84K miles, A/T, radiator replaced by original owner recently, good heater, never in a major accident, pressure tested cooling system and is within specs, heater works until A/F expels out the overflow bottle (the overflow bottle cap was loose). After it overheated, the car is only 1/2 gallon low on A/F so I replaced the thermostat. I do not know if it was bad or not. Belts are OK. The water pump seems to flow the A/F, but seems to not move it too quickly. There is no smell out the exhaust, nor is there any A/F in the oil. I had the car idling on a 80 plus degree day last summer for an hour and there was no issues then. Cooling fans work correctly.
Has there been issues with this engine design??? Anybody have ANY ideas?

 

My friend had a similar problem with his 2003 Elantra. The car would dump a fair amount of coolant while driving on the freeway in a matter of a few seconds. It never happened while the car was idling, even during hot weather with the car up to temperature. The culprit ended up being the water pump.