* Water Pump: The most common cause. If the water pump is failing, it may still circulate coolant adequately at idle, but under the increased demand of driving (higher engine RPM and therefore higher heat generation), it can't keep up. The impeller inside might be worn or damaged.
* Thermostat: While a stuck-closed thermostat would cause overheating at idle *and* driving, a thermostat that's sticking *partially* open could allow sufficient flow at idle but restrict flow under higher engine loads.
* Radiator: A partially clogged radiator restricts coolant flow. Scale buildup, debris, or a collapsed core are possibilities. This would be more noticeable under higher demand.
* Radiator Cap: A faulty radiator cap might not maintain proper system pressure. This reduces boiling point, leading to overheating, especially under stress. Low pressure means less efficient coolant circulation.
* Coolant Leaks (subtle): A small leak, especially in a hose that's only under significant pressure during driving, might not be immediately obvious. Look carefully at all hoses and connections for any dampness or signs of weeping.
* Fan Clutch (if it's viscous): While you say the fan *switches* on and off correctly, a viscous fan clutch could be failing to engage properly under load, offering inadequate airflow when needed. This is less likely if the fan is electrically driven, but it's worth considering.
* Restriction in the cooling system: A blockage somewhere in the system, such as a clogged heater core, can severely impede flow.
Troubleshooting Steps:
1. Visual Inspection: Carefully examine all hoses, clamps, and connections for leaks or damage. Check the radiator for any signs of damage or debris.
2. Check Radiator Pressure: With the engine cold, remove the radiator cap (carefully!) and look for signs of scale buildup or debris inside the radiator.
3. Test the Thermostat: Remove the thermostat and place it in a pot of water on the stove. Observe the temperature at which it opens. It should open around the manufacturer's specified temperature.
4. Inspect the Water Pump: This requires more mechanical knowledge, but you can often see if the pump pulley is spinning freely. A seized or failing pump will often make noise.
5. Pressure Test the Cooling System: A professional mechanic can pressure test the system to identify leaks that aren't readily visible.
Because this overheating only happens under driving conditions, the issue is almost certainly related to insufficient coolant flow. If you're not comfortable performing these checks yourself, take your car to a qualified mechanic for diagnosis and repair. Ignoring overheating can lead to serious engine damage.
