Evaporative Emissions System: Description and Operation

Charcoal Canister:

SYSTEM DESCRIPTION

The Evaporative Emission Control System (EECS) uses the basic 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

Gasoline vapors from the fuel tank flow into the top of the canister. Any liquid fuel goes into a reservoir at the bottom of the canister to protect the carbon element. These vapors are stored in the activated carbon element. The canister is purged when the engine is running above idle speed. The Tank Pressure Control Valve (TPCV) opens the valve and allows fill-time vapor passage. Ambient air is allowed into the canister through an air tube. A vacuum relief allows vapor to travel from the canister back to the tank during cool down periods.

EVAPORATIVE CONTROL

The ECM operates a pulse width modulated solenoid valve which controls vacuum to the purge valve in the charcoal canister. Under cold engine or idle conditions, the solenoid is de-energized by the ECM, which closes the solenoid and blocks vacuum to the canister purge valve.
The canister is commanded by the ECM to purge when:

1. The vehicle is in Closed loop operation, the throttle switch is opened, and coolant temperature is above 80~C (176~F).
2. Open loop caused by a diagnostic disable from both O2 sensors.