Evaporative Emissions System: Description and Operation

EVAPORATIVE EMISSIONS

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 (ORVR only) 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 EVAP test port for test purposes.

The fuel vapors are routed:
- from the fuel tank through the fuel vapor vent valves.
- to the EVAP canister(s) through a vapor line.
- to the engine when the EVAP canister purge valve is opened by the 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 EVAP canister:
- is mounted above the spare tire on midship fuel tank equipped vehicle.
- is mounted on the LH frame rail on aft-of-axle fuel tank equipped vehicles.
- is located under the hood on the left inner fender.
- contains activated carbon.
- stores fuel vapors.

The fuel tank filler cap:
- relieves system vacuum below 3.8 kPa (15.26 inches H2O).

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

The EVAP canister purge valve:
- is located in the engine compartment.
- is normally closed.
- regulates the purging of the EVAP canister.
- is controlled by the PCM.

The EVAP system monitor (OBD II only):
- 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 and positive leak test methods to test and activate the EVAP system.

The 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.