Overheating: Pontiac Sunfire

Have a mechanic check for combustion gases in the cooling system. A positive test indicates a leaking head gasket. The tester has a special dark blue liquid that will turn bright yellow in the presence of combustion gases.

Caradiooc

Typically thermostats stick open so the engine doesn't reach normal operating temperature. They rarely stick closed. If it did, you would not be adding coolant to the system. What CAN happen is the combustion gases getting into the cooling system can prevent the thermostat from opening if an air bubble collects under it. Hot air will not cause a thermostat to open. The sensing pellet must be hit with hot liquid.

This is confusing:

"I decided NOT to run the car after that and the car was fine. I drove it for another 30min".

Part of your observations suggests an inoperative fan but you said it's working. If it were a fan problem, the engine would not run hot at highway speeds. If this is indeed a head gasket problem, when it gets bad enough, you will see large bubbles in the reservoir shortly after starting the engine, and before it has time to get warm. It will look like the coolant is boiling but it won't be hot yet.

Caradiodoc

If the engine really is overheating, feel the air temperature from the heater. If it is real hot, the heater core is fine. Suspect poor circulation through the radiator. A loose or corroded water pump impeller always comes to mind as well as a plugged radiator, but a less known problem, especially if you live in a state where they throw a ton of salt onto an ounce of snow, is rotted cooling fins between the radiator tubes. That WILL lead to running hot on the highway because the radiator can't give up its heat. The heater will work great, in fact running the heater on high will bring the engine temperature down rather quickly.

Some GM engines need coolant to circulate through the heater core to get to the thermostat and cause it to open. In those, a plugged heater core can lead to overheating. The clue is when the engine is hot, the air from the heater will be cool.

None of this applies if you're adding coolant on a regular basis. If you don't see any leaks, you can add a small bottle of dye, then check various places with a black light. A common place to look is the tail pipe. If you find dye there, you will most often see white smoke too while the engine is running. That would be another sign of a leaking head gasket.

Caradiodoc