I went through this with the dealer and finally a helpful tech person pulled me aside and told me how to modify the thermostat so that it will cool the engine properly.
1. Remove thermostat and with a strong pair of pliers break off the centering arm. The arm was a nice idea, but the hole tends to fill up with gunk causing the thermostat to stay closed.
2.Using a 4mm drill bit drill 3 holes in the frame part part of the thermostat (make sure that it won't be covered by the rubber seal) that will allow a little of the coolant to constantly flow whenever the engine is running.
I did this last year and my 380SL NEVER even comes close to 100!
