DESCRIPTION
All passenger cars are equipped with a system for preheating the air entering the carburetor or fuel injection unit.
This device maintains incoming air temperature at a point where the carburetor or fuel injection unit can be calibrated much leaner to reduce hydrocarbon (HC) emissions. This also improves warm-up operations and reduces carburetor icing.
This system consists of an air cleaner assembly, integral air control door, vacuum control temperature sensor, vacuum motor(s) or thermostatic spring, heat shroud (on exhaust manifold), heated air tube and vacuum hoses.
Some models also use additional controls such as vacuum traps, cold weather modulators, vacuum check valves and vacuum delay valves.
Exploded View of Thermostatic Air Cleaner Assembly. Scheme 46
OPERATION
When the temperature of the air entering the air cleaner is less than the calibrated temperature of the temperature sensor, the sensor closes. This allows engine vacuum to operate the vacuum motor.
When engine vacuum is applied to the vacuum motor, the door closes off outside air. Air is then drawn into the air cleaner from around the exhaust manifold.
As air inside the air cleaner warms, the temperature sensor begins to close. This gradually bleeds off vacuum to the vacuum motor. As vacuum to the vacuum motor decreases, the air control door begins to open.
As the air control door opens, outside air is allowed to enter the air cleaner assembly. When the air entering the air cleaner reaches a predetermined temperature, the air control door opens completely. This completely closes off air from around the exhaust manifold.
Thermostatic Air Cleaner Assembly with Temperature Sensor & Vacuum Motor. Scheme 47
VACUUM CHECK VALVE & DELAY VALVE
A vacuum check valve and/or delay valve is used on the thermostatic air cleaner. During cold weather operation, and when engine is under full or hard acceleration, the valve will postpone opening of the air control door to outside air. The engine will continue to receive heated air for a short period of time to improve driveability.
VACUUM TRAP
Some vehicles are equipped with a built-in vacuum trap, designed to hold the air control door in the heated air position during full throttle if outside air temperature is below 70°F. The length of time that the air control door is held closed depends on outside air temperature. The vacuum trap can be identified by a check valve in the small orifice leg of the temperature sensor.
Thermostatic Air Cleaner Assembly Showing Air Flow into Carburetor. Scheme 48
COMPONENT TESTING
Note. See SPECIFICATIONS tables for correct specifications when performing the following tests.
VACUUM CONTROL TEMPERATURE SENSOR
- Tape a thermometer close to the vacuum control temperature sensor located inside the air cleaner. Leave wing nuts off top of air cleaner so that top can be quickly removed to read thermometer during test.
- With engine cold, temperature below vacuum control temperature sensor specifications, check air control door in air cleaner. It should be in fully open position (open to outside air).
- Start engine. As soon as engine starts, door should move to full heated air position (closed to outside air). Watch air control door.
- When door reaches fully open position, quickly remove air cleaner top and read thermometer. Compare thermometer reading with specifications.
- If reading is not to specifications, perform vacuum motor test. If vacuum motor is okay replace sensor.
| Application | Heated Air Temp.: °F (°C) | Fresh Air Temp.: °F (°C) | |
|---|---|---|---|
| Buick | 70 (21) | 162 (72) | |
| Cadillac | 77 (25) | 123 (50) | |
| Chevrolet | |||
| Cavalier & Corvette | 86 (30) | 131 (55) | |
| Chevette | 80 (27) | 100 (38) | |
| All Other Models | 77 (25) | 123 (50) | |
| Oldsmobile | 77 (25) | 123 (50) | |
| Pontiac | 77 (25) | 123 (50) | |
VACUUM CONTROL TEMPERATURE SENSOR SPECIFICATIONS
VACUUM MOTOR TEST
- Remove air cleaner from vehicle. Disconnect vacuum hose from vacuum motor. Apply 20 in. Hg vacuum to motor and pinch off hose.
- Vacuum should not leak down more than 10 in. Hg in 5 minutes. If vacuum motor does leak down, replace it.
- Connect a vacuum pump to vacuum motor. Apply specified amount of vacuum to vacuum motor to open or close heated air door. If door does not open or close at specified vacuum, replace vacuum motor.
| Application | Vacuum In. Hg |
|---|---|
| General Motors | 7 |
AIR CONTROL DOOR OPENING/CLOSING VACUUM