The thermostat on all MB's is like this, I believe, for some very specific reasons. First, nearly all cars have some provision for at least low flow during warm up, so the cold coolant is not forced to go through the radiator, and you get the engine as well as yourself warm sooner. The main drawback of such systems is that when the thermostat first opens after the engine and head especially, are somewhat warm is that in very cold climates coolant at ambient (sub zero) temperatures is suddenly admitted and thermally exercises the head and block materials. Second, the water pump flow rate is controlled by the thermostat, meaning in part load conditions the water pump is likely throttled on either the input or the output side, depending on the arrangement of components. This can lead to cavitation damage in the pump housing and impeller.

MB's system involves a thermostat with two moving parts. One is used to close the radiator bypass line, while the other is used to control the flow through the radiator. MB does this to ensure the water entering the engine is within a temperature range they determine is healthy for the engine. The placement of the whole affair in front of the pump is intended to ensure the cold water from the radiator is mixed with the warm water from the engine as thoroughly as possible. Additionally, the flow rate through the water pump at a given rpm is near constant since the thermostat never throttles down on a flow path without simultaneously diverting that flow volume to another.

Overall the system is very technically elegant and robust, with the intent to protect the engine while maximizing the passenger comfort on a cold day by minimizing the warm up period. The weak link is the thermostat though, as both flow control plates need to move according to design to maintain the intended flow conditions. As a result the system is sensitive to the selection and installation of the thermostat. Any binding due to shipping or handling damage, or deposits from coolant chemicals on the alignment sensitive parts and the thermostat will fail. In my 190E 2.3-16 the failure mode I have seen twice is the engine gets warm, reaching normal operating temperature, the thermostat opens and begins to regulate temperature, then gets stuck in the "send the water to the radiator" position, and the engine would run cold. Not good for mileage or the cat. The car would run so cool the mileage would drop 4 or 5 mpg.

Hope this helps, 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)