This is important for the following reason: in cold weather, the combustible mixture is formed better if the intake air is supplied heated. However, at higher outside temperatures, the engine does not need to be supplied with 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 on, it receives less air, and the combustible mixture becomes too rich, the consumption increases. So preheating is necessary for uniform engine operation only at low temperatures.
74 kW 4-cylinder engines with Mono-Motronic injection system: Smaller engines have vacuum-controlled intake air preheating. The vacuum is collected at the engine behind the throttle and acts on the membrane mechanism of the vacuum ignition timing controller, which in turn opens the intake air heating valve in the air filter inlet pipe.
Principle of operation
In the vacuum pipeline leading to the heated air valve, in the air filter housing, a temperature regulator is additionally included, which interrupts the free passage in this sleeve if the outside air temperature has reached a temperature of +35°to +45°C.
Engines from 85 kW: for these engines, the intake air heating 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 it completely cuts off the warm air at +5°C.
Faults
If the intake air heating does not function as it should, it is fraught with interruptions in operation.
Winter:
Poor operation of a cold engine after starting (at idle) during the warm-up period.
The engine runs unevenly, intermittently.
During the warm season:
Insufficient power, you can not develop 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 sleeve tips are accessible on the engine with Mono-Motronic injection from the underside of the air filter.
Disconnect the sleeve from the brass tip or the tip with a mark.
Use your mouth to forcefully draw air from the sleeve: the hot air damper should open or close with a sound.
If this does not happen: check the valve for ease of movement, the sleeve and the membrane mechanism of the vacuum ignition timing regulator for tightness.
Then check the temperature controller.
When checking, the air temperature must be below +20°C.
Remove the sleeve tip to the air damper at the temperature controller or hot air valve.
Remove the tip of the hose to the engine on the block for preparing a combustible mixture or on the body of the throttle assembly.
Use your mouth to strongly draw air from the sleeve: the sleeve must be «passable».
Heat the temperature regulator with a hair dryer to over 45°C. Again strongly draw air from the sleeve: now it should be closed.
Replace temperature controller.
Motors from 85 kW
Remove the air intake sleeve behind the headlamp cover.
Look into the nozzle of the sleeve, if necessary, highlight with a flashlight.
Depending on ambient temperature (see previous section) the air damper must block the access of cold or hot air.
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