Evaporative Emissions System: Description and Operation

Evaporative Emission (EVAP) Control System:

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
This system uses a basic two tube evaporative emission canister. The venting of vapors to the canister is controlled by an in-tank pressure control valve. The fuel vapors vent from the fuel tank through the tank tube to the canister. The EVAP canister purge is controlled by a remotely mounted normally closed pulse width modulated (PWM) solenoid valve.

The Powertrain Control Module (PCM) operates a normally closed PWM solenoid valve which controls vacuum to the purge valve in the charcoal canister. Under cold engine or idle conditions, the solenoid is turned "OFF" by the PCM, which closes the solenoid and blocks vacuum to the EVAP canister purge solenoid valve.

The PCM turns "ON" the solenoid valve and allows purge when:

^ Engine is warm.
^ After the engine has been running a specified time.
^ Above a specified road speed.
^ Above a specified throttle opening.


RESULTS OF INCORRECT OPERATION
Poor idle, stalling and poor driveability can be caused by:

^ An improperly connected or faulty EVAP canister purge solenoid valve. This would usually be accompanied by a start that is not as fast as normal (once started, the engine does not build up speed as fast as it normally would) and a puff of black smoke at the tailpipe.

^ Damaged 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 front fuel lines.
^ Cracked or damaged canister.
^ Disconnected, misrouted, kinked, deteriorated or damaged hoses.
^ Air cleaner or air cleaner gasket improperly seated.