* Low Coolant: This is the most common cause. Even a seemingly small leak can lead to overheating when the car is stationary and the engine isn't generating as much airflow to help cool the radiator. Check the coolant level in the overflow tank (when cold) and look for any leaks around the radiator, hoses, water pump, and engine block.
* Faulty Thermostat: A stuck closed thermostat prevents coolant from circulating properly, leading to overheating, especially when the engine isn't under load (like when stopped). When the car is moving, the engine's heat might be enough to force some circulation, masking the problem.
* Radiator Issues: A clogged radiator (from debris, sediment, or corrosion) restricts coolant flow. Similarly, a bent or damaged radiator fin can reduce its cooling efficiency. Inspect the radiator for damage and consider flushing it if it's dirty.
* Water Pump: A weak or failing water pump may not circulate coolant effectively enough at low RPMs. While it might manage to keep things cool at higher engine speeds, it fails when the engine is idling.
* Faulty Fan Clutch (even if the fan is spinning): While you say the fan is working, it might not be engaging strongly enough at the correct temperature. A faulty clutch could mean the fan isn't spinning fast enough to provide adequate cooling when stopped.
* Air Pockets in the Cooling System: Air trapped in the cooling system can prevent proper heat transfer. Make sure the system is properly bled of air. This often requires a specific procedure depending on the vehicle.
* Head Gasket: A blown head gasket allows combustion gases to leak into the cooling system, increasing pressure and leading to overheating. This is less likely if the car only overheats when stationary, but it's still a possibility. Testing for exhaust gases in the coolant is necessary to confirm this.
* Cooling Fan Speed: Even if the fan is spinning, it might not be spinning *fast enough*. Check the fan speed at operating temperature to ensure it's within the correct range. A faulty fan speed control module is a potential cause.
Troubleshooting Steps:
1. Check coolant level: This is the easiest and most important first step.
2. Inspect for leaks: Look carefully around all components of the cooling system.
3. Feel the upper and lower radiator hoses: When the engine is warm, the upper hose should be hot, and the lower hose should be noticeably warmer than the upper, but not scalding. If both are equally hot or the lower is cold, the thermostat might be stuck closed.
4. Check the radiator for clogs: Shine a light through it.
5. If all else fails, take it to a mechanic: Diagnosing overheating issues can be tricky, and a professional can use diagnostic tools to pinpoint the problem.
Remember safety first! When working with a hot engine, allow it to cool completely before touching any components. Also, be cautious of hot coolant.
