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 Thermostatic system is composed of the air cleaner body, a filter, sensor unit, vacuum diaphragm, damper door, associated hoses and connections. Heat radiating from the exhaust manifold is trapped by a heat stove and is ducted to the air cleaner supplying heated air to the carburetor. A movable door is the air cleaner case snorkel allows air to be drawn in from the heat stove (during cold engine operation) or from underhood air (during warm engine operation). The door position is controlled by the vacuum motor, which receives intake manifold vacuum as modulated by the temperature sensor.
Thermostatic Air Cleaner
See Figure 1
- Check the vacuum hoses for leaks, kinks, breaks, or improper connections and correct any defects.
- 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.
- Apply at least 7 in. Hg (48 kPa) of vacuum to the damper diaphragm unit. The door should close. If it doesn't, check the diaphragm linkage for binding and correct hookup.
- 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.
- If the diaphragm is holding vacuum and the system has source vacuum but the door still doesn't remain closed, the last choice is the air bleed valve located in the air cleaner assembly.
- The air bleed may be holding the damper door closed even after the vehicle has warmed up. Or the valve may not close the door at all. In any case, if the damper diaphragm and vacuum source are OK, and the door still remains closed, replace the air bleed valve.
The Thermostatic Air Cleaner (TAC) thermal valve was introduced in 1975 and is located on the air cleaner housing. When the engine is cold, the valve restricts the vacuum supplied to the air cleaner door vacuum motor, which slows the operation of the door down. Under wide-open throttle conditions, the door does not jump to the full cold air position, thus eliminating the flat spot that occurs during cold engine acceleration.