Engine temperature
 

My 95S420 runs close to 100C on the highway (80mph) in hot weather. Is this normal? My wife's C230 runs at constant 80C. I think 100 is too hot, but would like to know what other 420's run at.

It definitly should be running a touch above 80 like your C230, especially if cruising on the hwy doing 80 in normal (70ish) weather. Unless it's 100 degrees outside, I would have it checked out.
However, when you come to a stop, and the temperature goes to 100, I would find this ok, but not when cruising.

Most later cars have t-stats that run at 92deg so i was prepared to say you were about right. I looked up the t-stat for that motor and its only a 80 deg.

Based on that the car should probably run 190 or so. The temp reading on a t-stat is when they start opening.

Be sure your auxilary fan is working. Change the t-stat. Check the flow in the radiator. Thats the order we usually take if we suspect a motor to be running over t-stat temp without stress.

So THATS why the CLK always runs a little warm!