Evaporative Emissions: Notes

The Evaporative Emission (EVAP) system consists of the following components:

• Dust separator
• EVAP canister (located above the spare tire)
• EVAP canister bracket (located above the spare tire)
• EVAP canister purge valve (located on the left front fender)
• EVAP canister vent solenoid (located above the spare tire)
• Rear vapor tube and Fuel Tank Pressure (FTP) sensor assembly (located behind and along side of the fuel tank)
• Vapor tubes
• Vapor hoses

The EVAP system:

• is equipped with an On-Board Refueling Vapor Recovery (ORVR) system to control hydrocarbon emissions during refueling.
• prevents hydrocarbon emissions from reaching the atmosphere.
• stores fuel vapors (generated during vehicle operation, hot soak or vehicle refueling) in the EVAP canister until they can be consumed by the engine.
• routes the stored fuel vapors to the engine during engine operation.
• is controlled by the 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 EVAP canister purge valve.

The fuel vapors are routed:

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

The FTP sensor:

• monitors the pressure levels in the fuel tank.
• communicates the pressure reading to the PCM during the OBD II leak test.
• is located in the fuel vapor valve tube and is serviced as part of the fuel vapor valve tube assembly.

The EVAP canister:

• is located under the rear of the vehicle near the fuel tank.
• contains activated carbon.
• stores fuel vapor.

The fuel tank filler cap:

• relieves system vacuum below 3.5 kPa (14.0 in H₂ O).

The 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.
• is repaired as a separate item.

The electronic EVAP canister purge valve:

• is normally closed.
• regulates the purging of the EVAP canister.
• is controlled by the PCM.

The fuel pump and tank assembly contains the fill limiting vent valve and fuel vapor vent valve. The fill limiting vent valve prevents suspended liquid fuel from being drawn into the EVAP canister along with the fuel vapors.

The EVAP system:

• 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.
• will display a "Fuel Cap" message by the message center if the EVAP system detects a loose or missing fuel cap.

The EVAP dust separator:

• is attached to the EVAP canister bracket.
• prevents suspended dust and dirt particles from entering the EVAP system.
• is repaired as a separate item.

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 is executed:

• once the engine ON EVAP leak-check monitor is completed and the key is turned OFF. 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.