the new stat could be defective. Happened to me once. Check it or replace it. Drilling holes in the thermostat to allow some baseline coolant flow is one approach, or you can use a thermostat from the 1970's 450 models, which opens at 76 C instead of 80 C (the stock). I installed one due to some overheating in summer stop/go traffic here in tucson with the a/c on and it made a real difference.

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Der Panzermann und Fraulein Fahrvergnuegen

1991 420SEL 201K "The Big Blue One"
1985 300DT 205K chassis/285K engine nee California emissions "Goldbug"
1983 300TDT 255K "The Womble"
1983 300 DT 214K "Sea Sprite"-Rear-ended a truck
1983 300SD 285K "The Donor" Gave his life so that others can live
1980 500SL Euro 105K "Der Panzer"