Evaporative Emissions System: Description and Operation

Evaporative Emission System Schematic:

Charcoal Canister:

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 the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.


OPERATION
Gasoline vapors from the fuel tank flow into the tube labeled "TANK."
Any liquid fuel goes into a reservoir in the bottom of the EVAP canister to protect the integrity of the carbon bed above. These vapors are absorbed into the carbon. The canister is purged when the engine is running or commanded by the Powertrain Control Module (PCM).

The PCM operates a normally closed 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 an inches of water vacuum gauge. Under cold engine or idle conditions, the valve is not energized by the PCM, which blocks vacuum to purge the EVAP canister.

The PCM energized the purge valve and allows purge when:

^ Engine is warm.
^ After 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.

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 hoses.

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

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