Jim:

Thanks for the clear explanation. I looked the cooling system diagram and think your explanation is correct.

I found 60k miles is the point that a fresh thermostat is needed to endure optimum coolant temperature. I happen to have a spare thermostat for a 300E (bought 3 at one time and replaced two on two 300Es) and will replace the one in my W210 (58k miles).

Thanks again.

Bo