Tstat should be closed until some temp slope prior to tstat's rating (is it 80?)and then start opening, then, as temp drops it should start closing to allow some coolant to circ to maintain rated temp until load on engine relative to outside air temp begins to increase, causing it to open.

So, if temp goes down to tstat rating and holds going down long hill, then tstat is working near closed position (allowing some coolant flow likely), but if immediately goes above rating when headed back up by a significant margin and stays, then something isn't working correctly, right? Should increase some, then start regulating, IF all other things are equal - like radiator capacity, no air in coolant, no blown head gasket, pressurized system, viscous clutch fan, etc - 100C electric fan probably not in play, nor would the electric fan linked into air conditioning pressure switch, right?

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George Stephenson
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg