Evaporative Emissions: Notes

The Evaporative Emission (EVAP) system consists of the:

• EVAP  canister purge valve.
• EVAP  canister.
• dust separator and EVAP  canister vent solenoid assembly.
• fuel vapor tube assembly (includes the Fuel Tank Pressure (FTP) sensor).
• Easy Fuel ™ (capless) fuel tank filler pipe.

The EVAP  system:

• utilizes an On-Board Refueling Vapor Recovery (ORVR) system that captures the fuel vapors from the vehicle's fuel tank during refueling.
• 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 EVAP  canister purge valve.

The fuel vapors are routed:

• from the fuel tank through the fuel vapor vent valves in the fuel tank.
• to the EVAP  canisters through the fuel vapor tube assembly.
• to the engine when the EVAP  canister purge valve is opened by the PCM.

The FTP  sensor:

• is part of the 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 on the top of the fuel tank.
• is used to control the flow of fuel vapors entering the EVAP  system.

The EVAP  canister:

• is located midship under the RH side of the vehicle.
• contains activated carbon.
• stores fuel vapors.

Easy Fuel ™ (capless) fuel tank filler pipe:

• relieves system vacuum below -4.0 kPa (16.05 in H ² O).

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 serviced as part of the dust separator assembly.

The EVAP  canister purge valve:

• is normally closed.
• regulates the purging of the EVAP  canisters.
• is controlled by the PCM.
• is repaired as a separate item.

The EVAP  dust separator:

• is attached to the EVAP  canister.
• prevents suspended dust and dirt particles from entering the EVAP  system.
• and the EVAP  canister vent solenoid are serviced together as an assembly.

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 engine on EVAP  leak-check monitor:

• The engine ON EVAP  leak-check monitor is executed by the individual components of the enhanced EVAP  system. Intake manifold vacuum is utilized to reach a target vacuum on the EVAP  system. The FTP  sensor is used by the engine ON EVAP  leak-check monitor to determine if the target vacuum necessary to carry out the leak check on the EVAP  system has been reached. Once target vacuum on the EVAP  system is achieved, the change in EVAP  system vacuum over a calibrated period of time determines if a leak exists.

The Engine Off Natural Vacuum (EONV) EVAP  leak-check monitor:

• Once the engine ON EVAP  leak-check monitor is completed and the ignition is turned OFF, the EONV leak-check monitor is executed. The EONV EVAP  leak-check monitor determines if a leak is present when the naturally occurring change in the fuel tank pressure or vacuum does not exceed a calibrated limit during a calibrated amount of time. A separate, low-power consuming microprocessor in the PCM manages the EONV leak check. The engine OFF EVAP  leak-check monitor is executed by the individual components of the enhanced EVAP  system.