I recently found out that when you turn the steering wheel to lock, the RPMs do drop... and that this is normal. The power steering pump uses up to 9 horsepower, and the idle speed controller doesn't know when you are going to turn the wheel to lock, so it takes a second for the RPMs to correct itself.
I hope this helps atleast a little!
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!)