1977 Models

With engine coolant temperature below 68°F (20°C), start engine and allow it to idle. Disconnect Red stripe vacuum hose from mixture control valve.
Place hand under mixture control valve and reconnect vacuum hose. See Fig 2. No suction should be felt. If suction is felt, thermo valve or mixture control vale is defective and should be replaced.

Fig 1: Mixture Control Valve System (1978-79)
Courtesy of CHRYSLER GROUP, LLC
With engine coolant temperature above 68°F (20°C), disconnect vacuum hose from mixture control valve. Place hand under mixture control valve and reconnect vacuum hose.
A strong suction should be felt. If no suction is felt, thermo valve or mixture control valve is defective and should be replaced.

Fig 2: Testing Mixture Control Valve System (1977)
Courtesy of CHRYSLER GROUP, LLC