* Water Pump: The water pump circulates coolant through the engine. A failing water pump may work adequately at idle but not deliver enough flow under the increased demands of driving. This is a very common cause of overheating under load.
* Thermostat: A stuck closed thermostat prevents coolant from circulating until it reaches its opening temperature. This could cause overheating only while driving, especially if the engine is already quite warm. A new thermostat is inexpensive and easy to rule out.
* Coolant Leaks (subtle): Even with a new radiator, you might have a small leak elsewhere in the system – a hose, a crack in the engine block or head, or a failing water pump seal. A pressure test of the cooling system is necessary to find these.
* Radiator Fan: The electric cooling fan might not be turning on when needed. This is crucial at lower speeds or during stop-and-go driving. Check the fan's operation (it might need a new relay or the fan motor itself might be faulty). Some cars have a dual-stage fan; only one might be working.
* Low Coolant: Despite the new radiator, there might still be air trapped in the system, leading to inefficient cooling. Properly bleeding the system is crucial. Also ensure you are using the correct coolant mixture (check your owner's manual).
* Head Gasket: A blown head gasket can allow combustion gases into the cooling system, pressurizing it and causing overheating. This often results in white exhaust smoke and/or a milky, oily residue in the coolant. This is a more serious and expensive repair.
* Faulty Cooling System Sensor: A sensor that monitors coolant temperature might be malfunctioning, leading to incorrect readings and improper fan operation. A code reader can often help diagnose sensor issues.
Troubleshooting Steps:
1. Check the coolant level: Make absolutely sure the coolant is at the correct level and is the correct 50/50 mix of coolant and distilled water.
2. Check for leaks: Carefully inspect all hoses, clamps, and connections for leaks, both when the engine is cold and after running. Look for signs of coolant stains or residue.
3. Replace the thermostat: This is a cheap and easy part to replace, and it's a very common culprit.
4. Inspect the water pump: Check for leaks around the water pump and listen for unusual noises (bearing whine).
5. Test the radiator fan: Ensure the fan turns on when the engine gets hot.
6. Bleed the cooling system: Properly bleed the system to remove any air pockets.
7. Pressure test the cooling system: This will reveal even small leaks.
8. Have a mechanic perform a comprehensive diagnostic: If you're uncomfortable with these steps, a qualified mechanic can use diagnostic tools to pinpoint the problem more accurately.
Don't continue driving the car while it overheats; you risk serious engine damage. Address the problem promptly.
