Table of contents: 4-cylinder engines (Mono-Motronic) ↓ Checking the intake manifold heating ↓
4-cylinder engines (Mono-Motronic)
From the Mono-Motronic system's fuel mixture preparation unit, the air and fuel mixture enters the cylinders through the intake manifold. If the intake manifold is cold, a significant portion of the mixture condenses on the inner walls of the manifold and enters the combustion chambers not quite in the form of vapor. This misfortune can be helped by double heating of the intake manifold.
When the engine is cold, the electric heating of the intake manifold comes into operation. It consists of an electrically heated "hedgehog" that is screwed into the intake manifold below the fuel mixture preparation unit. Our "hedgehog" has only the needles in common with the natural animal. In the Audi-100, they serve to increase the surface of the heating element. This "hedgehog" heating only works up to a coolant temperature of approximately 65°C. Then the current is switched off.
The thermal switch (designated as "coolant temperature sensor - G62" on electrical diagrams) does not turn on the "hedgehog" directly. G62 supplies the Mono-Motronic control unit with temperature information, and the latter, via a relay in an additional relay block under the instrument panel, gives the order to supply power to the "hedgehog".
At higher temperatures, the intake manifold is heated by the coolant. To do this, an additional amount of the total coolant flow is directed through a special channel in the intake manifold.
Left: The intake air preheating temperature controller is located in the air intake housing on the 74 kW engine; from the outside, the thermostat can be easily identified by the connections of the sleeve to the hot air valve (1) and to the combustible mixture preparation unit (2).
Right: for heating the fuel-air mixture in the air intake sleeve of a 74 kW engine (with Mono-Motronic injection system) the "hedgehog" (1) is mounted. The connecting wire is shown at position 2.
Left: The variable intake manifold flaps of the 2.8-liter V6 engine are actuated by a diaphragm mechanism, indicated by the arrow.
Right: This "sequential intake manifold changeover valve" (2) determines whether rarefied air can enter the diaphragm mechanism. The valve is located on the right rear of the engine compartment, below the air flow meter (1).
Checking the intake manifold heating
Checking the "hedgehog": on a cold engine, disconnect the wire leading to the "hedgehog" under the intake manifold, using the plug connection (it is installed on the side of the fuel mixture preparation unit).
Connect an ohmmeter to the "hedgehog" and to the "ground" on the cable: if it shows 0.25-0.50 Ohm, then the "hedgehog" is OK.
If you don't have an ohmmeter, you should remove the fuel mixture preparation unit when the engine is cold, turn on the ignition and check with a wet finger whether the "hedgehog" needles are heating up.
If the "hedgehog" does not heat up, despite the correct resistance readings, the reason may be a faulty relay.
Checking the thermal switch: (see chapter "Mono-Motronic injection system").
Checking the relay. (Chapter "Body electrical system").