Mercedes C-Class

W203 since 2000 of release

Repair and operation of the car

+ Mercedes-Benz cars of class C (W-203)
+ Operation manual
+ Routine maintenance
+ Engine
- Cooling system and heating
   - Cooling system
      Antifreeze - a security measure from freezing
      Replacement of cooling liquid - general comments
      Removal and replacement of hoses of the cooling system
      Removal and installation of the thermostat
      Check of the thermostat
      Check of the cooling system under pressure
      Removal and installation of the pump of cooling liquid
      Removal and installation of the fan
      Removal and installation of a radiator
   + Systems of ventilation, heating and air conditioning
+ Power supply system and release
+ Engine electric equipment
+ Manual transmission
+ Automatic transmission
+ Coupling and power shafts
+ Brake system
+ Suspension bracket and steering
+ Body
+ Onboard electric equipment


Check of the thermostat

The thermostat at warming up of the engine puts a big contour of cooling into operation.

At the same time the thermostat continuously extends, from the and size to the b size.


1. Before removing and checking operability of the thermostat, be convinced that problems with cooling of the engine arose not owing to insufficient amount of cooling liquid or a weak tension of a driving belt. Be convinced of serviceability of the indicator located on the control panel.
2. If you noticed that the engine long gets warm up to the working temperature (according to indications of the indicator), the thermostat, perhaps, is jammed in open situation. In this case the thermostat should be replaced.
3. On the other hand, long warming up can be a consequence of lack of the thermostat. It could be made specially or inadvertently (forgot to establish after repair). Do not use the car without thermostat. The control system of the engine will keep the engine in the warming up mode longer time. During this time the engine will consume bigger amount of fuel, and the fulfilled gases will contain bigger amount of harmful impurity.
4. If the engine overheats, touch the top hose of a radiator. If it not hot, but the engine is overheated, the thermostat, perhaps, jammed in a closed position. Liquid does not get to a radiator and circulates on a small circle. In this case replace the thermostat.
5. If the top hose of a radiator hot, it means that the thermostat is open and liquid normally circulates on system. In this case study the recommendations provided in the section "Diagnostics of Malfunctions" of Introduction.



1. Remove the thermostat case.

2. Check position of the thermostat through a connecting branch pipe in the case. Measure the and size on the thermostat.

3. If the thermostat at the room temperature is abroach — it is faulty and is subject to replacement.
4. For rough check of operability of the thermostat suspend its case on a piece of a twine in a vessel with water. The case of the thermostat should not concern vessel walls. Begin to heat a vessel.
5. In process of water heating the thermostat has to open (it occurs just before water boiling). As temperature of boiling of water of 100 °C, the thermostat can not open completely (address Specifications). Take the thermostat and let's it cool down. In process of cooling it has to be closed completely.
6. If the thermostat does not open when heating or is not closed when cooling, replace it.
7. At installation of the thermostat keep in mind the following:

a) Carefully clear the interfaced surfaces and replace a sealing ring.
b) Place the thermostat in a casing, having correctly oriented.
c) Tighten bolts of fastening of a cover, having made the required effort.
d) Establish all hoses on former places then fill in cooling liquid (address the Head Routine maintenance).
e) Start the engine, warm up up to the working temperature and be convinced of lack of leaks.