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:

VAPOR CANISTER

Gasoline vapors from the fuel tank flow into the evaporative canister tube labeled "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. Ambient air is allowed into the canister through the air tube at the top. The air mixes with the vapor and is drawn into the intake manifold.



EVAPORATIVE CONTROL

The PCM 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 "ON" by the PCM, which blocks vacuum to the canister purge passage. The PCM uses an input from the oxygen sensor to regulate the purging of the canister.
The PCM turns "OFF" the solenoid valve and allows purging when the following conditions are present:

1. Engine is at normal operating temperature.
2. Engine has been running a specified time.
3. Above a specified road speed.
4. Greater than 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.