check transmission oil to see if milky. The built in radiator transcooler have been known to fail,head gaskets to one these engines can fail,when over heated.I run lower thermosat a 170F .Plenty heat for winter.
If transcooler try changing fluid after buying a external trancooler, or new radiator. Good Luck

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1999 w140, quit voting to old, and to old to fight, a god damned veteran, deutschland deutschland uber alles uber alles in der welt