According to my Haynes manual. The thermostat starts to open between 175f to 193f(80 to 90c). The stat. is fully open between 193f to 200f(90 to 94c). My experience with overheating in other cars was the thermostat. Just try removing it completely and see what happens. If you have a jelly thermometer or frying thermometer, you can test it in a pan of water on your stove and see what temp. it opens and closes. Also sometimes hoses colapse if they are old, not allowing the water to circulate properly.

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Gary
85 300D Ivory, 202,000
Beatus exsisto Jesu, verus et Deus verus Vir