Evaporative Emissions System: Description and Operation

Evaporative Emissions

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

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

- Dust separator (aft-of-axle and auxiliary fuel tanks)
- EVAP canister assembly (located above spare tire on midship fuel tanks, located on LH frame rail for aft-of-axle fuel tank and located mid frame on auxiliary fuel tank)
- EVAP canister bracket
- EVAP canister purge valve
- EVAP canister vent solenoid and dust separator assembly (midship fuel tank)
- EVAP canister vent solenoid (aft-of-axle and auxiliary fuel tanks)
- EVAP filter assembly (aft-of-axle and auxiliary fuel tanks)
- Fresh air hose (midship fuel tanks)
- Fuel Tank Pressure (FTP) sensor (located behind fuel tank in the vapor tube for midship fuel tank and located on the fuel tank for aft-of-axle and auxiliary fuel tanks)
- Vapor tubes

The EVAP system:

- prevents hydrocarbon emissions from reaching the atmosphere.
- stores fuel vapors in the EVAP canister that are generated during vehicle operation and fuel filling (On-Board Refueling Vapor Recovery (ORVR) only) 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 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 canister purge valve.

The fuel vapors are routed:

- from the fuel tank through the fuel vapor vent valves.
- to the EVAP canister(s) through a vapor tube.
- to the engine when the EVAP canister purge valve is opened by the PCM.

The FTP sensor (OBD II only):

- monitors the pressure levels in the fuel tank.
- communicates the pressure reading to the PCM during the OBD II leak test.

The EVAP canister assembly:

- is mounted above the spare tire on midship fuel tank equipped vehicle with a fresh air hose connection into the box sill.
- is mounted on the LH frame rail on aft-of-axle fuel tank equipped vehicles.
- is mounted mid frame on auxiliary fuel tank equipped vehicles.
- contains activated carbon.
- stores fuel vapors.

The fuel tank filler cap:

- relieves system vacuum below 4.7 kPa (19 in H2O).

The canister vent solenoid (OBD II only):

- is an assembly with dust separator on midship fuel tank.
- is located on the EVAP canister assembly.
- is normally open.
- seals the EVAP system for the OBD II leak and pressure tests.
- is mounted to the EVAP canister.
- is repaired as a separate item.

The EVAP canister purge valve:

- is located in the engine compartment.
- is normally closed.
- regulates the purging of the EVAP canister.
- is controlled by the PCM.

The EVAP system monitor (OBD II only):

- 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:

- 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.