Is the coolant system a closed system? 2006 aveo HB

[FlaAveo5]

oh. Ok. I didnt think of that, that the plastic could handle the temps. of a running engine. I just assumed that as the cars got more and more complex, the radiator cap,on top of the radiator, went the way of the way of most rear wheel drive and no air bags! And it did, it is now on the reservoir, instead of on top of the radiator.

That reservoir is under pressure, your right, I have opened it sometimes, when it is still 'warm' and gotten a loud HISS from it. Well I am going to read the coolant additive that I want to put in from Prestone, to make sure it is just to add more of the preventative chemicals in it, not to plug up holes or if things are wrong with the radiator. Just to add more lubricants, and anti corrosive chemicals to the coolant, to keep it runing in top shape. It is hot here in Fla.,it may be going into 'fall/winter,but it is still HOT all the time..