I am not sure where you looked to see it boiling, but water boils at a certain temperature /pressure, so if the pressure and temperature are right, it will boil. As the pressure increases, it takes longer to boil, but is hotter. So water at sea level will boil at 212 degrees, and as the atmospheric pressure goes down, ( car going higher into the mountains, higher elevation) the temperature also drops. So at say 6000 feet above sea level, the water might boil at 95 degrees or so, therefore boiling sooner.

The point is, your engine is hot enough to boil water easily, but the pressure in the radiator cap, keeps it from boiling too soon. And once the engine pressure gets to the set point on the cap, it will cause the cap to release coolant into the reservoir, and it will then start boiling, because the pressure outside the engine is less then inside, so it boils sooner.

You see, you are going from a higher pressure inside the engine where the water is real hot but not boiling, to a lower pressure outside the engine, where the same hot water can then start boiling, and it will.

The antifreeze-coolant has probably been diluted to the point were it's pretty well useless. Take your car to a mechanic and get it flushed our and put Propylene Glycol antifreeze which, when diluted to a 50:50 ratio is superior and should last double that of ethylene glycol. Propylene Glycol is not lethal or toxic like ethanol glycol is.

This condition is usually caused by a defective radiator cap.

If it was boiling then you don't have enough antifreeze.

I have a acura rsx type s and when I was using my car I notice the coolant was boiling also it had a leak somwhere. I turn it back on and drove the car for a bit but it wasn't overheating. Any idea?