Evaporative Emissions

Evaporative Emissions

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

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

- EVAP canister vent solenoid and dust separator assembly (located on the front of the EVAP canister)
- EVAP canister (located on top of the fuel tank for midship fuel tanks and on the LH frame rail for aft-of-axle fuel tanks)
- EVAP canister bracket
- EVAP canister purge valve (located on the intake manifold)
- Fuel Tank Pressure (FTP) sensor (located on the Fuel Pump (FP) module)
- Fuel vapor vent valve
- Vapor hoses
- 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 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 EVAP 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 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 On-Board Diagnostic (OBD) II leak test.

The EVAP canister:

- contains activated carbon.
- stores fuel vapors.

The fuel tank filler cap:

- relieves system vacuum below 3.5 kPa (15.09-in H2O).

The EVAP canister vent solenoid:

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

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 FTP 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.


Evaporative Emission (EVAP) Control System Vacuum Routing

4.6L - Midship Fuel Tank









5.4L, 6.8L - Midship Fuel Tank









4.6L - Aft-of-Axle Fuel Tank









5.4L, 6.8L - Aft-of-Axle Fuel Tank