The heater core on a Mustang is located in the evaporator case adjacent to the evaporator core, separated by a wall and an air door. It does not use a heater-control valve; it is controlled within the vehicle by the amount of opening of the air door. This door is controlled by a vacuum motor on earlier models and by an electrical servo (motor) on later-model vehicles. In essence, regardless of the method of activation, they work the same.
Raise the hood and check the radiator water level. Fill as necessary. Leave the cap off the radiator for the moment.
Start the engine and allow it to warm up to normal operating temperature. This test is to identify at what temperature the thermostat opens and if it is working properly. When the car is started, there should be no water circulating (look in the radiator filler neck to check). When the thermostat opens, water can be seen flowing through the cores of the radiator and the water level will drop slightly. As soon as this happens, the temperature gauge in the car should be checked. Use the temperature probe to cross-check the gauge for correct indication.
Check the block and cylinder head temperature as the engine warms up (use the temperature probe). Check the inside temperature gauge to see if it matches the temperature probe. When the thermostat opens, check the temperature of the water in the radiator by probing the water in the radiator filler neck.
Inspect the radiator filler neck to see if water is flowing as soon as the engine is started. If so, this indicates that the thermostat is stuck open. When this happens, the engine will not warm up properly and this will cause the heater to fail to warm up sufficiently to supply heat.
Check the heater hoses for crimps that would restrict the flow of water to the heater core. Repair as necessary. As the engine warms up, feel the heater hoses to see if they are getting hot to the touch. If not, there is a restriction in one or both of the hoses. Shut the engine off and let it cool. Remove the hoses and check for debris. If one hose is hot and one is cold, the heater core has a restriction and should be replaced.
Check the operation of the interior fan when the engine has reached normal operating temperature. Make sure the fan operates on all speeds. If it does not operate on all speeds, but does operate, the blower motor resister is bad.
Check the vents for proper opening on all levels as the control panel is manipulated through the different positions. It the doors fail to operate and the air will not come out of all the different vents, the control panel or door operating motor is bad.
