I agree - t-stat is probably gone.

My 240D ran at about 82C most of the time with a new t-stat. Be careful when you remove the screws that hold on the cover - they can break. Hit them with PB Blaster the day before and tap them before trying to remove.

It's not a big deal if they break - I was able to retap the hole and use a larger screw when mine broke.

Might as well drain and replace the coolant and replace the radiator hoses while you're at it - not a hard job. When you refill the radiator, park uphill if possible - it makes getting the air out a bit easier.

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1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer