I had the exact same problem when I first got my 300d. Going up the mountains, the temp would climb, but never over 100.

When I flushed the old stuff, Ol' Turbo went back to normal operating tempuratures.

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1985 500SL Euro w/ AMG bits 130k
1984 300SD Turbodiesel 192k
1980 240D Stick China 188k
2001 CLK55 AMG 101k
2007 S600 Biturbo 149k Overheated Project, IT'S ALIVE!!!