Evaporative Emissions System: Description and Operation

The Evaporative Emission Control (EVAP) system is designed to prevent of fuel vapors being emitted to the atmosphere from a vehicles fuel system. In order to meet Federal and State Regulations for vehicle evaporative emission control, all gasoline powered engines are required to be equipped with this system.
The EVAP system consists of a fuel tank pressure/vacuum filler cap, a fuel tank vapor vent valve, a charcoal "carbon" canister, and a canister purging control device.
The EVAP system directs fuel vapors, generated in the fuel tank, through the vapor valve and vapor lines to the Charcoal Canister. Activated charcoal within the canister holds and stores the fuel vapors, until they are purged into the induction system and become part of the air/fuel mixture. As the fuel vapors are being purged from the charcoal canister, fresh air is drawn into the canister from under the fresh air inlet cap, located on top of the canister.

Typical Combined EEC-IV And Purge Ported System:

The Combined EEC-IV and Purge Ported System uses a computer controlled solenoid valve for canister purging. In addition, a second vacuum/vapor line is Teed into the solenoids vacuum hose, from just before the solenoid, and is connected to a port at the throttle body. Canister purging is permitted via this second vacuum/vapor line, after the engine has reached operating temperature and the throttle plate is past the idle position.
This combined system is more commonly seen on heavy duty model vehicles.