Emission Control Systems: Description and Operation

The Positive Crankcase Ventilation (PCV) system vents harmful blow-by fumes from the engine crankcase into the engine air intake for burning with the fuel and air mixture. The PCV valve limits the fresh air intake to suit the engine demand and serves to prevent combustion from back firing into the crankcase Thus, the benefits from the PCV system include the ability to:
^ maximize the oil cleanliness by venting moisture and corrosion from the crankcase.
^ protect against crankcase explosions.
^ automatically regulate the ventilation system air flow to the engine air intake as necessary by engine operating conditions.

The PCV valve is located in the upper intake manifold.

The Exhaust Gas Recirculation (EGR) system recirculates a portion of the exhaust gases into the intake manifold under average vehicle driving conditions to reduce combustion temperatures and exhaust gas Oxides Of Nitrogen (NOx) content. The amount of exhaust gas recirculated varies from zero with a cold engine, to a fixed rate for a hot engine with intermediate load and low engine speed. The exhaust gas flow rate increases steadily as coolant temperature rises above 65°C (150°F).

The system uses a control solenoid, activated by the Powertrain Control Module (PCM), to control the EGR valve. The components that affect the EGR system are:
^ Camshaft Position (CMP) sensor.
^ EGR vacuum regulator solenoid.
^ EGR modulator valve.
^ EGR Temperature (EGRT) sensor.
^ EGR valve.

Engine Coolant Temperature (ECT) sensor.
^ Mass Air Flow (MAF) sensor.
^ PCM.
^ Throttle Position Sensor (TPS).

The EGR system can be deactivated, aside from component malfunction, by the PCM and sensor inputs to the PCM. To deactivate the EGR system, the vacuum supply to the EGR valve is eliminated. The PCM activates the EGR control solenoid to disable the EGR system.

The solenoid controls the EGR valve by controlling vacuum supply to the EGR modulator valve. If the PCM and the sensor inputs determine that the EGR valve needs to be closed, then the PCM deactivates the EGR control solenoid. When deactivated, the EGR control solenoid vents vacuum to the atmosphere so that the EGR valve remains closed.

An EGR Temperature (EGRT) sensor is threaded into the bottom of the upper intake manifold. The EGRT sensor detects the EGR flow and notifies the PCM with an input signal. The PCM can then determine the EGR valve position and regulate the EGR valve.

The EGR modulator valve monitors exhaust back pressure to control vacuum to the EGR valve. When exhaust back pressure is too low, vacuum is cut off to the EGR valve.

Each vehicle is equipped with a decal containing control data that applies specifically to that vehicle and engine. The specifications provided on the decal are critical to servicing systems.