why?
Because it is under pressure and the result will be just like taking the radiator cap off a conventional radiator when it's hot -- the boiling liquid will spew out in all directions and you'll very likely end up with some serious scalding at least on your hands, and possibly on your face and upper body.
But don't take my word for it ... go ahead and try it.
i knew that. i thought maybe it had something to do with it being closed system and didn't want air into it.
Just the opposite. Since there is nowhere for "excess" coolant to go when it expands due to heat, there HAS to be air in the system to provide for expansion. Air compresses ... liquids do not. If you fill the plastic tank full and bleed all air out of the system, the plastic tank will explode (or rupture).
This is true, I found out somehow one time. :rotfl2: