Evaporative Emissions System: Description and Operation

Evaporative Emission Control System Operation

The basic evaporative (EVAP) emission control system is a duty control linear type and uses the EVAP emission canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon (charcoal) 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 airflow and consumed in the normal combustion process.

Gasoline vapors from the fuel tank flow into the tube labeled TANK. These vapors are absorbed into the carbon. The canister is purged by the powertrain control module (PCM)/engine control module (ECM) when the engine has been running for a specified amount of time. Air is drawn into the canister and mixed with the vapor. This mixture is then drawn into the intake manifold.

The PCM/ECM supplies a ground to energize the EVAP emission canister purge solenoid valve. This valve is Pulse Width Modulated (PWM) or turned on and off several times a second. The EVAP emission canister purge PWM duty cycle varies according to operating conditions determined by mass airflow, fuel trim, and intake air temperature.

Poor idle, stalling, and poor driveability can be caused by the following conditions:
- An inoperative EVAP emission canister purge valve.
- A damaged canister.
- Hoses that are split, cracked, or not connected to the proper tubes.