[s-cars] still looking for missing boost

[Theodore Chen]

symptoms: inconsistent low boost.  sometimes i get full boost or
close to it.  other times i get only a few psi (which i assume
is due to the wastegate spring).   it is inconsistent and changes
during the course of a drive.  it has occasionally even changed
during the same acceleration run.  sometimes it feels like it's
spooling slowly and then the boost suddenly comes on.

things done so far:
1.  replaced all of the little boost hoses between intake, 
fuel pressure regulator, tee, moisture trap, and ECU.  used 
bicycle pump to verify that it would hold at least 20 psi, and 
a mityvac showed no leaks.
2.  all hoses in intake tract (turbo to XC, XC to IC, IC
to TB) are samco with t-bolt clamps, and have been checked.
3.  bypass valve is a forge, and its operation has been
verified with a mityvac.  i also tried to blow through it
to confirm that it wasn't leaking.
4.  replaced little boost/vacuum hose from manifold to bypass
valve.
5.  output tests run and WGFV (wastegate frequency valve)
clicks.
6.  ECU checked with VAG-COM.  no codes except for engine
position sensor (the engine wasn't running so it's ok).
7.  pressurized the hose from the WGFV to the WG to 20 psi
with a bicycle pump.  it leaked down quickly to about 10 psi,
held, and then suddenly leaked all the way down, with an
audible sound from the wastegate.  this happened every time.
i'm not sure if this is normal.  is it?

can a WGFV pass the output test and still be bad?  is it
possible for the WGFV to fail intermittently?  or work well
enough to pass the output test but not cycle rapidly when
commanded to do so by the ECU?

i'm not sure what to try next.  i could replace the O2 sensor,
since i'm not sure it has ever been replaced.  the car has
nearly 132k miles now.

by the way, i now have a hoppen-sourced silicone turbo to XC
hose available if anybody needs to buy one.  perfect condition.
i wasted time swapping it, but at least i eliminated one more
possibility.

-teddy



		
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