Exhaust Gas Recirculation (EGR) System

Fig. 39 EGR System Operations Schematic:

Fig. 40 EGR System Operations Chart:

SYSTEM DESCRIPTION

This system is used to dilute the incoming air/fuel mixture with recirculated exhaust gases. Dilution of the incoming mixture lowers the peak flame temperature and thus limits the formation of nitrous oxides (NOx).

EGR VALVE

This is a vacuum operated valve which opens and closes a passage from the exhaust manifold to the intake manifold. This allows flame temperature in the combustion chamber to be lowered by diluting the incoming mixture with exhaust gases. The EGR valve is controlled by the vacuum modulator.

VACUUM MODULATOR

This valve controls the amount of exhaust gases that are allowed into the intake manifold by sensing load conditions. Exhaust back-pressure is sensed through a tube on the underside of the modulator. This pressure opens and closes various vacuum passages in the upper part of the modulator valve.

THERMOSTATIC VACUUM SWITCHING VALVE

The thermostatic vacuum switching valve (TVSV) is a temperature sensative valve that controls the vacuum that is used to operate the EGR valve. It opens when the engine is warm (154° F) and closes when the coolant temperature is below 122° F.

CHECK VALVE

The check valve regulates vacuum that is applied to the EGR valve (after the TVSV has opened), according to the operation of the vacuum modulator. As the vacuum modulator opens and closes, a pressure differential is created at the check valve. When the vacuum modulator opens to atmospheric pressure, the check valve closes allowing the EGR valve to close, (Manifold vacuum does not escape). When the vacuum modulator closes its atmospheric port, the check valve opens allowing vacuum to be applied to the EGR valve and exhaust gasses are recirculated.