If my heat gauge is reading all the way hot but motor and coolant is only warm to the touch and no smoking or boiling over would it be the thermostat?
Probably Not. The heat gauge uses a ECT Sensor to indicate the engine coolant temperature. Sometimes if there is an air pocket the sending unit will read hot until the cooling system is properly bled. Once the system is properly bled you can use an infrared hand held temperature gun to check the accuracy of the gauge.
The first thing I would check would be the coolant level. Also confirm the temperature gauge sending unit wire is connected. If it has become disconnected it will read full hot all the time with Key in ON position.
If you just worked on it and are now having the issue, check around the area you were working to make sure you didn’t accidentally leave a wire disconnected.
You can check the the sensor by removing it and using checking the resistance ohms.
|Application||Resistance (Min-Max) Ohms||Temp ºF (ºC)|
|EX||15000 – 20000||-4ºF (-20ºC)|
|5000 – 7000||32ºF (0ºC)|
|2000 – 4000||68ºF (20ºC)|
|900 – 1200||104ºF (40ºC)|
|300 – 400||176ºF (80ºC)|
|100 – 300||248ºF (115ºC)|