What Causes High Water Temperature When Idling with AC On?

[MacCarson]

Here are the specific reasons for high engine water temperature when idling with AC on: 1. Insufficient coolant: Coolant leaks or low coolant levels can cause the engine water temperature to rise. In this case, check the coolant level and top it up if necessary. Recheck after a few days to see if the level drops significantly, and locate any leaks if present. 2. Cooling fan malfunction: When the vehicle runs at high RPM, heat cannot be effectively transferred to the antifreeze, impairing heat dissipation and causing the antifreeze temperature to rise, potentially leading to boiling. 3. Water pump failure: Water pump issues include seized bearings and damaged impellers. While seized bearings are rare, impeller damage—such as cracks or detachment from the pump shaft—can slow coolant circulation, often resulting in engine overheating. 4. Thermostat failure: The thermostat automatically adjusts the flow of coolant into the radiator based on temperature, regulating the cooling system's efficiency to maintain the engine within an optimal temperature range. A faulty thermostat may prevent the cooling system from entering a full circulation mode, leading to elevated water temperature.