This may sound silly but how do you know the guage is working correctly at all? If it came out of a car that already overheated I would not trust it. It should be running at about 85C for it's normal temperature. It could be that it's reading is off and you dont really have an actual overheating problem at all. I would certainly want to remove it and check it independently against another thermometer before proceding with more complicated stuff.

- Peter.

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2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S