[Biturbos4] Strange loss of coolant

[Ti Kan]

Hi folks,

Thanks for comments from Al S., Andrew Lundy, Kneale Brownson, Geraldo
Candela, John Lag see, Mark Rosenkrantz, Chris Semple, Dave Eaton and
Wylie Bean.

Here's an update to the problem.

After driving for a few days (over 100 miles) with apparently no
more coolant leaks, today I noticed a leak again.  It occurred right
after shutting off the car from an ordinary 20-minute drive in
about 85 deg. F weather.  The puddle below the car suggests no more
than about one quart lost, and that's how much I had to refill to
get the level up to normal again.  Then, I was able to drive around
for the rest of the day without any further leaks.

While looking around under the hood, what I do know is that the leak is
not coming from the coolant reservoir or the cap.  Everything looked
dry, there is no G12 residue anywhere that I could see, and the sound
absorbing liner under the hood was also dry with no sign of hot steam
on it.  Of course the engine bay in an S4 2.7T biturbo is quite cramped
and one couldn't see much through the gaps.  I did not remove the
under-engine cover to look at the bottom side.

I also know that it's not a problem with the electric radiator fans.
They operate just fine when I turn on the A/C, and also when the after-run
auxiliary coolant pump is activated.  Btw, I am running a lower than stock
temperature switch for the auxiliary coolant pump and fans, so that it
helps to cool things down after shutting off.

It's also not likely to be an internal leak (i.e., into the combustion
chamber).  There is no smoke out the tail pipes, the engine oil
level looks normal, with good color with no sludge on the dipstick.

So right now I think I've narrowed it down to two possibilities:

1. A pin-hole on a hose somewhere that leaks only when above a certain
   pressure.
2. Electric auxiliary coolant pump

#1 would obviously explain why I lose coolant only occasionally,
but would #2 also exhibit a similar symptom?  FYI my auxiliary
coolant pump was replaced a year and 3 months ago.  That would be
a really short life if it had gone bad...

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

> 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?
>
> -Ti