[Biturbos4] Strange loss of coolant

[Ti Kan]

Hi folks,

My 2001.5 S4 did something weird that I cannot explain, maybe
someone out there would have an idea what happened.

A few days ago I drove home as usual, the car ran fine without
any signs of trouble.  It was a very hot day so I had my A/C
running.  Upon arrival, I parked it outside, and noticed a small
trail of water behind the car.  It didn't concern me at the
time, thinking that it was just condensation from the A/C.

About 3 hours later I leave to go run an errand.  As soon as
I started the car, the auto-check system displayed the big
"low coolant" warning symbol.  I shut off the car to check
under the hood.  Sure enough, the coolant reservoir was dry.
I had checked the level recently, so this is not due to a
slow leak.  However, looking around underhood, I cannot see
an obvious place where the leak might be.  The first thing
that comes to mind was hoses, the expansion tank, or the
expansion tank cap.  There was no sign of coolant or wetness
anywhere that I could see, except on a small hose that runs
from the fuel pressure regulator to the intake manifold.  The
hose looks like a vacuum hose to me, it has a woven cloth
exterior, but it was wet.  I don't think the hose itself was
the culprit, but perhaps the coolant leaked from something
and got this hose wet.

I also examined the radiator area, the heater hoses, the
plenum area where the heater hoses go to the heater core.
I also checked the interior floor carpet... all dry.  There
was no musty smell in the cabin.

Hmm.  OK, so I refilled the expansion tank back to the proper
level with distilled water and G12 coolant mix.  The total
amount I had to pour in was a little over 1/3 gallon water
and the same amount of G12, making it 2/3 gallons total.
The cooling system capacity was not listed in the owners manual,
so I searched for it on the net, and found it to be 1.58 gallon.
This means that the system lost 2/3 gallons, but still had about
0.9 gallons in it.

That's enough coolant loss, more than what would only get a
small hose wet.  I dried that hsoe with a paper towel just to
see if it gets wet again.

I drove the car for 2 days now, and the coolant level
is staying pretty much where I filled it.  I don't see any
signs of leak, no puddle under the car, no trail of water behind
the car.  It seems as if the problem cured itself.  That small
hose feels a little damp.  I'm not sure if it's new leak
or it's still coolant from before.  The cloth texture of
the hose seem to retain some water even after I wiped it.

I am still baffled about this, and carry distilled water and
G12 in the trunk just in case, but what could cause an apparent
big loss of coolant and then act as if it never happened?

Sorry about the long post.

-Ti
-- 
2003 A4 1.8T
2001.5 S4 2.7T
1984 5000S turbo
1980 4000 2.0