Evaporative Emissions System: Description and Operation

Evaporative Emission System Schematic:

PURPOSE
The basic Evaporative Emission (EVAP) control system 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, fuel vapors are purged from the carbon element by intake air flow and consumed in the normal combustion process.

OPERATION
The solenoid used with this canister uses pulse width modulation to control purge. This means that the Powertrain Control Module (PCM) opens and closes the solenoid many times a second.

The PCM operates a Normally Closed (N/C) purge valve which controls the vacuum to purge the charcoal canister. Although the purge valve is connected to ported vacuum, the purge valve is commanded "ON" at idle. An idle purge passageway in the port allows purge at idle that can only be measured with a flow meter or inches of water vacuum gauge. Under cold engine, the valve is not energized by the PCM, which blocks vacuum to purge the EVAP canister.

The PCM energizes the purge valve and allows purge when:
^ The engine is warm.
^ The engine has been running a specified time.

The PCM feedback purge system is used on this application. The PCM uses inputs from the Oxygen Sensor (O2S) to regulate the purging of the canister. The PCM also has diagnostic capability to set a Diagnostic Trouble Code (DTC) 27 in the event of a malfunctioning EVAP canister purge valve.

RESULTS OF INCORRECT OPERATION
Poor idle, stalling and poor driveability can be caused by:
^ Inoperative EVAP canister purge valve.
^ Damaged EVAP canister.
^ Hoses split, cracked and/or not connected to the proper tubes.

Evidence of fuel loss or fuel vapor odor can be caused by:
^ Liquid fuel leaking from fuel lines, or fuel pump.
^ Cracked or damaged EVAP canister.
^ Disconnected, misrouted, kinked, deteriorated or damaged vapor hoses, or control hoses.

If the EVAP canister purge valve is always open, the canister can purge to the intake manifold at all times. This can allow extra fuel at idle or during warm-up, which can cause rough or unstable idle, or too rich operation.

If the EVAP canister purge valve is always closed, the canister can become over-loaded, resulting in fuel odor.