Enhanced EVAP System Operation

The basic Evaporative Emission ([1][2]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.

The [1][2]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 is Pulse Width Modulated (PWM) or turned ON and OFF several times a second.

The evaporative leak detection diagnostic strategy is based on applying vacuum to the EVAP system and monitoring vacuum decay. The fuel level sensor input to the PCM is used to determine if the fuel level in the tank is correct to run the [1][2]EVAP diagnostic tests. To insure sufficient volume in the fuel tank to begin the various diagnostic tests, the fuel level must be between 15 percent and 85 percent. The PCM monitors system vacuum level via the fuel tank pressure sensor input.