Evaporative Emissions System: Description and Operation

Evporative Emission Control System Schematic:

Evaporative Emission Canister:

PURPOSE:

To prevent the discharge of fuel vapors into the atmosphere under all operating conditions.


OPERATION:

The ECM operates a normally open pulse width modulated solenoid valve which controls vacuum to the evaporative emission canister. Under cold engine or idle conditions, the solenoid is turned "ON" by the ECM, which closes the solenoid and blocks vacuum to the EVAP canister.

The ECM turns "OFF" the solenoid valve and allows purge when:

Engine is warm.
After the engine has been running a specified time.
Above a specified road speed.
Above a specified throttle opening.

The system uses an EVAP control valve (fuel tank pressure control valve) to control the flow of vapors from the fuel tank to the EVAP canister.

Tank Pressure Control Valve:

The EVAP pressure control valve, located in the engine compartment, is a spring biased diaphragm valve, normally closed. When the vapor pressure in the fuel tank exceeds 6.0 kPa the valve will open allowing the vapors to vent to the canister and then be purged. When the vapor pressure drops sufficiently, the vapor pressure control valve will close, thus keeping the vapors in the fuel tank. The control vacuum tube of the control valve is connected into the EVAP canister line to prevent contamination from entering the valve.