The EGR system is used to control oxides of nitrogen (NOx) by recycling some exhaust gas through the combustion process. A small amount of exhaust gas is admitted into the intake manifold by the EGR valve.
Exhaust gas flow is a function of the pressure differential between the exhaust and intake systems. It is controlled by the vacuum operated EGR valve located on the intake manifold. The valve is open at closed throttle (idle) and closed at wide open throttle.
The EGR control unit monitors coolant temperature and engine speed to determine if the EGR control unit output signal should be sent to open the EGR valve. The control unit de-energizes the EGR control solenoid, which in turn shuts off the vacuum to the EGR valve under cold start and warming-up conditions. During these periods, exhaust gas recirculation is not desirable. As the coolant warms up to normal, vacuum is allowed to operate the valve proportional to throttle position. If the throttle opens beyond a preset level, a vacuum reducer is engaged, temporarily limiting vacuum to the EGR valve and effectively turning it off during the wide open throttle period. The vacuum signal to the valve is also reduced at high altitude, reducing tailpipe smoke.