Intake air heating (Audi 80 B4)

The automatic intake air preheating unit is located on the inlet side of the air filter housing. The air preheating unit is designed to provide the engine with air at a temperature optimal for normal combustion of the mixture. In cold weather, the combustion of the air-fuel mixture is more efficient if the air entering the combustion chamber is preheated. At the same time, at high air temperatures, there is no need to preheat it. On the other hand, preheated air is larger in volume than the same amount of cold air. Since the engine can only take in a certain volume of air, when the preheating is on, it takes in less air and the air-fuel mixture is richer. This means that fuel consumption increases. This is necessary for smooth engine operation at low ambient temperatures.

In four-cylinder engines, the intake air is heated by means of low air pressure generated behind the throttle valve. From there, through a corresponding hose, this low pressure acts on a diaphragm valve, which opens the flap for supplying heated air in the intake manifold (see illustration 6.0).



The intake air temperature sensor on the air filter housing is connected to the warm air supply flap circuit and shuts off the warm air supply at an ambient air temperature of +35 - +45°C (mono-Motronic injection system) or at +20 -+30°C (digifant injection system) (see illustration 6.0a).





The position of the warm air supply flap in cars with five- and six-cylinder engines is regulated by a thermostat, which ensures full opening of the flap at ambient temperatures of -20°C. As the ambient temperature increases, the warm air supply flap gradually closes. It closes completely at an ambient temperature of +5°C.

If the air heater does not function properly, this can lead to engine malfunctions. In the cold season, this is expressed in the following:

• poor idle speed of a cold engine after starting it in the warm-up phase;
• poor engine transition to another operating mode. The engine "chokes".

In the warm season

• reduction in effective engine power. The car does not accelerate to its maximum speed;
• increased fuel consumption.

To check the air heating on vehicles with four-cylinder engines:

1. Remove the hose from the brass or knurled fitting.

2. Blow through the hose with your mouth. You will hear the valve open on exhalation and close on inhalation. If the valve does not work this way, check its ease of movement, and the hose and membrane valve for leaks. If everything is normal, the next step is to check the intake air temperature sensor itself.



3. Disconnect the hose from the throttle valve fitting. Blow air through the hose with your mouth. The hose should not be blocked.

4. Heat the intake air temperature sensor with a hair dryer to a temperature above 45°C.

5. Blow out the hose. It should be blocked. Otherwise, replace the intake air temperature sensor.

Air heating in cars with five- and six-cylinder engines is checked as follows.

6. Remove the air supply hose on the air cleaner cover.

7. Shine a flashlight into the intake manifold. Depending on the ambient air temperature, the damper should open or close access to heated air (see illustration).