Evaporative Emissions System: Description and Operation

Evaporative Emission Control System Schematic:

PURPOSE
The basic Evaporative Emission Control System (EECS) used on all vehicles is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon (charcoal) storage device (canister) to hold the 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.


OPERATION
This system uses a basic two tube canister. The venting of vapors to the canister is controlled by an in-tank pressure control valve. The fuel vapors vent from the fuel tank to the tank tube of the canister. This canister is placed upside down and located in the wheel well of the car. The canister purge is controlled by a remotely mounted normally opened Pulse Width Modulated solenoid.

The Electronic Control Module (ECM) operates a normally opened 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 turned "ON" by the ECM, which closes the solenoid and blocks vacuum to the canister purge valve.

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

^ Engine run time after start more than 3 minutes.
^ Coolant temperature above 176°F (80°C).
^ Vehicle speed above 24 kph (15 mph).
^ Throttle "OFF" idle.

Also, if the diagnostic test terminal is grounded with the engine stopped, the purge solenoid is de-energized (purge "ON").