Subaru Coupes/Sedans/Wagons 1985-1996 Repair Guide

Hot Air Control System

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See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Hot air control system

The hot air control system is used on carbureted vehicles only. The purpose of the system is to reduce HC emissions and improve engine performance during warm-up. This is accomplished by deflecting either cool outside air or warm engine heated air into the carburetor, depending upon engine operating conditions.

The system works automatically be means of a temperature sensor and vacuum motor. The temperature sensor detects the inlet air temperature, and controls the flow of vacuum to the vacuum motor. Together they regulate the air control valve, mounted in the air horn of the air cleaner.

The possible combinations of inlet (underhood) air temperatures and vacuum readings, and the resulting valve operation, are as follows:



Underhood temperature below 100°F (38°C) with vacuum below 1.57 in. Hg (5.30 kPa) at motor diaphragm: valve closed and cold air admitted.
 
Underhood temperature below 100°F (38°C) with vacuum above 6.30 in. Hg (21.27 kPa) at motor diaphragm: valve closed and hot air admitted.
 
Underhood temperature between 100-127°F (38-53°C) with varying vacuum at motor diaphragm: valve open and hot/cold air mixture admitted.
 
Underhood temperature above 127°F (53°C) with no vacuum at motor diaphragm: valve open and cold air admitted.
 

TESTING



Air Control Valve

See Figure 2

Click image to see an enlarged view

Fig. Fig. 2: Using a mirror to inspect the air control valve assembly

  1. If the car is running, turn off the engine.
  2.  
  3. Place a mirror at the end of the air cleaner assembly.
  4.  
  5. Inspect the position of the air control valve. The proper position is with the fresh air vent open and the hot air inlet closed.
  6.  
  7. If the position is not as described, check the air control valve linkage for any sticking.
  8.  

Vacuum Motor
  1. Keeping the inspection mirror in position, make sure the engine is off.
  2.  
  3. Remove the vacuum hose from the vacuum motor assembly.
  4.  
  5. Connect a separate piece of the same diameter and type of hose to the now vacant vacuum motor. Insert the other end of the new hose into your mouth and draw in a breath, creating a vacuum in the vacuum motor.
  6.  
  7. Check the position of the air control valve. The fresh air vent should be closed and the hot air inlet open.
  8.  
  9. Now, pinch the hose attached to the vacuum motor so that the vacuum is not instantly lost. The valve position should be maintained for more than 30 seconds.
  10.  
  11. If the motor does not function properly, it should be replaced.
  12.  

Temperature Sensor

WARNING
Engine must be cold before starting this test.

  1. Keep the mirror in position, as described above.
  2.  
  3. Start the engine, and keep it idling.
  4.  
  5. Check the position of the air control valve immediately after starting. The fresh air vent should be closed and the hot air inlet open.
  6.  
  7. Continue to watch the air control valve as the engine warms up to normal operating temperature. The fresh air vent should gradually open.
  8.  
  9. If the sensor does not function properly, replace it.
  10.  

REMOVAL & INSTALLATION



Temperature Sensor
  1. Remove the air cleaner cover and filter assembly.
  2.  
  3. Using a pair of pliers, flatten the clip securing the vacuum hose to the temperature sensor and remove the hose.
  4.  
  5. Now pull the same clip completely away from the sensor. Lift the sensor off the air cleaner.
  6.  

The gasket between the sensor and air cleaner is glued to the air cleaner and should be removed.

  1. Remove and discard the gasket. Clean both mating surfaces.
  2.  

To install:

Always install a new clip when the temperature sensor is reinstalled.

  1. Install a new gasket to the air cleaner surface.
  2.  
  3. Install the sensor and attach a new clip.
  4.  
  5. Install the air cleaner cover and filter.
  6.  

Vacuum Motor
  1. Remove the retainer screws securing the vacuum motor to the air cleaner assembly.
  2.  
  3. Disconnect the vacuum motor valve shaft from the air control valve, and remove the vacuum motor from the air cleaner.
  4.  

To install:
  1. Install the vacuum motor valve shaft on the air control valve.
  2.  
  3. Install the vacuum motor on the air cleaner and tighten the screws securely.
  4.  

 
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