Evaporative Emissions System: Description and Operation

Typical Combined Purge Port Evaporative Emission System:

PURPOSE
To reduce HC, the fuel vapors are vented from the fuel tank to the carbon canister and stored there until they can be purged to the engine for burning.

OPERATION
Fuel vapors trapped in the sealed fuel tank are vented through the orificed vapor valve assembly in the top of the fuel tank. The vapors leave the vapor assembly through a single vapor line and continue to the carbon canister (located in the engine compartment or along the frame rail) for storage until they are purged to the engine for burning. With a purge port system, the vapors flow from the carbon canister to the throttle body. With a PCM controlled EVAP system, the flow of vapors from the canister to the engine is controlled by a Canister Purge (CANP) solenoid or vacuum controlled purge valve. Purging occurs when the engine is at operating temperature and off idle.