My car normally runs perfectly, but if it sits still for a few minutes, the temperature gauge goes up and I get a warning light. If the car is moving, the gauge is normal. If I turn on the heat full-blast, the gauge drops to normal. It hasn't actually steamed up yet or given any other indication, but when I catch it happening I either turn the car off, or turn on the heater. The car is due for an oil change, but the level is fine and it doesn't look terrible. The coolant was low. I can't really say how low exactly, but about the capacity of the reservoir. However, adding coolant hasn't changed anything. Also, I haven't caught it leaking anything yet. The radiator fans are running. I don't know if this could be relevant, but this has been happening more or less since I recharged the A/C. The A/C hadn't been working for almost as long as we've owned the car, about three years. But turning the A/C on or off makes no difference. Any ideas?
The only way to determine the root cause of your problem is to have the vehicle's warning light inspected firsthand by a professional, such as one from YourMechanic.
An engine that is running hot is less likely to overheat when it is moving because cool air is being pushed through the radiator at the front of the vehicle. Likewise, turning on the heater draws heat out of the engine compartment and into the interior of the vehicle.
The first thing that comes to mind is the engine cooling fan, since a failed fan would be the most likely cause of a vehicle that overheats at idle, but not while moving. But since you said the fan is working, I would look at the water pump next. Since it is driven off the engine, the water pump flows more at higher RPMs. This is just a guess though, and really, any part of your cooling system could be to blame.
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