Question2004 Hyundia Sonata car has over heated 2 times in the last month. Check of coolant fluid revealed systems was approx 3/4 gallon low the first time. Overheated the following week System was approx 1/2 gallon low. Can find no leaks, Check of Oil and Trans fluid show no sign of missing coolant. Exhaust system appears ok no white smoke or smell, also no sign of coolant.
AnswerIf there are no external leaks, the thermostat is usually the culprit. What happens when the car overheats is that the coolant expands to the point that you begin losing it from the reservoir.
I'd recommend topping the coolant and letting the car run (a/c off) with the radiator cap off. You'll want to check for coolant flow when the car reaches operating temperature. Because of the design of this radiator, it may be difficult to tell when this occurs by looking in the radiator filler neck. Another way to check for coolant flow is to feel the radiator hoses. If they're both the same temperature, the coolant is flowing.
A short while after coolant flow begins, the radiator fans should cycle. Let the car run until they cycle at least two times. This will indicate that your fans are working properly, and the fact that they cycle on and off supports the idea that you have coolant flow.
If everything seems to be behaving normally, you should probably recheck for coolant leaks, paying special attention to areas that may be difficult to see such as the water pump weep hole. In some cases, you'll only see a trail of white crust (coolant residue) from the leak. If you cannot find any leaks, I'd recommend starting with replacing the thermostat and then seeing what happens after that's done.
