Temperature Gauge Issue

Something else required that hasn't been mentioned... hot water. If the thermostat is missing or stuck open it could take a very long time to make that gauge move.
As the sender heats its resistance should slowly change to somewhere around 30 ohms. this slowly warms the bimetal beam inside the gauge. Beam slowly bows to lift the needle. Instant zero ohms test can make a OEM limiter and/or gauge do things they wouldn't do in normal conditions.
If you have ohms meter you can test the sender before removing it, assuming its bad. Thermometer to check water temperature also.