What Causes High Engine Coolant Temperature at Idle?

[LeKing]

When the engine coolant temperature is excessively high at idle, it's necessary to check whether the thermostat is malfunctioning and unable to fully open, which can restrict coolant flow. Alternatively, the radiator may be rusted, impeding proper coolant circulation and heat dissipation. Methods for diagnosing a faulty coolant temperature sensor are as follows: Multimeter Testing: Use a multimeter to test for open or short circuits, which indicate abnormalities. Hot Water Method: When exposed to hot water, the sensor's resistance should vary with temperature changes—this is a normal condition. However, the specific pattern of change must be compared against the sensor's specifications. Temperature Simulation: Use a digital resistor simulator to simulate coolant temperature and compare it with actual conditions. Infrared Thermometer: Measure the actual temperature of the coolant temperature sensor using an infrared thermometer and compare it with the temperature gauge readings. Multimeter Resistance Test: Measure the resistance value of the coolant temperature sensor with a multimeter. When heated with a hairdryer, the resistance value should decrease.