Evaporative Emissions System: Description and Operation

Evaporative Emission Control System:

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 evaporative canister 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. 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 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 "ON" 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 "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.

Tank Pressure Control Valve:

FUEL TANK PRESSURE CONTROL VALVE

The fuel tank pressure control valve, located in the engine compartment, is a biassed spring diaphragm valve, which is normally closed. When the vapor pressure in the fuel tank exceeds 1" Hg (6.0 kPa), the valve will open. Fuel vapors are now free to vent to the canister. This valve tends to hold vapor in the tank instead of using the vapor canister when the tank pressure is low.