Evaporative Emissions System: Description and Operation

EVAP Component Layout (Typical):

EVAP System Operation:

PURPOSE
The Evaporative Emission (EVAP) control system stores vapor generated by the vehicle and regulates its consumption during normal driving operation. The main purpose of the EVAP system is to prevent fuel vapor from dispelling into the atmosphere.

DESCRIPTION
A overview of system components follows. Additional component information is located under each component heading.

The EVAP System has five major components:

1. EVAP Charcoal Canister - Stores hydrocarbon vapor.
2. EVAP Solenoid - Regulates cannister vapor flow.
3. Throttle Body - Provides canister purge vacuum.
4. Fuel Tank - Contains generated hydrocarbon vapor.
5. Air Separator - Supplies the EVAP Charcoal Canister clean, dry air.

OPERATION
During vehicle operation, vapor generated inside the fuel tank is transferred to and stored by the charcoal canister. When certain engine parameters are met the Powertrain Control Module (PCM) commands the solenoid to energize and the stored vapors are consumed by the engine during the combustion process. Under cold or 'Open Loop' conditions the PCM de-energizes the solenoid and canister purge is disabled.

CANISTER PURGE
The PCM shall enable canister purge when all of the conditions below are met:

^ Maximum of 150 seconds after 'Closed Loop' is enabled (time varies depending on coolant temperature at key on).
^ Coolant temperature is above 80°C (176°F).
^ When the PCM is not enabling injector shut-off for Traction Control and Acceleration reduction.
^ Throttle switch is open.
^ Vehicle speed is above 20 mph.
^ Engine speed is greater than 1100 RPM.
^ Engine metal over temp/low coolant condition is not present.

EVAP SOLENOID FAILURE
Should the EVAP solenoid fail closed, electrically or mechanically, the canister charcoal bed could become saturated allowing vapors to escape to the atmosphere. The vapors should not enter the passenger compartment because of the rear mounted canister.