Exhaust Gas Recirculation: Description and Operation

EGR System Testing:

EGR System Operation


The EGR system routes exhaust gases from the exhaust manifold to the intake manifold during specified engine operating conditions. The EGR valve is opened by back pressure modulated manifold vacuum to let exhaust gas flow into the intake manifold. The exhaust gas then moves with the air/fuel mixture into the combustion chamber. Since there is less air/fuel mixture to burn in the combustion chamber during EGR operation, the temperature is reduced enough to limit NOx formation. If too much exhaust gas enters, combustion will not occur. Because of this, very little exhaust gas is allowed to pass through the valve, especially at idle.
The system consists of an EGR valve solenoid, backpressure modulated transducer and the EGR valve.
The vacuum supply for the transducer and EGR valve is controlled by the EGR Valve Solenoid. The solenoid is controlled by the Electronic Control Unit. The ECU monitors engine coolant temperature and other operating conditions to determine when EGR operation is desired.
NOTE: If the electrical connector to the EGR solenoid is disconnected the EGR valve will operate at all times resulting in poor engine performance and reduced driveability during certain operating conditions.

EGR Vacuum Transducer:

When the EGR solenoid is de-energized by the ECU (no voltage applied), engine vacuum flows through the EGR valve solenoid to the vacuum transducer through a connecting hose. The transducer is connected to the engine exhaust system by a small hose that connects to the base of the EGR valve. This hose is used to sense exhaust manifold back pressure. When the EGR solenoid is not energized and the engine back pressure entering the EGR inlet is strong enough to close the transducer bleed valve, vacuum will be supplied to the EGR valve and EGR operation will begin. If back pressure is not strong enough to close the transducer bleed valve, the transducer will bleed off the vacuum preventing EGR operation.