The Exhaust Gas Recirculation/Coolant Spark Control (EGR/CSC) system is used on most 1974 and later models. It regulates both distributor spark advance and the EGR valve operation according to coolant temperature by sequentially switching vacuum signals.
The major EGR/CSC system components are:
When the engine coolant temperature is below 82°F (28°C), the EGR/Ported Vacuum Switch (PVS) valve admits carburetor EGR port vacuum (occurring at about 2500 rpm) directly to the distributor advance diaphragm, through the one-way check valve. At the same time, the EGR/PVS valve shuts off carburetor EGR vacuum to the EGR valve and transmission diaphragm.
When engine coolant temperature is 95°F (35°C) and above, the EGR/PVS valve is actuated and directs carburetor EGR vacuum to the EGR valve and transmission instead of the distributor. At temperatures between 82-95°F (28-35°C), the EGR-PVS valve may be open, closed, or in mid-position.
The Spark Delay Valve (SDV) delays vacuum to the distributor advance diaphragm by restricting the vacuum signal through the SDV valve from a predetermined time. During normal acceleration, little or no vacuum is admitted to the distributor advance diaphragm until acceleration is completed, because of (1) the time delay of the SDV valve and (2) the re-routing of EGR port vacuum if the engine coolant temperature is 95°F (35°C) or higher.
The check valve blocks off vacuum signal from the SDV to the EGR/PVS so that carburetor spark vacuum will not be dissipated when the EGR/PVS is actuated above 95°F (35°C).
The 235°F (113°C) PVS is not part of the EGR/PVS system, but is connected to the distributor vacuum advance to prevent engine overheating while idling (as on previous models). At idle speed, no vacuum is generated at either the carburetor spark port or EGR port and engine timing is fully retarded. When engine coolant temperature reaches 235°F (113°C), however, the valve is actuated to admit intake manifold vacuum to the distributor advance diaphragm. This advances the engine timing and speeds up the engine. The increase in coolant flow and fan speed lowers engine temperature.