This system consists of a heat shroud which is integral with the right side exhaust manifold, a hot air hose and a special air cleaner assembly equipped with a thermal sensor and a vacuum motor and air valve assembly.
The thermal sensor incorporates an air bleed valve which regulates the amount of vacuum applied to the vacuum motor, controlling the air valve position to supply either heated air from the exhaust manifold or air from the engine compartment.
During the warm-up period when underhood temperatures are low, the air bleed valve is closed and sufficient vacuum is applied to the vacuum motor to hold the air valve in the closed (heat on) position.
As the temperature of the air entering the air cleaner approaches approximately 115°F (46°C), the air bleed valve opens to decrease the amount of vacuum applied to the vacuum motor. The diaphragm spring in the vacuum motor then moves the air valve into the open (heat off) position, allowing only underhood air to enter the air cleaner.
The air valve in the air cleaner will also open, regardless of air temperature, during heavy acceleration to obtain maximum air flow through the air cleaner.
Non Vacuum-Operated Air Door Test
See Figure 1
- Unfasten the temperature sensing valve and snorkle assembly from the air cleaner. Place it in a container of cold water. Make sure that the thermostat is completely covered with water.
- Place a thermometer, of known accuracy, in the water. Heat the water slowly and watch the temperature.
- At 105°F or less, the door should be closed (manifold heat position).
- Continue heating the water until it reaches 130°F. The door should be fully open to the outside air position.
- If the door does not open at or near this temperature, check it for binding or a detached spring. If the door doesn't open or close properly, the sensor is defective and must be replaced.
This usually requires that the entire snorkle assembly must be replaced.
Vacuum-Operated Air Door Test
See Figure 2
- Either start with a cold engine or remove the air cleaner from the engine for at least half an hour. While cooling the air cleaner, leave the hood open.
- Tape a thermometer, of know accuracy, to the inside of the air cleaner so that it is near the temperature sensor unit. Install the air cleaner on the engine but do not fasten its securing nut.
- Start the engine. With the engine cold and the outside temperature less than 90°F, the door should be in the `heat-on' position (closed to outside air).
Due to the position of the air cleaner on some cars, a mirror may be necessary when observing the position of the air door.
- Operate the throttle rapidly to 1 / 2 - 3 / 4 of its opening and release it. The air door should open to allow outside air to enter and then close again.
- Allow the engine to warm up to normal temperature. Watch the door. When it opens to the outside air, remove the cover from the air cleaner. The temperature should be over 90°F and no more than 130°F; 115°F is about normal. If the door does not work within these temperature ranges, or fails to work at all, check for linkage or door binding.
If there is no binding and the air door is not working, proceed with the vacuum test below. If these indicate no faults in the vacuum motor and the door is not working, the temperature sensor is defective and must be replaced.
Vacuum Motor Test
Be sure that the vacuum hose that runs between the temperature switch and the vacuum motor is not pinched by the retaining clip under the air cleaner. This could prevent the air door from closing.
- Check all of the vacuum lines and fittings for leaks. Correct any leaks. If none are found, proceed with the test.
- Remove the hose which runs from the sensor to the vacuum motor. Run a hose directly from the manifold vacuum source to the vacuum motor.
- If the motor closes the air door, it is functioning properly and the temperature sensor is defective.
- If the motor does not close the door and no binding is present in its operations, the vacuum motor is defective and must be replaced.
If an alternate vacuum source is applied to the motor, insert a vacuum gauge in the line by using a T-fitting. Apply at least 9 in.Hg of vacuum in order to operate the motor.