Emission Control Systems: Description and Operation

ENGINE EMISSION CONTROL

CAUTION: Do not remove any part of the engine emission control system. Operating the engine without the engine emission control system intact will reduce fuel economy and engine ventilation. This will weaken engine performance and shorten engine life.

The engine emission control system consists of:
- positive crankcase ventilation (PCV) system
- exhaust gas circulation (EGR) system
- evaporative emission (EVAP) system

Positive Crankcase Ventilation (PCV) System
The PCV system:
- uses intake manifold vacuum to ventilate blow-by fumes from the crankcase.
- returns the fumes to the intake manifold for combustion.

The PCV valve:
- varies the amount of blow-by gases returned to the intake manifold based on available engine vacuum.
- prevents the entry of combustion gasses backfiring into the crankcase.

The PCV system consists of:
- crankcase ventilation tube
- positive crankcase ventilation valve
- crankcase vent oil separator (2.0L Zetec only)

Exhaust Gas Recirculation (EGR) System
The EGR system:
- returns a small amount of exhaust gas into the engine for recombustion.
- reduces the overall combustion temperature and
- provides a significant reduction of oxides of nitrogen (NOx).

The EGR valve:
- is electronically controlled by the powertrain control module (PCM) (12A650).
- controls the vacuum to the EGR valve.

The EGR valve:
- is connected to the exhaust manifold by the EGR valve to exhaust manifold tube.
- controls recirculated exhaust gas flow to the intake manifold.
- monitors the flow of recirculated exhaust gases through the EGR valve to exhaust manifold tube.
- returns a signal to the powertrain control module.

The amount of recirculated exhaust gas depends upon:
- engine rpm.
- intake manifold vacuum.
- exhaust back pressure.
- engine coolant temperature.
- throttle position.

The vehicle emission control information (VECI) decal:
- is located on the hood.
- lists the exhaust emission control system information.

Evaporative emission (EVAP) system
The evaporative emission (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 powertrain control module(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 vapor management valve.
- has an EVAP test port for test purposes.

The fuel vapors are routed:
- from the fuel tank through the fuel vapor vent valve and the fuel vapor control valve.
- to the EVAP canister through a vapor line.
- to the engine when the EVAP vapor management valve is opened by the PCM.

The fuel tank pressure (FTP) sensor:
- monitors the pressure levels in the fuel tank.
- communicates the pressure reading to the PCM during the OBD II leak test.

Fuel vapor vent (FVV) valve assembly:

CAUTION: The fuel vapor vent valve cannot be removed or repaired on vehicles equipped with composite fuel tanks.

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

For removal and installation, refer to FUEL TANK.

The fuel vapor control valve (Duratec Engine only):
- prevents the flow of liquid fuel into the EVAP canister or up to the vapor management valve during refueling.
- prevents the collection of liquid fuel in the fuel vapor hoses by overfilling the fuel tank.

The evaporative emissions (EVAP) canister:
- is located under the vehicle along the left rocker panel.
- contains activated carbon.
- stores fuel vapors.

The fuel tank filler cap:
- relieves system pressure above 17 kPa (68.25 inches H2O).
- relieves system vacuum below -3.25 kPa (-13.05 inches H2O).

The canister vent solenoid:
- is normally open.
- seals the EVAP system for the inspection and maintenance (I/M 240) test and the OBD II leak and pressure tests.
- is mounted to the EVAP canister.

The evaporative emission (EVAP) check valve:
- vent path is normally closed.
- opens during refueling allowing displaced air to vent to the atmosphere through a more direct and lower-resistance path.
- is mounted to the EVAP canister bracket.

The evaporative emission (EVAP) vapor management valve:
- is normally closed.
- regulates the purging of the EVAP canister.
- is controlled by the PCM.

The evaporative emission (EVAP) system test port:
- is a part of the EVAP vapor management valve.
- is used to connect the Evaporative Emissions System Leak Tester to the EVAP system.

The evaporative emission (EVAP) dust separator:
- is attached to the EVAP canister bracket.
- prevents suspended dust and dirt particles from entering the evaporative emission system.
- is repaired as a separate item.

The evaporative emission system monitor:
- is a self-test strategy within the powertrain control module (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 EVAP running loss system leak test:
- utilizes intake manifold vacuum to test the system and involves several stages.