Evaporative Emissions System: Description and Operation

The Evaporative Emission (EVAP) system:
^ prevents hydrocarbon emissions from reaching the atmosphere.
^ stores fuel vapors in the 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, using various sensor inputs, calculates 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 EVAP canister purge valve.
^ has an EVAP test port for test purposes.

The fuel vapors are routed:
^ from the fuel tank through the fill limit valve and fuel vapor vent valve. The fuel vapor vent valves are part of the fuel tank assembly and cannot be serviced separately.
^ to the EVAP canisters through a vapor line.
^ to the engine when the EVAP canister purge valve is opened by the PCM.

The Fuel Tank Pressure (FTP) sensor:
^ 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) canisters:
^ are located on top of the fuel tank.
^ contain activated carbon.
^ store 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:
^ is normally open.
^ seals the evaporative emissions system for the inspection and maintenance (I/M 240) test and OBD II leak and pressure tests.

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

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

The evaporative emission dust separator:
^ prevents suspended dust and dirt particles from entering the evaporative emission system.
^ is serviced as a separate item.

The evaporative emission system monitor:
^ is a self-test strategy within the 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 leak test methods to test and activate the EVAP system.

The EVAP Running Loss System leak test:
^ utilizes intake manifold vacuum to test the system and involves several stages.