Honda Accord engine coolant temperature high

I have a 1990 Honda Accord. The coolant temperature, if normal, is right at the middle of the gauge. Recently, it has been as high as 4/5 of the gauge. I took it to a mechanics, thinking I might need
a new water pump. He checked it, told me the water pump was not leaking, and did not change the water pump.
The thermocouple is fairly new, less than two years.
What might be wrong?
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It would help to know how it behaves. Does it warm up in about 5 minutes but climb steadily to 4/5 scale instead of half scale then level off? Does it warm up to half scale, level off, then rise more slowly to 4/5 scale? Is the reading steady or does it waver, and if it does, how much and how fast? Does it read higher at idle or at cruising speed, or doesn't matter?
And although it may sound like a dumb question, I suppose the coolant level is okay and the coolant isn't disappearing? And the heater does still work?
All these things help rule out some possibilities and identify the most likely suspects.
Mike
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Michael Pardee wrote:

and don't check the coolant by looking at the expansion bottle! /always/ look inside the radiator after removing the cap.
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did you replace the thermostat with a honda or oem equivalent one ? most aftermarket place tend to sell the "regular" thermostat vs honda failsafe. "regular" version ususally failed closed so the engine can get overheat. failsafe die open
if it is a failsafe version than i have no idea other than the coolant might be low

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I agree with the other posters that told you that the thermostat needs to be changed--use a thermostat made by Honda. You should also check the radiator water level--when the engine is cold. When I was very young--I made the mistake of checking the radiator water when the engine was hot and I have NEVER made that mistake again. I only check it in the morning before I start the car.
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