Hi folks, if you know could you please explain what is the meaning of the temperature rating of the thermostat. What is the correct one for the W124 2.6L engine? For example these are some of the ratings I have seen 192degree, 180degree, 167degree. Why there are differences? What is the physics behind it? thx

Simple. The rating of the thermostat regulates the coolant temperature and by association, the engine temperature. For most conditions, the unit from the factory is optimum, but for racing sometimes a lower temperature is better, and a hotter unit might be better for winter conditions.

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There was a long discussion of this in the last couple of weeks. Do a search.
Your thermostat is probably and 87 degree Celcius, but you should check with parts suppliers. You want Wahler or Behr brand.

Steve

It is important you stick with a factory spec. thermostat (same rating) as the fuel injection system operation is directly linked to the temperature sensor to adjust proper fuel/air ratio.