Why is the engine temperature high when the radiator is full of water?
3 Answers
Here are the reasons why the engine temperature is high when the radiator is full of water: 1. Excessive scale buildup in the radiator and engine water jacket: Too much scale can lead to poor heat dissipation, causing the water temperature to rise. External dirt on the radiator or blockages between the air conditioning condenser and the radiator can also result in poor radiator cooling, leading to high water temperature. 2. Thermostat failure preventing it from opening: A damaged thermostat can prevent coolant from smoothly entering the radiator to form a large circulation loop, causing the water temperature to rise quickly. If the thermostat cannot open or does not open flexibly, the coolant cannot circulate through the radiator, resulting in excessively high temperatures or intermittent high temperatures. 3. Water pump damage or drive belt slippage causing poor pump performance: If the water pump cannot rotate normally to generate sufficient water pressure, it will reduce the flow rate of the coolant. The circulation of coolant relies on the water pump to apply pressure, forcing cooled water from the radiator into the engine water jacket and pushing out the heated coolant. If the water pump is not functioning properly and cannot generate adequate pressure, the flow rate of the coolant circulation decreases, leading to poor heat dissipation and excessively high coolant system temperature.
The radiator is full but the engine temperature is high. I've seen this situation many times. It's not just about whether the coolant is sufficient, the key issue lies in the circulation problem. There could be several possible causes: the thermostat is stuck in the closed position, blocking the coolant flow, which is quite common; the water pump is broken and can't pump the liquid out for circulation and cooling; the radiator is clogged with dirt, like leaves or dust; the fan isn't spinning, possibly due to a sensor or relay failure; or there are air bubbles in the cooling system, creating hot spots. I recommend checking the thermostat and water pump first, as these are the most common troublemakers. If the dashboard temperature rises while driving, stop the car immediately, turn off the engine, and try turning on the heater to alleviate the situation. Don't push it—engine damage is expensive to repair. Regular radiator flushing and coolant replacement can prevent this, just once a year, saving you both hassle and money.
I've been driving for over a decade and encountered several instances of a full radiator with skyrocketing temperatures. One time it was due to a faulty thermostat that remained completely closed without allowing any ventilation, which gave me a whole day of trouble. Another friend's car had a water pump issue where the worn-out impeller couldn't circulate water properly; there was also air trapped in the system causing localized overheating. A non-functioning fan is another common problem—just check the fuse and fan motor. Driving with high temperatures is risky; if the dashboard warning light turns red, pull over and turn off the engine immediately to avoid engine seizure. During regular maintenance, pay attention to the coolant's condition and avoid mixing different brands to prevent chemical reactions. To save money, you can replace the thermostat yourself—it's quite simple, just get a set of tools and do it yourself. Safety comes first; address any issues promptly without delay.
