Exhaust Gas Recirculation: Description and Operation

Exhaust Gas Recirculation:

PURPOSE
The Exhaust Gas Recirculation (EGR) system is used to lower NOx (Oxides of Nitrogen) emission levels caused by high combustion temperature and excessive oxygen. This is accomplished by reducing combustion temperature and displacing oxygen.

The EGR valve feeds small amounts of exhaust gas back into the combustion chamber.

OPERATION

EGR Valve
The EGR valve is a negative backpressure valve. It varies the amount of exhaust gas flow into the manifold depending on manifold vacuum and variations in exhaust backpressure. The diaphragm has an internal vacuum bleed hole which is held closed by a small spring when there is no exhaust backpressure. Engine vacuum opens the EGR valve against the pressure of a large spring. When vacuum combines with negative exhaust backpressure, the vacuum bleed hole opens and the EGR valve closes.


EGR Control
Vacuum control utilizes a vacuum solenoid which is controlled by Pulse Width Modulation (PWM). The PCM turns the solenoid "ON" and "OFF" many times per second and varies the amount of "ON" time ("PULSE WIDTH") to vary the amount of vacuum applied to the EGR valve.

The PCM uses rpm and information from the following sensors to regulate the EGR solenoid:

^ Coolant Temperature Sensor (CTS)
^ Intake Air Temperature (IAT) Sensor
^ Throttle Position (TP) Sensor
^ Manifold Absolute Pressure (MAP) Sensor
^ Park/Neutral (PNP) switch
^ Vehicle Speed Sensor (VSS)

Grounding the [1][2]DLC output/field service enable terminal, with the ignition "ON" and the engine not running, will energize the solenoid and allow vacuum to the EGR valve.

EGR Valve Identification:

Identification
Negative backpressure EGR valves will have an "N" stamped on the top side of the valve after the part number. Positive backpressure EGR valves will have a "P" stamped on the top side of the valve, after the part number. Port EGR valves have no identification stamped after the part number.

When replacing an EGR valve, always ensure the correct part number in the parts catalog or supplemental bulletin.