The EGR system reduces oxides of nitrogen. This is accomplished by recirculating some of the exhaust gases through the EGR valve to the intake manifold, lowering peak combustion temperatures.
Whenever the engine coolant is below 122°F (50°C), the thermostatic vacuum switching valve (TVSV) connects manifold vacuum to the EGR vacuum modulator and at the same time to the EGR valve.
The EGR vacuum modulator controls the EGR valve by modulating the vacuum signal with an atmospheric bleed. This bleed is controlled by the amount of exhaust pressure acting on the bottom of the EGR vacuum modulator (diaphragm).
Since recirculation of exhaust gas is undesirable at low rpm or idle, the system limits itself by sensing the exhaust flow. Under low load conditions, such as low speed driving, the exhaust pressure is low. In this state, the diaphragm in the modulator is pushed down by spring force and the modulator valve opens to allow outside air into the vacuum passage. The vacuum in the line is reduced, the EGR valve does not open as far, and the amount of recirculation is reduced.
Under high load conditions or high rpm driving, the exhaust pressure is increased. This pushes the modulator diaphragm upwards and closes the bleed valve. A full vacuum signal is transmitted to the EGR valve; it opens completely and allows full recirculation. The slight reduction in combustion temperature (and therefore power) is not noticed at highway speeds or under hard acceleration.
Prizm vehicles also control the EGR with a vacuum solenoid valve (VSV), 1989-92 or exhaust gas recirculation solenoid valve (EGR SV), 1993. These devices allow the ECM to further control the EGR under certain conditions. The ECM will electrically close the VSV or EGR SV if the engine is not warmed up, the throttle valve is in the idle position or if the engine is under very hard acceleration. Aside from these conditions, the Prizm EGR operates in accordance with the normal vacuum modulator function.
The price seems high, i would get another opinion or two.
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Thanks for asking!
My resources show the egr valve to be around $200.......So i think that their price is too high.