Table of contents: Operating principle ↓ Malfunctions ↓ Checking the intake air heating… ↓ Engines with power from 85 kW ↓
The automatic intake air heater is located on the inlet side of the air filter housing. Its task is to maintain the desired temperature of the air sucked in by the engine.
This is important for the following reason: in cold weather, the combustible mixture is formed better if the intake air is supplied preheated. However, at higher outside temperatures, the engine does not need preheated air: the same amount of warm air has a larger volume than cold air. Since the engine can only take in a certain amount of air, when the intake air preheating is turned on, it receives less air, and the combustible mixture becomes over-rich, the consumption increases. So preheating is necessary for uniform engine operation only at low temperatures.
4-cylinder engines with 74 kW power and Mono-Motronic injection system: in the smaller engines, the intake air preheating is controlled by vacuum. The vacuum is collected at the engine behind the throttle valve and acts on the membrane mechanism of the vacuum advance regulator, which in turn opens the intake air preheating valve in the air filter intake manifold.
Operating principle
In the vacuum line leading to the heated air valve, in the air filter housing, an additional temperature regulator is included, which interrupts the free passage in this sleeve if the outside air temperature reaches a temperature of +35° to +45°C.
Engines with a power of 85 kW and above: in these engines the intake air preheating valve is controlled via a thermostat, which ensures full access of warm air at -20°C. As the temperature rises, the valve slowly closes until it completely blocks the access of warm air at +5°C.
Malfunctions
If the intake air heater does not function as it should, it can cause problems with operation.
In winter:
Poor cold engine performance after starting (at idle) during the warm-up period.
The engine runs unevenly and intermittently.
In the warm season:
Insufficient power, cannot reach maximum speed.
Increased fuel consumption.
Checking the intake air heating (4-cylinder Mono-Motronic engines)
The temperature regulator is located in the air intake housing. Its hose ends are accessible on the engine with the Mono-Motronic injection system from the bottom of the air filter.
Disconnect the sleeve from the brass tip or the tip with the mark.
Use your mouth to draw in air from the sleeve: the hot air valve should open or close with a sound.
If this does not happen: check the valve for ease of movement, the sleeve and membrane mechanism of the vacuum ignition advance regulator for leaks.
Then check the temperature controller.
During the check, the air temperature should be below +20°C.
Remove the end of the hose to the air damper at the temperature regulator or at the hot air valve.
Remove the hose tip to the engine on the fuel mixture preparation unit or on the throttle body.
Use your mouth to draw in air from the sleeve: the sleeve should be "passable".
Use a hair dryer to heat the temperature regulator to more than 45°C. Again, suck air strongly from the sleeve: it should now be closed.
Replace the temperature controller.
Engines with power from 85 kW
Remove the air intake sleeve behind the headlight cap.
Look into the hose nozzle, and use a flashlight if necessary.
Depending on the ambient temperature (see previous section) the air damper must close the access of cold or hot air.
(The original text is available on the website: «AudiManual.ru»)