My daughter has 2011 Aveo with approx. 27,000 miles. It quit accelerating and seemed to be overheating. She put more coolant in. Last week it did it again, we had it towed to a dealership. They tried to duplicate it overheating by driving it and did a pressure test on it - but found nothing. They replaced the thermostat - as they figured it might be that - and that was covered under the powertrain warranty (no cost to me). Today I drove it to return it to her approximately 80 miles. As I was driving I noticed that the engine temperature would fluctuate between normal and cold, then I would get either real hot or cold air from the air vents. When I reached her workplace I heard a noise like the fluid in the radiator was boiling. No warning lights came on. Any ideas? We took it to another dealership (closer to her) to have them look at it. If it's the heater core, how many hours should it take to replace it? The new dealer said it would be about 10 hours to replace it, which seems high to me. If it's the heater core, it won't be covered under the powertrain warranty, so we'll go somewhere else to get fixed, as the dealer will be priced horribly. Any info would be appreciated. Thanks.