Evaporative Emissions System: Description and Operation

EVAP System:

EVAP Pressure/Purge Diagnostic Station:

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
The EVAP purge valve allows manifold vacuum to purge the canister. The Powertrain Control Module (PCM) supplies a ground to energize the EVAP purge valve (purge ON ). The EVAP purge valve control circuit is Pulse Width Modulated (PWM) or turned ON and OFF several times a second. The PCM controlled PWM output is commanded when the appropriate conditions have been met:
- Engine coolant temperature above 60°C (14O°F).
- After the engine has been running about 3 minutes on a cold start or 3 seconds on a warm start.
- The vehicle is operating in closed loop fuel control.

Canister purge PWM duty cycle varies according to operating conditions determined by mass air flow, fuel trim, intake air temperature, and throttle position. Canister purge will be disabled if TP angle increases to above 70%. Canister purge will be re-enabled when TP angle decreases below 66%.

The EVAP vacuum switch is a normally closed switch positioned in the purge line between the canister and the EVAP purge valve. The EVAP vacuum switch will open when vacuum increases to greater than 5 inches of water in the purge line. The EVAP vacuum switch is used by the PCM to monitor EVAP purge valve operation and purge system integrity. The EVAP vacuum switch should be closed with no vacuum present (0% EVAP Canister Purge PWM). With EVAP Canister Purge PWM at 25% or greater, the EVAP vacuum switch should open.

Diagnosis
Poor idle, stalling and poor driveability can be caused by the following conditions:

^ Malfunctioning EVAP purge valve.
^ Damaged canister.
^ Hoses split, cracked and/or not connected properly.

An incorrect EVAP purge system flow should set a DTC P0441. A continuous purge condition with no purge commanded by the PCM should set a DTC P1441.

The EVAP system purge/pressure diagnostic station is used to clear carbon released into the EVAP system by the canister. Carbon released into the EVAP system may cause EVAP system components to be inoperative. If diagnosis indicates that an EVAP system component needs to be replaced, the system should be checked for released carbon. A released carbon condition must be corrected before replacing any EVAP system component. Procedures for checking the EVAP system for released carbon and for correcting a released carbon condition are in the Diagnostic Aids for the applicable DTC tables.