Evaporative Emissions System: Description and Operation

PURPOSE
The evaporative emission control system prevents the emission of fuel tank vapors into the atmosphere. When fuel evaporates in the fuel tank, the vapors pass through vent hoses or tubes to the charcoal filled EVAP canister. The EVAP canister temporarily holds the vapors. Through the EVAP purge solenoid, the powertrain control module (PCM) allows intake manifold vacuum to draw vapors into the combustion chambers during certain engine operating conditions. The plunger solenoid regulates vapor flow from the canister to the engine.

OPERATION
The EVAP purge solenoid regulates the rate of vapor flow from the EVAP canister to the throttle body. The PCM controls solenoid operation. During the cold start warm-up period and the hot start time delay, the PCM does not energize the solenoid. When de-energized, no vapors are purged. The PCM de-energizes the solenoid during open loop operation.
The engine enters closed loop operation after it reaches a specified temperature and the programmed time delay ends. During closed loop operation, the PCM energizes and de-energizes the solenoid approximately 5 to 10 times per second, depending on operating conditions. The PCM varies the vapor flow rate by changing solenoid pulse width. Pulse width is the amount of time the solenoid de-energizes.