Exhaust Gas Recirculation: Description and Operation

EGR System:

The exhaust gas recirculation system (EGR) controls formation of oxides of nitrogen (NOx) by reintroducing a measured amount of exhaust gas into the combustion chamber through the intake manifold. The EGR system is controlled by the ECM through an EGR vacuum switching valve and modulator (transducer).

EGR VALVE

This valve is controlled by a flexible diaphragm which is spring loaded to hold the valve closed. Vacuum applied to the top side of the diaphragm overcomes spring pressure and opens the valve in the exhaust port, allowing exhaust gas into the intake manifold and thus to the cylinders.

EGR MODULATOR (TRANSDUCER)

Exhaust gas back pressure operates a diaphragm in the EGR modulator to open and close the valve. The opening and closing of this valve controls the vacuum transmitted to the EGR valve.
Under low load conditions exhaust pressure is low, allowing the modulator spring to open the vacuum passage to outside air. Less vacuum is transmitted to the EGR valve and only a small amount of exhaust gas enters the intake manifold.
Under high load conditions exhaust pressure is high, pushing the modulator diaphragm up to close its valve. More vacuum is transmitted to the EGR valve and a larger amount of exhaust gas enters the intake manifold.

EGR VACUUM SWITCHING VALVE (VSV)

Under any one of the following conditions, the ECM closes the VSV vacuum passage and the EGR valve remains closed:

1. When engine coolant temperature is low.
2. When throttle valve is at idle position.
3. When manifold air temperature is low.

Other than the above, EGR valve opens and closes in accordance with EGR modulator operation.