Great explanation Mark.  Thank you.

If the "air" is indeed steam, then wouldn't it return to liquid form once
the system cools?

On Mon, Sep 9, 2013 at 1:59 PM, mark drillock <mdrillock@cox.net> wrote:

> 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?
>>
>>
>>