Evaporative Emissions

Evaporative Emissions

NOTE: The vehicle vacuum routing diagrams are contained in the Description and Operation subsection of the Engine Emission Control system.

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

- EVAP canister (located in front of the fuel tank)
- EVAP canister purge valve (located on the intake manifold)
- EVAP canister vent solenoid and dust separator assembly (located on the front of the EVAP canister)
- Fuel Tank Pressure (FTP) sensor and vapor tube assembly (located on top of the fuel tank)
- Vapor tubes
- Fresh air hose (located on top of the transmission)

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 EVAP canister purge valve.

The fuel vapors are routed:

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

The FTP sensor:

- measures the pressure levels in the fuel tank continuously.
- serves as an input to the PCM.

The EVAP canister:

- contains activated carbon.
- stores fuel vapors.

The EVAP canister vent solenoid:

- is controlled by the PCM.
- is normally open.
- seals the EVAP system for the inspection and maintenance (I/M 240) test and OBD II leak and pressure tests.

The EVAP canister purge valve:

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

The EVAP dust separator:

- prevents suspended dust and dirt particles from entering the EVAP system.
- is serviced as a separate item.

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 leak test methods to test and activate the EVAP system.
- a check engine light will illuminate if the EVAP system leaks.

The Engine On EVAP leak-check monitor:

- Uses intake manifold vacuum 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.