In the last two days I've seen my heat meter goes above the normal, but only when car is stopped. Long red light, two minutes in car with engine on it starts overheating. But when I go, even in slow traffic, the arrow goes back to normal value. The radiator and coolant bottle are full. Last time I had heating problems, the thermostat was replaced. I'm taking a car to a shop tomorrow, but I can't think what can cause this behavior.
- The water pump's impeller fins are corroded and worn down.
- Blockages in the radiator. Feel the surface of the radiator for cold spots.
If the car is overheating only when sitting still, it's most likely the cooling fan not working. When you're moving, air is flowing though the radiator, but when you're not, you need the fan to push or pull air through the fins to keep the engine cool.
You indicated in the comments that it's got electric fans, and that they do work. You could have failed sensors or controllers, meaning the car isn't turning the fans on because it doesn't think it needs to. Check if the fans are running when the car is idling, stopped, and warm (do it quickly so it doesn't overheat!). If they're not spinning, then that's most likely the issue. If there's a temperature sensor in your lower radiator hose, try unplugging it and see if the fans come on. If the fans were tested off the car, there could also be a physical blockage when installed preventing them from turning.
Start off with simple inspections then move up.
Does the fan work?
- If yes, check fluid levels
- If fan doesn't work and fluid is ok, the fan is the problem
- If both work it might be a faulty sensor or broken tach (if sensor is ok)
Stop the car and see if fluid begins boiling.
- If it doesn't boil after 20 mins of idle it might be an electrical problem. If the fan is ok, fluid ok and not boiling, it is usually an electric problem.