GM Firebird 1967-1981 Repair Guide

Thermostatic Air Cleaner (THERMAC)


See Figures 1 and 2

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Fig. Fig. 1: Cutaway view of a typical THERMAC air cleaner and vacuum diaphragm motor

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Fig. Fig. 2: Diagram of the 3 modes of operation of the THERMAC vacuum motor

All late model engines utilize the THERMAC system (in 1978 it was called TAC, but was the same). This system is designed to warm the air entering the carburetor when underhood temperatures are low, and to maintain a controlled air temperature into the carburetor at all times. By allowing preheated air to enter the carburetor, the amount of time the choke is on is reduced, resulting in better fuel economy and lower emissions. Engine warm-up time is also reduced.

The Thermac system 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 case snorkel allows air to be drawn in from the heat stove (cold operation) or from underhood air (warm operation). The door position is controlled by the vacuum motor, which receives intake manifold vacuum as modulated by the temperature sensor.


  1. Check the vacuum hoses for leaks, kinks, breaks, or improper connections and correct any defects.
  3. With the engine OFF, check the position of the damper door within the snorkel. A mirror can be used to make this job easier. The damper door should be open to admit outside air.
  5. Apply at least 7 in. Hg of vacuum to the damper diaphragm unit. The door should close. If it doesn't, check the diaphragm linkage for binding and correct hookup.
  7. With vacuum still applied and the door closed, clamp the tube to trap the vacuum. If the door doesn't remain closed, there is a leak in the diaphragm assembly.