My car overheated on the drive in to work today. I'll describe the symptoms...
I've been dealing with a minor coolant leak for about a month now. I top it off once per week, about 1L of fluid. The leak is coming from the upper corner of the radiator (I ordered a new one 2 days ago).
Just recently (2 weeks) I've noticed that when I stop the car, the coolant is boiling. If I open the hood I can see it bubbling up out of the radiator hose into the coolant reservoir. It didn't used to do that.
Also, sometimes the heat doesn't work, especially if the coolant is low. It just blows out cold air. Then I hear a click, and I have hot air again.
Today, nothing was out of the ordinary at first. I just happened to look down at the temp guage and it was edging up to the red. Normally, it sits dead even in the middle. So I turned on the heat and the gauge came down a little. But the heat quit working and the car was blowing cold air for 5 minutes.
After 5 minutes, I hear a sudden thump, then a loud series of thumping noises. I can tell that it's a ton of boiling coolant rushing into the reservoir. I pull the car over and the coolant is boiling like crazy. Car won't start. I let it sit there and cool down.
Once the temp comes down, I try again. Exact same situation. Cold air from the heater, a thump, and a sudden rush of boiling coolant. Then the heat works for a few minutes and then gets cold.
What the heck is wrong? Thermostat? Water pump?
edit: the radiator fan does come on once the car warms up, and when I turn on the AC.