overflowing coolant

Fisher, Scott Scott_Fisher at intuit.com
Tue Aug 20 09:43:23 EDT 2002

(the "> >" is Frank's original question, the ">" is Brett's response...)

> > Can this be due to straight water
> > or a bad thermostat which I just replaced?
> Probably more likely that your expansion tank cap is leaking
> pressure, so that's allowing the water to boil.

Exactly -- in fact, the laws of cause and effect suggest that it's more
likely that an overflowing reservoir leads to replacement with straight
water than the other way 'round. :-)

Something else: I learned on the list several years ago that Audi
expansion-tank caps are designed to be replaced if there is ever an
overheating incident.  That is, once the coolant boils to the point that it
spews out the cap, buy a new cap.  They have a pressure-relief system built
in that is not intended to be re-used (as with the familiar "Stant" radiator
caps at the auto-parts chains).

The important question is where the leaks are coming from.  If they're
coming from around the overflow cap, then replace the overflow cap and don't
fill up to the cap -- just fill up to the "max" line on the tank.  You need
the air space to allow for the coolant's expansion when it heats up.  If the
leaks are coming from elsewhere in the tank, you need a new tank.  They do
occasionally split from age and heat-cycling, and they're a critical part of
the system.

You may also find (as I did when I started going through the cooling system
in my '83 CGT when I first bought it four years ago) that the small hose
going TO the overflow tank has a leak in it.  In my car, that hose had a
minuscule split just under the hose clamp -- not enough for a visible leak,
but enough that it was losing coolant (and more important, losing pressure)
through the split.  The roadside fix for this used two of the blades on my
Swiss army knife: cut off the split portion of the hose, then screw the hose
clamp back on.  (Then drive to the auto parts store and buy a new hose...  I
swear these cars KNOW when you're kludging them, and a good one will let you
get away with it but only for as long as it takes you to get the right

In short, the cooling system is only as strong as its weakest link.  After
spending some time searching for successive weak links, I finally gave up
and replaced all the hoses, the thermostat, and the cap.  Did that two years
ago and have had no problems with cooling since, even in 100 degree heat
while climbing the Siskiyous at 80 mph with the air conditioning on, which I
do twice a month during the summer on my drives from Portland to San

Brett's cautions about the long-term use of straight water are well taken.
There's one exception, which may not apply to you: most of my friends who
work corner stations in the SCCA really appreciate it when you run straight
water (with a lubricant/corrosion protector like Water Wetter to avoid two
of the problems Brett mentions) if you're going to be driving on track.
They tell me that spilled coolant is every bit as slippery as oil.  This is
a hazard not only to the other cars that come charging into the impact zone
(what's that yellow thing they're flapping at me, can't they see I'm
racing???), but also to my pals in worker-whites who may have to run out and
pull you from the wreckage while slipping and sliding in a puddle of green
goo.  (Can you tell I'm having dinner with a couple of corner workers before
the Great Big Sea concert tonight? :-)

--Scott Fisher
  Tualatin, Oregon

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