1977-78 Models
When engine coolant temperature reaches 203°F (95°C), a thermo valve (located at water jacket of intake manifold), opens automatically to relieve intake manifold vacuum, being applied on the retard side of distributor vacuum control unit, to the atmosphere to suspend the spark timing retard. The thermo valve returns to the closed position as coolant temperature becomes lower than 203°F (95°C).
The thermo valve contains two valves which open and close according to coolant temperature. One valve controls intake manifold vacuum to distributor and the other controls EGR operation.