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.

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)

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.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

So THATS why the CLK always runs a little warm!

__________________
2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)