I’m not sure about other models, but the thermostat on my 83 240D is on the return water back to the engine. If you think about it, by placing the thermostat on the return to the engine and not on the water leaving the engine, you are always cooling your engine with a minimum of 80 degC water (176 degF), which certainly isn’t very cold. Even if the radiator could cool the engine more, the thermostat placement won’t let it. In engines where the thermostat is in the water leaving the engine (before the radiator), the radiator is allowed to cool the water farther than the 80 degC, maybe down to 70 degC?, since the Tstat has no idea and doesn’t care how cold of water it sends to the engine at the discharge of the radiator. We don’t have that ability on these engines, however.

Another consequence to the MB placement of the thermostat is that when it closes, the water in the radiator may cool to a point lower than 80 degC. Later, when the Tstat “sees” this cold water it shuts itself, which isn’t always what the engine needs. Also having the cooled water flowing over the Tstat (including the bypass water) makes it think the engine is cooler than it is. Basically, these engines have to heat up quite a bit to the point where the water going into the radiator is 100+ degC to raise the water temp at the Tstat to the 94? DegC fully open point. (depending on your actual outside temp, speed, etc. of course)

I’m sure being good diesel engine engineers MB wanted to always make sure they had a hot engine for good combustion, but it does make for a roller coaster of a ride for the temperature on these engines. There is something to be said for a Tstat on the engine discharge with hot water directly off of the engine pumped up against the Tstat. It knows exactly how hot your engine is.

JohnM 83 240D 4 sp 151K