Evaporative Emissions

EVAPORATIVE EMISSIONS

NOTE: The vehicle emission vacuum routing diagrams are contained in the Description and Operation subsection of the Engine Emissions Control. Refer to Emission Control Systems.

The evaporative emission (EVAP) system consists of the:
- EVAP canister purge valve.
- EVAP canister.
- EVAP canister vent solenoid (includes the dust separator).
- fuel vapor tube assembly (includes the fuel tank pressure [FTP] sensor).
- fuel tank filler cap.

The EVAP system:
- prevents hydrocarbon emissions from reaching the atmosphere.
- stores fuel vapors in the EVAP canisters, 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 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 vapor tube.

The fuel vapors are routed:
- from the fuel tank through the vapor vent valve (internal to the tank).
- to the EVAP canisters through a vapor tube.
- from the EVAP canister to the engine when the EVAP canister purge valve is opened by the PCM.

The FTP sensor (part of fuel vapor tube assembly):
- monitors the pressure levels in the fuel tank.
- communicates the pressure reading to the PCM during the OBD II leak test.

The fuel vapor tube assembly:
- is located midway under the vehicle along the LH frame rail.
- is used to route the flow of fuel vapors entering the EVAP system.

The EVAP canister:
- is located on the underside of the vehicle directly below the driver seat.
- contains activated carbon.
- stores fuel vapors.

The fuel tank filler cap:
- is sealed against the fuel tank filler pipe and does not relieve system pressure.

The EVAP canister vent solenoid:
- is normally open.
- seals the EVAP system for the inspection and maintenance (I/M 240) test and OBD II leak and pressure tests.
- is mounted to the EVAP canister bracket.

The EVAP canister purge valve:
- is normally closed.
- regulates the purging of the EVAP canister.
- is controlled by the PCM.
- is repaired as a separate item.
- is mounted to the cowl on a bracket.

The EVAP dust separator:
- is attached to the EVAP canister vent solenoid mounted on the EVAP canister bracket.
- prevents suspended dust and dirt particles from entering the EVAP system.
- is repaired as an assembly with the EVAP canister vent solenoid.

The EVAP 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 and positive 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.