Evaporative Emissions: Notes

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).

The 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 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 a 2-piece 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 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 intake manifold.

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 evaporative emission 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.

For additional information on the operation of the EVAP  system monitor, refer to the INTRODUCTION .