Diesel Engine EGR

Fig. 49 Diesel engine EGR valve:

Exhaust gas recirculation is used to control emissions at idle and low engine speeds.
Diesel engines use an EGR valve similar to the ported vacuum type used on gasoline engines, Fig. 49, that has an increased flow capacity to compensate for the reduced pressure differential between intake and exhaust manifolds. In addition, a normally open Exhaust Pressure Regulator (EPR) valve is used on V8 engines to increase back pressure and EGR flow at idle.
Although control mechanisms vary depending upon year, engine and application, all diesel engines use vacuum from the engine vacuum pump to operate the EGR and EPR valves. Because diesel engines only require exhaust recirculation at smaller throttle openings, the control mechanisms are designed to provide maximum recirculation at idle. Recirculation is progressively reduced, or stopped completely as the throttle opening increases.

Fig. 50 Exhaust Gas Recirculation (EGR) system V8-379 engines with light duty emissions 1983-87 except Calif.:

Fig. 51 EGR system operation chart. V8-379 engines with light duty emissions 1983-87 except Calif.:

V8-379 ENGINES

1983-87 V8-379 Except Calif.
Operating vacuum for the EGR and EPR valves is controlled by an injection pump mounted Throttle Position Switch (TPS) through separate EGR and EPR solenoid valves, Fig. 50. The solenoid valves are energized through separate contacts in the TPS which are spaced to vary valve operation depending upon throttle position. In the normal (de-energized) position, the EGR solenoid is open and allows vacuum to reach the EGR valve, and the EPR solenoid is closed blocking operating vacuum.
At idle, the EGR solenoid is de-energized and the EPR solenoid is energized, both valves receive full vacuum pump output, and maximum recirculation occurs, Fig. 51. As the throttle is moved to the off idle position, the EPR contacts open de-energizing the solenoid, the EPR valve opens, and normal recirculation occurs. When the throttle is moved another 5°, 10° or 15° (depending upon the TPS operating cam installed), the EGR solenoid is energized and operating vacuum to the EGR valve is blocked.



1984-87 V8-379 California & All 1988 V8-379 Engines
EGR and EPR valve operation is controlled by the Diesel Electronic Control (DEC) system. The DEC system consists of an Electronic Control Module (ECM), EGR/EPR solenoid assembly, Engine Speed Sensor (ESS), Manifold Absolute Pressure (MAP) vacuum sensor and Throttle Position Switch (TPS). The ECM monitors signals from the 3 sensors and controls exhaust recirculation through the EGR/EPR solenoid assembly.
The EGR/EPR assembly controls EGR and EPR valve operation by regulating operating vacuum supplied by the vacuum pump. The assembly consists of an EGR vent solenoid which acts as a vacuum bleed in the EGR valve circuit, and a normally closed EPR solenoid valve. Energized, the EGR solenoid vents EGR operating vacuum to atmosphere, and the EPR solenoid opens to provide EPR valve operating vacuum. The ECM energizes the EPR solenoid only at idle, and controls EGR valve operation by varying the ``on'' and ``off'' time of the EGR solenoid.
The ECM calculates the amount of recirculation necessary based on throttle position and engine speed, and monitors EGR valve operating vacuum through the MAP sensor. When the ECM calculates that no recirculation is necessary, the EGR solenoid remains energized to rapidly vent EGR valve operating vacuum. When the ECM calculates the need for more recirculation, the solenoid is de-energized and the EGR valve receives full vacuum pump output. At idle, when the need for exhaust recirculation is greatest, the ECM energizes the EPR solenoid and the EPR valve closes.