I don't drive through Death Valley, but I've driven MB's of all models through plenty of 108 degree, or so, days with no coolant temp trouble at all. I think what the poster is concerned about is the temp gauge moving around a little.

The reason is related not to the thermostat, but the way that coolant is routed through the thermostat. On American cars the thermostat simply regulates coolant flow through the thermostat so it flows or it doesn't. On most MB's there is a bypass sort of arrangement that I can't describe and don't understand. That is why an MB engine will have coolant warm enough at the heater core in a very short time after starting the engine on a cold or cool morning. It seems to route the coolant through the heater while the thermostat is closed and then allows it through the radiator when it opens.

I realize that this is a poor explanation, but it is unlike other cars that I've seen.

This does NOT make the cooling system inadequate and I have no worry whatsoever crossing death valley in any MB as long as the cooling system is in good order. The only concern I would have is seeing that the fuel tank is filled before starting across.

Have a great day,