Both of my MBs had this issue when I bought them. A new T stat fixed the first one and the other is getting a new one put in this weekend.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine