| Fig. 1: Typical EGR system on trucks without electronic
engine control
|
| Fig. 2: EGR system used on trucks equipped with electronic
engine control
|
Oxides of nitrogen are formed under conditions of high temperatures and high pressure. By eliminating one of these conditions, the production of NOx is restricted. The exhaust gas recirculation system (EGR) reintroduces a small portion of the exhaust gases into the combustion chamber with the intake charge, thus reducing spark combustion temperature. The EGR system is used on 1976 Mazdas, and all 1977 and later trucks.
Components used through 1978 include an EGR control valve, a three-way solenoid valve, a vacuum amplifier, and a water thermo switch. The control valve, operated by engine vacuum, opens to allow exhaust gases into the intake manifold, and closes to shut them off. The solenoid valve regulates vacuum to the EGR valve, It is governed by the thermo switch. At coolant temperatures below 122°F (50°C) — 131°F (55°C) for 1976 only — the thermo switch is closed. This closes the vacuum passage in the solenoid valve, cutting vacuum to the control valve which prevents exhaust recirculation when the engine is cold. At temperatures above 131°F (55°C), the thermo switch opens, allowing intake manifold vacuum to raise the control valve diaphragm, allowing recirculation of exhaust gases. The vacuum amplifier supplies varying amounts of vacuum to the control valve through the solenoid valve, opening or closing it during acceleration or at varying engine speeds.
A slightly different EGR system is used on 1979 and later models. Components include the EGR valve, a water thermo valve, and a vacuum amplifier. The three-way solenoid valve is used on 2.0 liter engines only. The EGR valve is the same vacuum operated unit used in earlier years, and operates in the same manner. The water thermo valve controls the EGR valve, except on California models; on California models, the thermo valve actuates the No. 2 relief valve in the air control valve, and the air control valve regulates the EGR vacuum signal. The thermo valve is closed when coolant temperatures are below 115°F (46°C). Above that temperature, the valve opens, allowing vacuum to be transmitted to the EGR valve. The vacuum amplifier performs the same function as in earlier years.
NOTE: 1,970 Mazdas sold in California (1979–80) have two water thermo valves. One is for the EGR system and the other is for the Spark Timing Control System. The thermo valve used in the EGR system has two vacuum hoses; one runs to the vacuum amplifier, and the other runs to the EGR valve.
