1978 Models
- With engine at normal operating temperature, attach tachometer and timing light to engine. Remove rubber cap from distributor vacuum retard unit. See Figure.
- With rubber cap removed, timing should advance. Reinstall cap and recheck timing. Timing should retard. If timing remains advanced with cap in place, replace thermo valve.
- With rubber cap still in place, cover radiator to increase engine coolant temperature. With temperature above 203°F (95°C), timing should advance. If ignition timing does not advance, replace thermo valve.