Re : 91 200q Boost and WG springs (kinda long)

[Dave Lawson <dlawson@ball.com>]

Hi Mike,
>
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. 
>
The spring tension does make a difference as you found out. One key
difference
between the 10V turbo and 20V turbo cars is where the WGFV operates. On
the 
10V cars, the WGFV switches between intake mainfold boost and vacum
(from the
turbo inlet) on a rpm based lookup duty cycle and then applies this air
signal
to the top side of the wastegate, where the spring resides. The
wastegate actuation 
pressure, taken from before the throttle body is applied to the bottom
side of the
wastegate.

On the 20V engines, the WGFV operates on the wastegate actuation
pressure which
is still routed to the bottom side of the wastegate. With this setup the
air connection on
the top side of the wastegate isn't connected to anything and has
ambient pressure present
in the top side. When the WGFV operates on the actuation pressure, it
bleeds off pressure
to produce a more intake manifold boost pressure. And to reduce intake
manifold 
boost pressure, it applies more pressure to the actuation side of the
wastegate which 
opens the WG valve and dumps the exhaust gases from the exhaust manifold
to the downpipe,
bypassing the turbine section of the turbocharger.

With the stiffer spring installed the ECU was probably trying to reduce
the boost pressure
by changing the WGFV duty cycle, but at the max setting it still
couldn't reduce the boost
pressure, which the stiffer spring was allowing. The ECU probably
flagged this as a 
sensor/actuator fault and triggered your check engine light.

>
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. 
>
The MAC02 in the ur-q and the MAC11 in the 5000/200 turbo cars have this
check.
I would assume the 91 also does this, but not absolutely sure.

>
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?
>
I can't comment on this as I don't drive a 20V turbo regularly.

>
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. 
>

See above.

HTH
-
Dave Lawson