Thermac System

Written by: Mike Ervin

If you are still getting crappy gas mileage and loss of power even after you have done the normal tune-up things.  The Thermac system just might be the problem, it was on my truck.  The thermostatic air cleaner (Thermac) system uses a damper assembly in the air cleaner inlet, controlled by a vacuum motor to mix preheated and cold air entering the air cleaner.  This is necessary to maintain a controlled air temperature into the carburetor.  The vacuum motor is controlled by a temperature sensor in the air cleaner.  The preheating of the air cleaner inlet air allows leaner carburetor and choke settings, while maintaining good driveability.

If you live in an extremely cold climate you need this system.  Where I live it doesn't get all that cold so I removed the air door from the snorkel all together.  I wanted all the air flow that I could get and still use the factory air cleaner assembly.  I also converted mine into a dual snorkel, but that is another story.  Removing the Thermac system will cause your engine to take longer to warm up, so be prepared for this if you decide to do this.


  • Either start with a cold engine or remove the air cleaner from the engine and allow to cool for at least 30 minutes.  While cooling the air cleaner, leave the hood open.
  • Tape a thermometer inside of the air cleaner so that it is near the temperature sensor unit.  Install the air cleaner cover but don't put on the wing nut.
  • Start the engine.  With the engine cold and the outside temperature less than 90*F, the air door inside the snorkel should be closed to outside air.
  • Operate the throttle lever rapidly to 1/2-3/4 WOT 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 air cleaner cover, 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 binding is not present and the air door is not working, proceed with the vacuum tests below.  If these indicate no faults in the vacuum motor and the door is not working, the temperature sensor is bad and must be replaced.

Vacuum Motor Test

  • 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.
  • Check all the vacuum lines and fittings for leaks.  Fix any leaks.  If you don't find any, let's do the test.
  • Remove the hose which runs from the sensor to the vacuum motor.  Run a hose directly from a manifold vacuum source or use a hand held vacuum pump to the vacuum motor.
  • If the motor closes the air door, it is functioning properly and the temperature sensor is bad.
  • If the motor does not close the door and no binding is present in its operation, the vacuum motor is bad and must be replaced.

Vacuum Diaphragm Motor

If you need to replace the vacuum motor, follow the steps below.

  • Remove the air cleaner and disconnect the vacuum hose from the motor.
  • Drill out the two spot welds with a 1/16 in. drill bit, then enlarge the hole as required to remove the retaining strap.  Be careful no to damage the snorkel tube.  Remove the retaining strap.
  • Lift up motor, cocking it to one side to unhook the motor linkage at the control damper assembly.

Installation of new motor.

  • Drill a 7/64 in. hole in the snorkel tube at center of vacuum motor retaining strap.  Reattach the vacuum motor linkage into the control dampener.  Reattach the vacuum hose to the vacuum motor.

Temperature Sensor

Follow the steps below to change the temperature sensor.

  • Remove the air cleaner and disconnect the vacuum hoses at the temperature sensor.
  • Pry up tabs on temperature sensor retaining clip.  Remove clip and temperature sensor from the air cleaner.  Note the position of the temperature sensor before removing.

Installation of new sensor.

  • Install the temperature sensor in the same position as when removed.  Install retaining clip tabs into position.  Reconnect the vacuum hoses to the temperature sensor.