I recently installed thermostat valve part number 17600-85811 rated 82C. I noticed valve opening when temp gauge reaches 10% and fan comes on at 40% on its way from C to H. Issue is that, while continuous driving at highway speeds the temperature gauge again comes back to 10% and stays around there until speed is slowed down or stopped then it starts rises up to 45% and again fan comes on. If I continue to drive at high speed the fan never comes on and needle remain near to C while in optimum situation it should show almost constant temperature near to half way to H.
My car has a manual transmission.
It sounds like the thermostat you have may be the wrong temperature rating. Try to use the 91 degree thermostat. That should keep your temperature closer to the midpoint on the gauge. The air rising across the radiator while driving is cooling the engine coolant very well, but by having the 82c thermostat, it will always try to cool to that temperature before closing and allowing the motor to heat up again.
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