Reference Manual

Chapter 10: Emissions Equipment: THERMAC


A heated intake air system is used to give good driveability under varying climatic conditions. By having a uniform air inlet temperature, the fuel system can be calibrated to reduce exhaust emissions and to eliminate throttle valve icing.


The THERMAC air cleaner operates by heated air and manifold vacuum. Air can enter the air cleaner from outside the engine compartment or from a heat stove built around the exhaust manifold. A vacuum diaphram motor, built into the air cleaner snorkle, moves a damper door, to admit hot air from the exhaust manifold, outside air, or a combination of both. Inside the air cleaner is a temperature sensor that reacts to air intake temperature and controls the amount of vacuum going to the motor.

Thermostatic Air Cleaner - "VIN 9"

The "VIN 9" applications use a dual-snorkle air cleaner assembly. The added snorkle contains a vacuum motor that is operated directly by intake manifold vacuum. This vacuum motor will start to open at 15.2 kPa (4.5" Hg) and be fully open at 5 kPa (1.5" Hg). The other components of this air cleaner assembly are the same as those used on the "VIN Y" engine.

Charcoal Absorber

The 5.0L, "VIN 9" application uses an air cleaner assembly containing an activated charcoal absorber. It will be permanently mounted inside of the area that contains the air filter element. The absorber acts as a barrier preventing fuel vapors from passing through the air cleaner element and then into the atmosphere.

Results of Incorrect THERMAC Operation

Hesitation during warm-up can be caused by:

Lack of power, sluggish, pinging or spongy, on a hot engine can be caused by:

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