
2. After the engine has been running a specified time.