Robin and Paul,
  The boiling temp decreases with pressure, but the pressure would
  decrease
  with altitude.
  What happens with the freezing level at attitude with reduced pressure?
  George ( down under)

The freezing temperature doesn't change much (if any) due to changes in
pressure.

I think the interesting concept is that as pressure is reduced, the
boiling point (temperature) can get very close to the freezing point
(temperature).

Matt-

 Matt, Paul, and All,
I may be crazy but aren't you talking backwards? Boiling TEMPERATURE
increases with pressure. Therefore the problem is that at altitude your
system un pressurized will boil SOONER and stop removing the evil heat
demon from your engine! Glycol water mix boils at a higher temperature
than water alone, preventing true boiling. This is kind of like driving
on ice, while you're moving but not skidding you had control, but once a
skid starts you can play hell regaining control. Once boiling starts
there is a double whammy of bubbles in the system preventing pumping and
lack of significant heat transferred to the coolant. This is obvious I
know, but it seems everyone is running with this.
Bill Jepson

Actually the freeze temperature of water decreases with increase in
pressure - the reason ice skates work is you focus pressure on the ice
to form and instantaneous layer of water under the blade - and it
instantly freezes are you remove the pressure by passing by. It is also
why a bottle of liquid under pressure and below the freeze temp at 1
atmosphere will freeze when you remove the lid and the pressure...
Here's the phase diagram for water that shows this
http://www.lsbu.ac.uk/water/phase.html

-dave



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