* Water Pump: A weak or failing water pump might not circulate coolant effectively enough at idle. When the engine is running faster (while driving), the increased RPMs can help overcome the pump's weakness, leading to adequate heat. This is a very common cause of this specific symptom.
* Thermostat (despite replacement): While you replaced it, there's a small chance the new thermostat is defective or not seated properly. It's also possible there's air trapped in the cooling system, preventing proper coolant flow, even with a working thermostat. Bleeding the cooling system (removing air pockets) is crucial after any thermostat replacement.
* Coolant Level/Leaks: Low coolant levels will obviously affect heating performance. Inspect the coolant level in the overflow reservoir (when the engine is cold) and check for any leaks around the radiator, hoses, water pump, and engine block.
* Radiator Fan: While seemingly unrelated to heat, a malfunctioning radiator fan could cause overheating at idle, especially in warmer weather. Overheating could restrict coolant flow to the heater core.
* Heater Core: A partially clogged or restricted heater core could reduce flow, making it more noticeable at idle when coolant flow is already lower. While less likely than the water pump, it's a possibility.
* Blend Door Actuator: This controls the air flow between the heater core and outside air. While it wouldn't directly affect the *temperature* of the air coming from the heater core, a malfunctioning actuator could be directing cold outside air into the mix even when the heater is turned up.
Troubleshooting Steps:
1. Bleed the Cooling System: This is the first thing you should do. There are usually bleed screws on the engine or radiator; consult your owner's manual for the correct procedure. Make sure the coolant level is topped off correctly (refer to your manual for the correct type of coolant).
2. Check Coolant Level: Ensure the coolant level is within the normal range in the overflow reservoir (when the engine is cold).
3. Inspect for Leaks: Carefully examine all hoses, clamps, and the water pump for any signs of leaks.
4. Listen for Water Pump Noise: A failing water pump often makes a whining or groaning noise, especially when the engine is cold.
5. Check Radiator Fan Operation: Ensure the radiator fan is turning on when the engine is at idle and the temperature gauge gets close to the operating range.
6. Verify Thermostat Operation (again): If possible, monitor the upper radiator hose. It should get hot relatively quickly after the engine starts. If it remains cold for an extended period, the thermostat might still be the problem.
7. Consider a Pressure Test: A pressure test on the cooling system can reveal small leaks not readily visible. A mechanic can perform this test.
If you've checked these and still have the problem, a mechanic should inspect the water pump and potentially the heater core. The water pump is the most likely culprit given your symptoms.
