Why does the car's water temperature fluctuate between high and low?

[DelWalker]

Here are the reasons why the car's water temperature fluctuates between high and low: 1. Faulty water temperature sensor. Since the water temperature sensor is installed on the engine block or cylinder head water jacket and directly contacts the coolant, it measures the engine coolant temperature. If the water temperature sensor fails or malfunctions, the engine ECU will detect an unreliable output signal and intervene by using a substitute value, which is also to protect the engine. This results in unstable readings on the car's water temperature gauge, along with difficulties in cold starts and acceleration. 2. Damaged thermostat. The thermostat automatically adjusts the amount of water entering the radiator based on the coolant temperature, changing the water circulation range to regulate the cooling system's heat dissipation capacity and ensure the engine operates within the appropriate temperature range. You can check by touching the thermostat's inlet and outlet to see if their temperatures are the same. If the thermostat is damaged, it can also affect the engine's normal operating temperature, causing the water temperature gauge to fluctuate. 3. Cooling system malfunction. Check if there is a leak in the engine cooling system or if the water pump is still functioning properly, as these can also cause the coolant temperature to fluctuate. 4. Short circuit in the water temperature gauge heating coil; the gauge needle may be rubbing against the surface. 5. Scale buildup on the contact surface between the water temperature sensor and the coolant. 6. Aging thermistor. 7. Unstable voltage regulator operation.