[BiturboS4] help diagnosing problem...

[California Fields]

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If you recall, I asked you guys about the following problem:  I noticed that
my boost levels peak only to 16-17 psi (when floored at lower rpm) and then
level off around 14-15psi (when floored during mid-hi rpm) and start falling
to 10-12psi (at rpm above 5000). Now, I realize that the boost normally drops
off as you get high in rpm, but when I first got my chip the spikes were
18-19psi and leveled to about15-16psi and fell just below 15psi at high rpm.
So, I'm obviously not getting the same boost now as I was before.  Also, I
could swear that my turbos sound louder (more siren like noise) especially
when cold...but that's more subjective.  To top that, I raced another S4 on
the street a few days ago. The situation didn't allow for a direct
side-by-side comparison, but it was close enough. Basically, he pulled on my
through 2nd and third gears fairly strongly. Everything on the outside of the
car looked stock, so I'm assuming he just had a chip. I didn't get a chance to
ask him.  Seeing as I have the GIAC-X, I'd think that another chipped S4
shouldn't pull like that.

Well, what's new? I decided to let the dealer take a look at the car since my
20k service interval was approaching.  I swapped back the stock chip last
night and drove the car around a bit.  The boost seemed normal - spikes to
9psi and leveling off to 7psi. So, the boost hasn't changed over the months at
least with the stock chip.

The dealer said they didn't find anything wrong with the turbo system. I
specifically asked them to check for intake and boost leaks and they said
nothing was found. They said they checked the TBB and nothing was wrong.  No
DTCs reported.
What could be causing my loss of boost with the GIAC chip? Is it the chip? Or,
could there be a leak that is only noticeable with the aftermarket chip but
not the stock? Or, are the turbos starting to fail but are able to appear fine
under stock boost levels?

Any ideas would be helpful. I'm going to try and get it in for a dyno.

Thanks, Carter.
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Carter Fields, San Francisco
2001.5, GIAC-X, Miltek 3" cat-back, Strat. shifter, Strat. DVs, A-pillar boost
gauge
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