See Figure 1
The EGR/CSC system is used on most vehicles. 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:
- A 95°F (35°C) EGR Ported Vacuum Switch (EGR-PVS)
- Spark Delay Valve (SDV)
- Vacuum Check Valve (VCV)
When the engine coolant temperature is below 82°F (28°C), the EGR-PVS admits carburetor or throttle body EGR port vacuum (occurring at about 2,500 rpm) directly to the distributor advance diaphragm, through the one-way check valve. At the same time, the EGR-PVS shuts off EGR port vacuum to the EGR valve and transmission diaphragm.
When engine coolant temperature is 95°F (35°C) and above, the EGR-PVS is actuated and directs EGR port vacuum to the EGR valve and transmission, instead of to the distributor. At temperatures between 82-95°F (28-35°C), the EGR-PVS may be opened, closed or in mid-position.
The SDV delays carburetor or throttle body spark vacuum to the distributor advance diaphragm by restricting the vacuum signal through the SDV for a predetermined time. During normal acceleration, little or no vacuum is admitted to the distributor advance diaphragm until acceleration is completed. This is accomplished because of the time delay of the SDV and the rerouting of the EGR port vacuum when 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 spark port 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/CSC system, but is connected to the distributor vacuum advance to prevent engine overheating while idling. At idle speed, no vacuum is generated at either the carburetor/throttle body spark port or at the EGR port, and engine timing is fully retarded. When engine coolant temperatures 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.
- With a cold engine, the passage between the upper and middle ports should be open, and the passage between the lower and middle ports should be closed.
- With the engine at or above the EGR-PVS temperature (typically 95°F), the passage between the lower and middle ports should be open, and the passage between the upper and middle ports should be closed.
Spark Delay Valve
- Disconnect the vacuum lines which run to the spark delay valve and connect a hand vacuum pump.
Operate the pump and note if vacuum can be achieved.
- A valve with one black or white side and one colored side is good if vacuum can be built up in one direction, but not in the other direction, and if the vacuum slowly decreases.
- A valve with both sides the same color is good if vacuum can be built up in both directions before visibly decreasing.
Vacuum Check Valve
- Connect a hand vacuum pump to the vacuum (black) side of the valve.
- Apply 16 in. Hg (54 kpa) of vacuum.
- If vacuum remains above 15 in. Hg (50.7 kpa) for 10 seconds, the valve is acceptable.