Evaporative Emissions System: Description and Operation

The evaporative emission system:
^ prevents hydrocarbon emissions from reaching the atmosphere.
^ stores fuel vapors in the Evaporative Emissions (EVAP) canister that are generated during vehicle operation or hot soak, until they can be consumed by the engine during normal engine operation.
^ routes the stored fuel vapors to the engine during engine operation.
^ is controlled by the Powertrain Control Module (PCM) which uses various sensor inputs to calculate the desired amount of purge flow. The PCM regulates the purge flow, induced by the application of intake manifold vacuum, by varying the duty cycle applied to the canister purge valve.
^ has an evaporative emission test port for test purposes.

The fuel vapors are routed:
^ from the fuel tank through the fuel vapor vent valves.
^ to the evaporative emissions (EVAP) canister(s) through a vapor line.
^ to the engine when the evaporative emission (EVAP) canister purge valve is opened by the powertrain control module (PCM).

The Fuel Tank Pressure (FTP) sensor (OBD II only):
^ monitors the pressure levels in the fuel tank.
^ communicates the pressure reading to the PCM during the OBD II leak test.

The evaporative emissions (EVAP) canister:
^ is located under the hood on the left inner fender.
^ contains activated carbon.
^ stores fuel vapors.

The fuel tank filler cap:
^ relieves system pressure above 14 kPa (56.21 inches H2O).
^ relieves system vacuum below 3.8 kPa (15.26 inches H2O).

The canister vent solenoid (OBD II only):
^ is normally open.
^ seals the evaporative emissions system for the OBD II leak and pressure tests.
^ is mounted to the evaporative emissions canister.
^ is repaired as a separate item.

The evaporative emission canister purge valve:
^ is normally closed.
^ regulates the purging of the evaporative emissions (EVAP) canister.
^ is controlled by the powertrain control module (PCM).

The evaporative emission system monitor (OBD II only):
^ is a self-test strategy within the powertrain control module (PCM) which tests the integrity of the EVAP system.
^ monitors the EVAP system for leaks.
^ monitors electronic EVAP components for irrationally high or low voltages.
^ monitors for correct EVAP system operation.
^ uses negative and positive leak test methods to test and activate the EVAP system.

The evaporative emission (EVAP) system test port:
^ is located in the EVAP canister purge valve.
^ is used to connect the Evaporative Emissions System Leak Tester to the EVAP system.