Contents: All engines except F3P 🠛 F3P engine 🠛
Note: This procedure only applies to engines with a carburetor or central fuel injection system.
1. For the engine to operate efficiently, the temperature of the air entering the intake system must be maintained at a certain level. (This does not apply to engines with a distributed fuel injection system, where the system compensates for changes in air temperature).
2. The air filter has two air sources, one is outside the engine compartment (cold air), and the other one - from the casing on the exhaust manifold (heated air). The wax thermostat controls the damper inside the inlet duct. When the ambient air temperature is below a certain level, the flap directs warm air away from the exhaust manifold housing. As the ambient temperature rises, the damper opens and begins to let in cold air from outside the car.
3. The temperature control system is shown in the accompanying illustrations (see illustrations).
25.3a Air temperature control system - C- and E-series engines: A - Cold inlet air; B - Hot inlet air; C - Control valve; D - To the carburetor; 2 - Thermostat.
25.3b Air temperature control system - F3P engine:A - To the vacuum capsule; B - From the intake manifold; 1 - Exhaust manifold; 2 - Intake manifold; 3 - Air intake cover; 5 - Air filter; 6 - Thermostat/vacuum capsule; 7 - Cold inlet air; 8 - Hot inlet air; 9 - Manifold vacuum connection.
All engines except F3P
4. To test the system, remove the air cleaner housing as described in the relevant section of Section 4, separate the temperature control unit, and immerse it in cold water (temperature no higher than 20°C). After no more than 5 minutes, the damper should close the cold air duct. Now increase the water temperature to 40°C, the damper should close the hot air channel. If the damper is not working properly, first check that it is not stuck. It is not possible to adjust the damper; in case of malfunction, the unit should be replaced.
F3P engine
5. This system is distinguished by the fact that the position of the valve is determined not only by temperature, but also by the vacuum in the intake manifold. The combined thermostat and vacuum capsule unit in the air filter inlet duct closes the cold air duct at temperatures below -10°C and closes the hot air duct at temperatures above +20°C. However, at high vacuum in the manifold (more than 200 mbar) the cold air duct will be closed again at temperatures up to 35°C. At temperatures above 35°C, the vacuum supply to the capsule is interrupted by a thermostat valve installed in the air intake cover, and the hot air duct closes (see illustration).
25.5 Schematic diagram of a temperature-controlled vacuum valve: A - To the vacuum capsule; B - From the intake manifold; C - Valve ball; D - Bimetallic plate.
6. Checking this system is very difficult for a car enthusiast; start by inspecting all vacuum hoses.
7. The thermostat and vacuum capsule can be checked as described for the central injection system, but to check the operation of the unit at low temperatures, place it in the freezer (see illustration), and to check its operation at high temperatures, warm it up with a hair dryer.
25.7 Thermostat/Vacuum Capsule Schematic (6): A - From the vacuum valve; B - Thermostat.
8. To check the operation of the thermostat valve, disconnect the hose from the manifold side. Attach a piece of plastic or rubber hose to the valve and create a vacuum in it using a hand pump. When the valve temperature is below 35°C, the vacuum should move the flap, closing the cold air channel. At higher temperatures, the vacuum should have no effect. The movement of the damper cannot be seen with the air filter installed, but it can be heard.