> It's not true that it's (coolant overflow tank) not
> used in normal driving, or that it only gets
> coolant in it in an overheating situation.

It is my understanding that as the coolant warms,
pressure is created in the system.  The coolant system
is designed to operate under pressure as pressure
increases the boiling point of liquid & helps reduce
coolant boiling.  I have driven my van in both sub
zero & over 50 degrees C weather and when the van is
moving (read: great amounts of air moving over the
radiator) this keeps the coolant at a temp that the
reservoir cap does not open.  Only when I slow to a
stop on a hot day with a very hot engine and the heat
begins to build, does the radiator cap get enough
pressure to open.  At least this is my experience and
understanding.  Correct me if I'm wrong.  I would
guess that if, at normal operating temps, there is
flow to the overflow tank, then maybe the reservoir
cap is not holding pressure.

This is not a difficult experiment. note/mark the
coolant level in the overflow tank and the drive in
Arizona/Texas in the summer (Egypt anyone???), drive
hard up hill on a hot day and then stop, run back and
look at the coolant level in the overflow tank.  Keep
watching as the engine idles, soon the cap will let go
and the level in the overflow tank will rise.  Once,
on a hot day, my water pump belt jumped its track and
the engine overheated so badly, and the coolant
expanded so much, that it was shooting out of the top
of the overflow tank's 3 little holes.

Anyway, I've been wrong before, but I have had my
cooling system fail and I keep a real good eye on it,
and this is my experience.  Malcolm

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