Intake Air Preheating

The automatic intake air preheating device is located on the inlet side of the air filter housing. Its function is to maintain the required temperature of the air drawn in by the engine.

This is important for the following reason: in cold weather the fuel mixture forms better if the intake air is preheated. However, at higher outside temperatures the engine does not need preheated air: the same quantity of warm air has a larger volume than cold air. Since the engine can only draw in a certain volume of air, when intake air preheating is active it receives less air, the fuel mixture becomes too rich and fuel consumption increases. Therefore, preheating is necessary for smooth engine operation only at low temperatures.

Operating Principle
Models with 4-cylinder engines: on smaller engines, intake air preheating is controlled by vacuum. Vacuum is taken from the engine behind the throttle valve and acts on the diaphragm of the vacuum ignition timing advance unit, which in turn opens the warm air flap valve in the air filter intake connection.

A temperature regulator is additionally installed in the vacuum line leading to the warm air valve, inside the air filter housing. It interrupts the vacuum line when the outside air temperature reaches +35 to +45°C (Mono-Motronic) or +20 to +30°C (Digifant).

Malfunctions
Models with 5- and 6-cylinder engines: on these engines the warm air valve is controlled by a thermostat, which provides full access of warm air at –20 °C. As the temperature rises, the valve slowly closes until, at +5 °C, it completely shuts off access of warm air.

If intake air preheating does not function correctly, operational problems may occur. In winter:
Poor engine operation after starting, during the so-called warm-up period.

The engine runs unevenly, with misfires.

In warm weather:
Insufficient power, maximum speed cannot be reached.

Increased fuel consumption.