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, the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.

Fuel Vapor Canister:

VAPOR CANISTER

This evaporative system uses a two tube canister. The venting of vapors to the canister is controlled by an in-tank pressure control valve. The canister purge is controlled by a remotely mounted, normally opened, pulse width modulated solenoid. The canister is located in the left rear wheel well of the vehicle.


EVAPORATIVE CONTROL

The ECM operates a solenoid valve which controls vacuum to the purge passage in the charcoal canister. Under cold or idle conditions, the solenoid is turned "ON" by the ECM which blocks vacuum to the canister purge passage.
The ECM turns "OFF" 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.
3. Above a specified road speed.
4. Above a specified throttle opening.

IN-TANK PRESSURE CONTROL VALVE

The in-tank pressure control valve, a combination roll-over, integral pressure, and vacuum relief valve, is located with the fuel sending unit in the fuel tank. When vapor pressure in the tank exceeds 1" Hg (5 kPa) the valve opens and allows vapors to vent to the canister and then to be purged. When the tank pressure drops below the opening point of the valve, it will close. This allows vapors to be trapped in the fuel tank. The valve provides vacuum relief to protect against vacuum build up in the fuel tank, and roll over protection to prevent liquid fuel from entering the canister during normal driving maneuvers.