I believe mine is failing open as well. In 60-70 degree weather, coolant temp refuses to go above 80, even with A/C on. On the highway, at around 75 mph, it drops to about 78 or 79.

Also, the engine is a much louder when coolant is below about 85-90 degrees. The engine roars, as if the fan clutch is engaged. As soon as the temp reaches about 90, it becomes very quiet.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black