FWIW: My '86 300E w/152,000 on the "clock" exhibits the same behavior. My trusted mechanic says that this engine is noted for running on the hot side and not to worry about it as long as it doesn't get up into the red zone. Still, at 100°f ambient, in slow traffic, it gives one pause. I stopped worrying about it.

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62 220sb
67 250S
72 280SE 4.5
74 280C
77 300D
82 240D
85 190E 2.3
86 300E RIP 12/28/09
85 300SD
92 300D 2.5
00 E320 Current
Over 1,000,000 miles in Benzes, Since66

....and a whole passel of BMW 2002 and Tii