There are 2 major choices for the "air" that appears to force the
coolant out into the overflow tank in your case. Either you have a hot
spot that is boiling the coolant and making steam, or you have some
failure that is letting high pressure combustion gases seep into the
cooling system and displacing the coolant. A third variation would be if
you were foolishly running straight water then steam is going to happen
much more easily. The coolant 50/50 mix doesn't boil until it is about
50 degrees hotter than straight water does. If the cap is bad and you
are running straight water then that is a double whammy since a
pressurized system raises the boiling point. A leaking cap valve reduces
the system pressure so boiling is more likely to occur with a bad cap.

Whether the "air" is combustion gases or steam, its source location is
away from the coolant tank. So when it first appears in the system it
adds rapid pressure to the system that will want to escape somehow.
Typically the pressure will escape through the overflow nipple of the
cap and into the overflow tank, pushing liquid out since that is what is
in the tank at that moment. The "air" will later end up gathered at the
main tank. If it had originated at the main tank it would be the first
thing forced out the cap nipple but that is not the case. Thus the
source is elsewhere.

Mark

Tom Carchrae wrote:
> This is not a different issue - this is my cooling issue....  :)
>
> I have a question to ask the experts regarding the bad cap theory.  I get
> how the fluid ends up in the overflow tank - but if it just a cap, where
> does the air/gas come from the displaces the coolant - and why does it not
> pull the coolant back into the pressure tank when it cools?
>
>