Evaporative Emissions System: Description and Operation

Evaporative Emission Control System Description

EVAP System Operation

The evaporative emission (EVAP) control system used is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon, charcoal, storage device, or 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 canister. These vapors are absorbed into the carbon. The canister is purged by the 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 ECM supplies a ground to energize the evaporative emission (EVAP) canister purge solenoid valve. This valve is pulse width modulated (PWM) or turned ON and OFF several times a second. The evaporative emission (EVAP) canister purge solenoid valve PWM duty cycle varies according to operating conditions determined by engine load, fuel trim, and intake air temperature.

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

* An inoperative evaporative emission (EVAP) canister purge solenoid valve
* A damaged canister
* Hoses that are split, cracked, or not connected to the proper tubes

Evaporative Emission Canister

The Evaporative Emission (EVAP) canister is an emission control device containing activated charcoal granules. The EVAP emission canister is used to store fuel vapors from the fuel tank. Once certain conditions are met, the engine control module (ECM) activates the evaporative emission (EVAP) canister purge solenoid valve, allowing the fuel vapors to be drawn into the engine cylinders and burned.