Exhaust Gas Recirculation: Description and Operation

Description
The EGR system is a Differential Pressure Feedback (DPFE) EGR system. Differential pressure feedback EGR is a subsonic closed loop EGR system. The differential pressure feedback EGR system:

^ monitors EGR flow rate by the pressure drop across the metering orifice.

^ uses an EGR backpressure transducer as the feedback device.

^ uses the EGR valve only as a pressure regulator, rather than a flow metering device.

Construction
The Differential Pressure Feedback EGR (DPFE) system consists of a D.P.F. EGR Sensor, EGR Vacuum Regulator (EGR V.R.) Solenoid, EGR valve, Orifice Tube Assembly, Powertrain Control Module (PCM) and connecting wires and vacuum hoses.

Features
The EGR system is designed to:

^ reintroduce exhaust gas into the combustion cycle.

^ lower combustion temperatures.

^ reduce the formation of oxides of Nitrogen (NOx).

The amount of exhaust gas reintroduced and the timing of the cycle varies by calibration. Timing and volume are controlled by the following:

^ engine rpm

^ engine vacuum

^ exhaust system back pressure

^ engine coolant temperature

^ throttle position

Exhaust Gas Recirculation Valve:

The EGR valve is vacuum actuated. The vacuum hose routing diagram is shown on the Vehicle Emission Control Information (VECI) decal.






Operation
When the strategy determines the EGR should be enabled, the PCM outputs a duty cycle to drive the EGR V.R. solenoid. The EGR V.R. solenoid responds by delivering some of the manifold vacuum it receives to the EGR valve, and vents the rest to atmosphere.
Once the vacuum signal applied to the EGR valve becomes sufficient to overcome the EGR valve spring force, the valve begins to open allowing exhaust gases to enter the intake system.
This flow of EGR gases must also pass through a metering orifice located in the orifice tube assembly. A pressure drop is created across the orifice that is proportional to the rate of EGR flow entering the intake manifold.
The DPFE sensor picks up the differential pressure signal across the orifice and provides the POM with an analog signal as feedback information on the rate of EGR flow. The POM uses this feedback to adjust the EGR V.R. duty cycle and achieve the desired EGR flow.

The DPFE system is continuously monitored by the OBD II system for component integrity, system functionality and faults that can cause the emission levels to exceed federal regulations. 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) has also been stored in memory.