91 200q Boost and WG springs (kinda long)

["Mike Miller" <MikeMilr@email.msn.com>]

I have been chasing an intermittent low boost problem for several months on
my 91 200q. It would usually max out at about 1.6 bar, but occasionally
would actually get to 1.8. The difference in power is *very* noticeable. It
would spool much faster when it was going to 1.8 and the turbo "whine" was
much higher pitched. I noticed recently that if I had the car fully warmed
up, parked it for about 10 minutes, then drove it again, I would get 1.8
boost for about the first minute. By this behaviour, I guessed that
something was warming up (heat soaking?) while the car was sitting and
cooling off again once I was moving. Well, I had a stuck open thermostat.
Replaced that, MF switch and *all* water hoses (~$700) and boost was
noticeably better, but still not to 1.8 consistently.
A while back I had purchased a wastegate out of an 86 5k from a lister . I
was cleaning it up and decided to see if there were any differences in
springs between mine and the old one. Well, mine was about 1/8 inch taller,
looked to be made of the same diameter wire, but was *quite* a bit less
stiff than the older one. So I put the older one in and went for a test
drive. YEE HAA - boost gauge reading 1.9 in 2nd gear (7k rpm) at the end of
the freeway onramp (tight 270 deg turn onto freeway) - hit 3rd and held at
1.8/1.9 clear to redline. After a couple days it still hits 1.8/1.9 just
about everytime in 2nd gear on up. (air temp around 60 dF)
Now the questions/comments:
I had always heard that on the 3B engine, the WG spring tension should not
make a difference because boost is controlled by WG frequency valve. Does
the mapping for the frequency valve just say to pulse the WGFV  x times at
y boost assuming a certain spring tension in the WG? Example - pulse 10/sec
@1.4, 15/sec @ 1.5, etc up to "pulse it with all you got, I am going into
overboost". This would make sense if my spring were "tired". The FV would
be pulsed enuf at a lower boost to hold the WG open against the weak
spring, preventing higher boosts. I am certain the WG is opening well below
max boost - this is easy to verify by just removing/clamping the hose to
the WG bottom - very rapid spool up and overboost cutout at maybe 1/4
throttle. Anyone know if there is a cutout for a turbo spooling up too fast
(boost rate of increase limit)?? I have hit a cutout at like 1.3 indicated
with the turbo spooling *very* quickly. 
Next - Now at max throttle/boost, the turbo whine varies like a siren. This
indicates to me that the FV is modulating the WG as it should be to
maintain max boost. Is this correct? Does anyone else hear the turbo whine
oscillating at WOT?
And finally - The only problem observed with the stiffer spring is that
after laying into it in 2nd and 3rd (taking both to near redline), about 2
seconds_after_I back off and resume "normal" driving, I get the check
engine light. Any ideas here? (not possible to dump codes on a 91 200q -
need the VAG 1551). If it was a knock problem, I would expect to see the ck
eng. lite while on WOT. I do not see the  ck eng. lite if on it for less
than 2 full gears (2nd  & 3rd). Haven't had the oppurtunity (yet) to try
approaching redline in 4th (~130)  :-) or holding high boost for more than
10 seconds or so. 

TIA,
mike miller
91 200q