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
- 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
- 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
- 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.
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.