Exhaust Gas Recirculation: Description and Operation

DPFE EGR System Diagram, OBD II:

PURPOSE
The Exhaust Gas Recirculation (EGR) system controls the Oxides of Nitrogen (NOx) emissions by recirculating small amounts of exhaust gases back into the combustion chamber to mix with the air/fuel charge. The combustion chamber temperature is reduced lowering NOx emissions.

OPERATION
The EGR system is a Differential Pressure Feedback (DPFE) EGR system. It is continuously monitored by the On Board Diagnostic II (OBD II) system for component integrity, system functionality and faults that can cause the emission levels to exceed federal regulations. The system is enabled only during part throttle modes when the engine is warm and stabilized. The EGR system is disabled by the Powertrain Control Module (PCM) whenever the TP sensor indicates closed throttle or wide open throttle. The disable function is necessary to avoid driveability concerns during idle and maximum power demands. While exhaust gases are being introduced, the PCM also compensates for changes in the air/fuel ratio by modifying injector pulse width and ignition spark advance. Once the EGR OBD II monitor determines that a failure has occurred, the Malfunction Indicator Lamp (MIL) illuminates to warn the driver and a Diagnostic Trouble Code (DTC) is also stored in memory. An on-demand test can also be requested to aid in the diagnosis.