Vacuum diaphragm controlling the valve regulating the temperature of the air entering the engine
The vacuum diaphragm is indicated by the arrow.
Location of the thermal valve in the air filter housing
The thermal valve is indicated by the arrow.
On engines with a carburetor, the thermostat maintains the temperature of the air entering the air filter within 25–35°C.
The damper in the air intake pipe provides the supply of hot or cold air. The damper is controlled by a vacuum diaphragm, which reacts to the vacuum in the intake manifold. The supply of vacuum to the damper is regulated by a thermovalve located in the air filter housing.
Examination
1. You should start checking the thermostat on a cold engine. When the engine is off, the thermostat flap should be completely closed. The flap position can be determined both from the filter side and from the air intake side.
2. Start the engine and check that the damper opens fully at idle speed, opening the supply of air heated by the exhaust manifold to the air filter.
3. If the thermostat flap remains closed after starting the engine, the thermostat and membrane should be checked.
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