Evaporative Emissions System: Description and Operation

The Evaporative Emission Control (EVAP) system is designed to prevent fuel vapors from being emitted into 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 purge control valve, which may be computer controlled or vacuum actuated.
The EVAP system directs fuel vapors, generated in the fuel tank, through the vapor valve and vapor lines to the 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.

Typical EEC-IV Evaporative Emission System:

An EEC-IV Controlled Purge System uses a purge control solenoid valve or a vacuum operated purge control valve.
Once the engine has reached operating temperature and the throttle plate is past the idle position, the Engine Control Assembly (ECA) opens the control valve, allowing vacuum to be applied to the canister, purging it of stored fuel vapors.