The one that I bought from the MB dealer (Wahler - Made in Germany) *it says on the T-stat* has no part numbers but at the back it says 80* 09 01
whatever the 09 01 means.

This one didn't open until the water temp reaches 98*C. In summer it's perfectly fine. In fall/winter the water temp was in the 98* C range and never seemed to open unless I drive the car hard. When I drive it nice and slow in the city the temperature would rise back to 98-100C

The Autozone one solved this problem (also a Wahler T-Stat - Made in Germany too)

I just reused the gasket that came with the Dealer's T-Stat and never bothered to return the dealer t-stat.

I just went to Autozone and I told them what car I have and they gave me a T-Stat. I think I paid around $40 or less (it was really cheap though)

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2008 BMW 335i Coupe