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Well if there are no leaks and the coolant level stays relatively the same, oil isn't milky, both fans come on....I'd look at the water pump. 180k is the time for the 2nd timing belt and water pump service.
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2015 Ford Mustang GT Fastback - Ingot Silver - 6M - Performance Package - Gibson Catback, JLT CAI, FR 47lb injectors, BAMA E85 tune, Eibach Sportline, BMR wheel hop kit, UPR oil separator, Steeda shifter bushing/bracket Team B.O.B.® - Ballaz on a Budget
u asked if old parts could be the problem, probably since u put in an old thermostat. so start there with a new thermostat. does sound like a bad thermostat, bad water pump or slowly leaking radiator. could be a leak in a hose. blown headgasket u would have trouble driving it. the car would not want to run or move. the symptoms of slim in the oil cap, white smoke come later. u would notice too alot of coolant is gone. water pumps make a bearing noise also.