Could be the coolant temperature sensor.
The temperature gauge will not work on a 1984 GMC Sierra truck if the sending unit is defective or the wiring has come loose. It is also possible for the gauge itself to fail, but this is less common.
Temperature Fluctuation simply
Your gauge could be bad but more than likely it is the temperature sending unit. It is a fairly inexpensive thing to fix.
change in temperature causes change in pressure
StUck in open position
Temperature.
It seems that your thermostat is not opening.
where is the temperature gauge sensor located in a 2005 f-150 fx4
The temperature sensor is probably bad
Loose connection or blown sensor !
bad temp sender usually. Bad gauge rarely.
Urine is the same temperature as the body it leaves. If the person has a fever, the urine will be a higher temperature. This is why there is a temperature gauge on the outside of a sample bottle.