Help....urq turbo problems

[ejfluhr@austin.ibm.com (Fluhr)]

My turbo experience is still somewhat weak, so I appeal to you guys for
some help...


The weekend of March 13th-14th I took the urquattro out to Texas World
Speedway for the track event.  The car ran great on Saturday, ~1bar of
boost pressure with fresh Mobil-1, and cold temps for the oil cooler and
intercooler.  On the 4th track session Sunday morning, however, I
noticed the boost pressure dropping.  Initially it just dropped to 13psi
of boost, and I thought perhaps it was heat soak due to the warmer temps
we were getting.  After a couple of laps, however, I knew something
was wrong because the boost pressure was dropping a couple of psi every
time around.  By the time it got around to ~7psi, I noticed some kind of
whistling/whooshing sound coming from the engine.  My first thought was
a blown intake hose...no big deal.  However, after taking the intake
tract apart, I could find no split or loose hoses.  The intercooler
seems to hold pressure well, and the turbo shaft has no noticeable
radial or axial play when I wiggle it with my fingers.  The turbo also
spins very smoothly.  The wastegate diaphram is not torn, and the
wastegate appears to function smoothly.  I do not think this could 
cause the problem, but at some point the center muffler developed a
crack around 1/2 of the outlet pipe.


Of course, as we all know, the Audi gods do not go lightly on us. 
I buttoned the car back together and went out to do some runs to see
if the problem still existed.  What happened next really baffles me.
I was pushing full throttle in second gear and the boost climbed up
to around 7psi, then the engine sort of lurched, and the boost pressure
jumped up to 13psi.  After another pass (or maybe the next pass), when
the boost pressure reached around 7psi, I heard some kind of screeching
sound from the engine.  Now the screeching is there anytime I reach
around 7psi of pressure.   I cannot tell if the sound is from air
escaping, or has something to do with the turbo bearings or impeller.

I worked on the vacuum system again this weekend, taking it apart,
checking everything, and putting it back together.  I did not find
anything seriously wrong, and the car idled better after I put it
all back together.  The frequency valve operates right around
50% duty cycle when idling hot, so the car is not running lean.  It
does drop to ~26% when the engine is steadily running at ~2000rpm,
and I am not sure why.  It should not affect the turbo, though.
The exhaust manifold does not appear to be warped or cracked...at
least I cannot hear anything from it while under the hood.

Rick Schwalenberg suggested that the cold-side impeller nut may be
a bit loose, and that the impeller is contacting the housing when
it wobbles under high boost pressure.  I will check it, but I
do not find any metal shavings or fresh oil in the turbo-to-intercooler
hose, which I would expect with such a problem.

So, any suggestions would be greatly appreciated.

Thanks,
Eric
'85 CGT, injured '82 urq 
---
Eric J. Fluhr                                Email:  ejfluhr@austin.ibm.com
High End Processor Design                    Phone:  (512) 838-7589
IBM Server Group                             Austin, TX