### Exhaust Gas Recirculation (EGR) System

Exhaust gas recirculation is used to reduce combustion temperatures in the engine, thereby reducing the oxides of nitrogen emissions.

An EGR valve is mounted on the center of the intake manifold. The recycled exhaust gas is drawn into the bottom of the intake manifold riser portion through the exhaust manifold heat stove and EGR valve. A vacuum diaphragm is connected to a timed signal port at the carburetor flange.

As the throttle valve is opened, vacuum is applied to the EGR valve vacuum diaphragm. When the vacuum reaches about 2 in. Hg, the diaphragm moves against string pressure and is in a fully up position at 8 in. Hg of vacuum. As the diaphragm moves up, it opens the exhaust gas metering valve which allows exhaust gas to be pulled into the engine intake manifold. The system does not operate when the engine is idling because the exhaust gas recirculation would cause a rough idle.

On some later models, a thermal vacuum valve inserted in the engine thermostat housing controls the application of the vacuum to the EGR valve. When the engine coolant reaches a predetermined temperature, the thermal vacuum valve opens and allows vacuum to be routed to the EGR valve. Below the predetermined temperature, the thermal vacuum valve closes and blocks vacuum to the EGR valve.

Most vehicles have a B.P.T. valve installed between the EGR valve and the thermal vacuum valve. The B.P.T. valve has a diaphragm which is raised or lowered by exhaust back pressure. The diaphragm opens or closes an air bleed, which is connected into the EGR vacuum line. High pressure results in higher levels of EGR, because the diaphragm is raised, closing off the air bleed, which allows more vacuum to reach and open the EGR valve. Thus the amount of recirculated exhaust gas varies with exhaust pressure.

Some early models use a V.V.T. valve (venturi vacuum transducer valve) instead of the B.P.T. valve. The V.V.T. valve monitors exhaust pressure and carburetor vacuum in order to activate the diaphragm which controls the throttle vacuum applied to the EGR control valve. This system expands the operating range of the EGR flow rate as compared to the B.P.T. unit.

Fig. 1: EGR system — carbureted models

Fig. 2: EGR system — CA20E and CA18ET engines

Fig. 3: Various emission and fuel injection component locations — 200SX and 200SX Turbo CA20E and CA18ET engines

Fig. 4: Various emission and fuel injection component locations — Stanza CA20E engine

Fig. 5: Various emission and fuel injection component locations — 240SX KA24E engine