The blinking light can be caused by either a real drop in coolant level
in the main tank, a false reading of a drop in the main tank, or a
problem in the dash. If you are sure that the system starts off with no
air in it but air appears after you drive, then you have a cooling
system problem causing a real drop in the coolant level.

The coolant expands when the engine warms up and the coolant gets hot.
Since there is so much coolant, 16+ quarts, the pressure rise in the
system from it all expanding forces a little coolant into the overflow
tank. The main tank has a pressure cap on it and after the pressure in
the system reaches that set pressure, some of the pressure is relieved
into the overflow tank. If there is air in the main tank, some of it
should be pushed into the overflow tank instead of coolant since the
hose from the main tank connects at the top, where air will collect.
When the engine cools down, the coolant in the system shrinks back to
cold size and this sucks from the overflow tank. Since the hose connects
to the bottom of the overflow tank it will suck liquid from it if there
is some in it. It only draws a little every time the engine cools, until
all the air is gone from the main tank. The system is designed to self
bleed, but can only do so gradually and over multiple cycles of heating
and cooling of the engine. The engine will NEVER suck coolant from the
overflow tank while the engine is running. That is why you must always
add coolant to the main tank if the main tank is too low.

If you are always getting more air in the main tank, something is wrong.
That air has to come from somewhere. There two likely sources of air in
the system. One is steam from boiling coolant. If you have just water or
very weak coolant, it will boil too easily where it touches the hottest
parts of the engine and make steam that will create a bubble in the main
tank. Also, if the pressure cap is bad it will let the coolant boil too
easily as well and make a bubble in the main tank The other likely
source is combustion gas leaking through the head sealing surfaces and
into the cooling system. The combustion gas pressure is over 1000 psi so
even a very tiny leak of this gas will make a big bubble in the system
and push coolant out into the overflow tank. This may well be your
problem. Since it is very expensive to fix this, it is good to eliminate
all the easy and cheaper causes before facing up to the worst case.

Mark

>
>
> i'm still confused as to why the fill tank would be full but the
> expansion tank will only read to be at minimum or under? when will the
> contents of the fill tank transfer to the expansion tank to fill it?