Most are also the fill and check containers as well as the purge tanks.
The air in the system rises into the tank. Then the "good" coolant is
drawn off the bottom, eliminating air pockets inside the engine.
As for overheating, my mothers little 1994 Saturn SL 2 had been overheating before I got it. It has many small failures. Rarely, the aux. fan will run. I found out that the aux. fan always runs when you turn on the air conditioning, so if the temp gauge gets near the red line I start the AC and I'm ok.
In theory someone could scrub it's little brain out and make it run right.