David Maurio wrote:
> RE:  In normal operation, the overflow tank will not be
>
>>used, except if the van overheats, then coolant flows
>>into the overflow tank, and then as the engine cools,
>>coolant will again be sucked back.
>
>
> Is this true?  Why is there a MIN line on the overflow tank???

Because you never want it to be completely empty.  If it completely
empties out air can be sucked back into the system.

It's not true that it's not used in normal driving, or that it only gets
coolant in it in an overheating situation.  Coolant expands as it gets
hotter, just like everything else.  The level will rise in the tank as
the engine gets hotter, and fall when it cools off.  (In fact, on some
cars the lines are marked "HOT" and "COLD" instead of "MAX" and "MIN".)
  Even seasonal temperature variations will affect it -- you'll see more
coolant in there in the summer than in the winter.  (If this sounds like
a thermometer, it's because it's exactly the same effect.)

I once had a Ford van with no overflow tank.  You had to leave two
inches of air space in the radiator to allow for expansion.  If you
topped it off, the first time the engine got warmed up it would pee a
bit of coolant onto the ground, and when it cooled down there'd be air
space in there again.  An overflow tank is a big improvement -- not only
do you not get coolant on the ground, but by moving coolant in and out
instead of air you limit the amount of free oxygen in the system,
reducing corrosion.

The original question was about a level that consistantly fell over
time.  To me this suggests a slow leak, possibly a pinhole in the
radiator.  It's normal to have to add a little coolant as the last bit
of air purges out of the system, but if it's been bled properly you
shouldn't have to keep adding for more than a couple days.

--

David Brodbeck, N8SRE
'82 Diesel Westfalia
'94 Honda Civic Si