I am pretty sure it sounds like your thermostat has failed... constantly running the coolant through the radiator.
The coolant temp setting for open/close (usually 87 d) is the "optimal" temperature for the engine. Running it at a much lower temperature can lead to ineffeciency and premature wear.
The part is 10 bucks... cheap fix!
2006 E350 w/ 155k miles (Daily Driver)
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!)