EGR System: Operation: Notes
Exhaust gases are picked up at the exhaust manifold. An EGR valve is placed between the exhaust and intake manifolds to control the flow of exhaust gases. The valve is actuated by a ported vacuum signal. The vacuum signal is taken from a port above the throttle plate in the carburetor. The vacuum signal line is interrupted by the Ported Vacuum Switch (PVS) which senses engine coolant temperature.
Federal model EGR valves are operated by ported vacuum (throttle opening). The ported vacuum signal in these vehicles is controlled by a 4-port Ported Vacuum Switch (PVS). No vacuum will reach the EGR valve at coolant temperatures less than 128°F (53.3°C).
On California models, and all 1987 1/2 EGR valves use a transducer in series with the vacuum supply line in addition to the 4-port PVS to control the vacuum signal.
The function of the transducer is to make the EGR valve more selective as to when it opens. The transducer, in effect, changes the exhaust pressure signal into a vacuum signal. By doing so, EGR operation is now determined not only by throttle opening, but also by engine load.
The transducer will vent the vacuum signal when the exhaust pressure is below a pre-determined value. This effect is accomplished by using a diaphragm to open or close an atmospheric vent port. When exhaust pressure is low, spring tension retracts the diaphragm to uncover the vent port. As exhaust pressure increases and acts on the diaphragm, spring tension will be overcome and the diaphragm will close the vent port, allowing vacuum to enter the EGR valve.