* Low coolant: This is the simplest and most common cause. Check your coolant level in the reservoir tank (when the engine is cold!). If it's low, you need to find the leak and refill it. Low coolant prevents proper heat transfer.
* Faulty radiator fan: The radiator fan is crucial for pulling air through the radiator to cool the coolant when the car is stationary or moving slowly. If the fan isn't working (either not turning on at all or not spinning fast enough), the engine will overheat while idling. Check if the fan comes on when the engine gets hot. You might hear it, or be able to feel the airflow if you put your hand near the radiator. A malfunctioning fan motor, relay, or temperature sensor could be the problem.
* Clogged radiator: Over time, debris, bugs, and other substances can clog the radiator fins, reducing its efficiency. This will hinder airflow and cause overheating, especially at idle. A visual inspection might reveal a heavily clogged radiator.
* Water pump: While a failing water pump might cause overheating at higher speeds too, a weak pump might not be able to circulate coolant effectively enough at idle, when the engine's demands are lower and the fan is the main cooling mechanism.
* Thermostat: A stuck closed thermostat will prevent coolant from circulating properly, leading to overheating, particularly at idle. A stuck open thermostat might cause the engine to take longer to warm up, but wouldn't directly cause overheating *while* idling.
* Coolant leaks: Small leaks might not be immediately obvious but can cause a slow coolant loss over time, leading to overheating. Check hoses, the radiator, the water pump, and the engine block for any signs of leakage.
Troubleshooting Steps:
1. Check coolant level: The easiest first step.
2. Check the radiator fan: Observe if it operates when the engine is hot.
3. Inspect the radiator for clogs: Look for any visible obstructions.
4. Feel the upper and lower radiator hoses: When the engine is hot, the upper hose should be hot, and the lower hose should be somewhat warm. If both are cold, there's likely a problem with coolant flow.
5. Listen for unusual noises: A squealing or grinding sound from the water pump could indicate a problem.
If you're not comfortable performing these checks yourself, take your Impala to a qualified mechanic. Overheating can cause serious engine damage if left unaddressed.
