[Biturbos4] Strange loss of coolant

[Ti Kan]

Thanks to all for further comments.  I have removed the under-engine
belly pan and waiting for a coolant leak to happen to again.  The
inside of the belly pan itself does not show conclusively where the
leak is from.  There is some G12 residue scattered toward the back
of the engine, but not localized enough to pinpoint where.  It's not
from the front of the engine.

So, the likely culprit is still one of the following:

1. Coolant reservoir or cap (despite no evidence of G12 seen)
2. Auxiliary electric coolant pump, or hoses to it.

Some additional observations: A small coolant hose from the
reservoir goes to a T-piece where it branches into two other
hoses.  There is some crusty deposit on that T-piece, and
I feel a slight dampness there.  But there is no sign of G12
residue right below it (where the engine valve cover plastic
trim and engine oil cap is).

Btw, Dan Haller suggested using a V6 Passat coolant reservoir, but
the part numbers are different. The 2.7T engine uses 8D0121403M
whereas the other 2.8 V6 Passat and Audis use 8D0121403L.  They
look similar in online photos but I don't know what the difference is.

To Geraldo Candelas, regarding the auxiliary electric coolant pump,
it was replaced 1 year and 3 months ago apparently with no change to
the hoses.

To Andrew Lundy and Ted Haug, the main water pump was also replaced
1 year and 3 months ago along with a timing belt job including the
thermostat and tensioner.  The thermostat housing was upgraded to
the metal one.

To Paul Caouette, I guess it's possible that a coolant hose isn't
inserted fully, but so far I don't see any.

I am waiting for another leak epsisode.  Hopefully it will be a
bit more telltale, now that I have the belly pan taken off.

Best regards,

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