Exhaust Gas Recirculation: Description and Operation

NOTE: An EGR system is used with certain Heavy Duty Cycle (HDC) engines only.

DESCRIPTION
The EGR system reduces oxides of nitrogen (NOx) in the engine exhaust and helps prevent spark knock. This is accomplished by allowing a predetermined amount of hot exhaust gas to recirculate and dilute the incoming fuel/air mixture. This dilution reduces peak flame temperature during combustion.

A malfunctioning EGR system can cause engine spark knock, sags or hesitation, rough idle, engine stalling and poor drive ability

Fig. 1 EGR System Component Location:

Fig. 2 EGR Valve And EGR Valve Control - Typical:

The system consists of:
^ An EGR valve assembly mounted to the intake manifold.
^ An EGR valve control containing a combination back-pressure transducer and an electric vacuum solenoid.
^ The powertrain control module (PCM) to control the electric solenoid portion of the valve control.

Fig. 3 EGR Tube:

^ An EGR tube connecting a passage in the intake manifold (near the EGR valve) to the rear of the right exhaust manifold.
^ Hoses to connect the various components.

OPERATION
When the PCM removes the ground signal to the electric solenoid portion of the valve control, EGR system operation starts to occur. The PCM will monitor and determine when to supply and remove this ground signal depending on certain engine temperatures, throttle positions and other engine operating conditions.

If the electrical connector to the EGR valve control is disconnected, or the electrical signal is lost, the EGR valve will operate at all times. This may result in; poor engine performance, rough idle speed and reduced driveability during certain operating conditions.

BACK PRESSURE TRANSDUCER
The EGR valve control also contains an internal pressure-type transducer. This transducer portion of the valve control is operated by exhaust back-pressure from the EGR valve. Exhaust is delivered to the EGR valve through the metal EGR tube. This connects it to the rear of the right exhaust manifold. A rubber hose connects the backpressure fitting on the EGR valve to the back-pressure fitting on the valve control.

When the ground signal is removed from the electric portion of the valve control (solenoid is not energized), and exhaust gas back-pressure entering the EGR valve inlet is high enough, back-pressure is supplied to the valve control. It then holds the bleed valve in the transducer closed. This allows engine vacuum to flow through the EGR valve control to activate and operate the EGR valve for exhaust gas recirculation. If back-pressure is not strong enough to close the bleed valve in the transducer, the valve control will bleed off engine vacuum preventing EGR valve operation.

The transducer measures and uses this exhaust back-pressure signal from the EGR valve to regulate and provide the correct amount of exhaust gas recirculation under all conditions.

Exhaust gas recirculation will begin in this order when:
^ The powertrain control module (PCM) determines that EGR system operation is necessary
^ The electrical portion of the EGR valve control is not energized (grounded) by the PCM.
^ Exhaust back-pressure entering the transducer in the EGR valve control is strong enough to close its bleed valve.
^ Engine vacuum is passed through the EGR valve control to the EGR valve.
^ The inlet seat (poppet valve) at the bottom of the EGR valve opens to dilute and recirculate exhaust gas back into the intake manifold.