GM Full Size Vans 1967-1986 Repair Guide

Thermostatic Air Cleaner



The use of carburetor heated air dates back to 1960 when it was first used on heavy trucks.

This system is designed to warm the air entering the carburetor when underhood temperatures are low. This allows more precise calibration of the carburetor.

The thermostatically controlled air cleaner is composed of the air cleaner body, a filter, sensor unit, vacuum diaphragm, damper door and associated hoses and connections. Heat radiating from the exhaust manifold is trapped by a heat stove and is ducted to the air cleaner to supply heated air to the carburetor. A movable door in the air cleaner snorkel allows air to be drawn in from the heat stove (cold operation) or from the underhood air (warm operation). Periods of extended idling, climbing a grade or high speed operation are followed by a considerable increase in engine compartment temperature. Excessive fuel vapors enter the intake manifold causing an over-rich mixture, resulting in a rough idle. To overcome this, some engines may be equipped with a hot idle compensator.


  1. 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 engine compartment hood open.
  3. Tape a thermometer, of known 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.
  5. Start the engine. With the engine cold and the outside temperature less than 90°F (32°C), the door should be in the heat on position (closed to outside air).

Due to the position of the air cleaner on some trucks, a mirror may be necessary when observing the position of the air door.

  1. Operate the throttle lever 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.
  3. 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 (32°C) and no more than 130°F (54°C). Normal temperature is about 115°F (46°C). If the door does not work within these temperature ranges, or fails to work at all, check for linkage or door binding.

If binding is not present and the air door is not working, proceed with the vacuum tests, given 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

Be sure that the vacuum hose which 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.

  1. Check all of the vacuum lines and fittings for leaks. Correct any leaks. If none are found, proceed with the test.
  3. 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.
  5. If the motor closes the air door, it is functioning properly and the temperature sensor is defective.
  7. If the motor does not close the door and no binding is present in its operation, 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 (62 kPa) of vacuum in order to operate the motor.