Bo,

The Mercedes-Benz cooling sytem design has a double acting thermostat. The point of the design is to avoid rapid, very high temperature changes in the water being circulated in the block and head. For example the radiator starts the day at ambient temperature which may be below zero, while the thermostat control that opens when the engine reaches its normal operating temperature, near two hundred degrees (F). Injecting zero degree water onto hot metal could cause the metal to fracture if not right away, then after a number of cycles.

The MB thermostat has two moving elements, that coordinate to make the water coming from the radiator mix with an appropriate amount of water coming from the hot engine to as it enters the water pump and gets sent to the engine. If the system gets old these two items get out of their coordinated movements, as happens to me and has happened to you, the engine never warms up. It usually happens after 70k miles or more for me so I just accept the fact that every so often I buy a thermostat and replace it. There is virtually no other explanation for the problem of not heating up right, as the engine running is surely making enough heat. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)