See Figure 1
The spark timing control system is designed to control the distributor vacuum advance under various driving conditions in order to reduce HC and NOx emissions. With the water temperature below 59°F (15°C) and above 122°F (50°C) the thermal vacuum valve is closed and the spark timing control system is activated. When the temperature is between the two temperatures, the valve is open and the system is not activated. Some early models are equipped with a vacuum delay valve, but all systems operate basically the same.
TESTING & INSPECTION
- Ensure that the vacuum hoses are properly connected and the distributor vacuum controller and linkage functions properly.
- Connect a timing light and start the engine when it is cold.
Check the spark timing when the system is activated.
Water temperature below 59°F (15°C)-the thermal vacuum valve is closed and the spark timing control system is activated
- If the timing does not change as specified, check the thermal vacuum valve.