[200q20v] Lack of boost very puzzling... long winded question

[Cordeiro, Alan]

Hi everyone,

I have been trying to figure out a lack of boost for the last many
months, not urgent, since the car has considerable performance even
at 1.5 bar... however I have run out of ideas.

The patient is a 200q20v, which seems to refuse to allow me more than
1.5 bar boost. When I purchased the car it only had 1.3-1.4, which
was cured by replacing the wastegate solenoid (open circuit failure).
Boost improved to 1.5 immediately, which caused failure of the accordion
hose shortly thereafter (weak with age, low boost prolonged its life).

In trying to cure an occasional stalling problem, I discovered a leak
in the bypass valve, replaced that, but still only see 1.6 bar on rare 
occasions. 

The only code that occasionally reappears is the idle switch, sometimes
it takes two or three days for this code to re-appear. I have put a 
meter across the throttle position potentiometer, and watch it run smoothly
from 0.3 to 4.7 volts. I also checked the engine temp sensor and the 
multifunction sensor was replaced by me to fix a "too low" temperature
reading after I replaced the thermostat with no noticeable improvement
in running closer to normal engine temperatures.

I have measured the output of the altimeter, it sits just over 4 volts,
seems about right for the Michigan area. By temporarily putting in a much
stronger spring, the gauge goes to 1.9 almost right away and the boost
cutoff
shuts me down, so the turbo is just fine. (It spools right up to 1.3 almost
as soon as the rpm gets over 1500, another good indication).

I have a 1.2 watt lamp across the wastegate solenoid, and watch the lamp
glow solid when I floor the loud pedal. Around 1.3 it begins to flicker
and by 1.5 it is pretty much totally off, eliminating all further boost.
So it would appear the CPU feels that for some reason it should limit the
boost to 1.5. BTW the gauge seems right, it reads 0.9 or 1.0 when I first
turn on the ignition, and runs 0.5 to 0.6 at idle, 0.7-0.8 cruising at
"typical freeway speeds".

I am using 93 octane, but I have experimented with lowering it to 87
just to see if the boost control point goes down some more, but it has no
effect. And the dash (added) check engine light never comes on after
the engine starts, and there is no evidence of knocking (even when I 
put in 87 octane).

Any ideas, does this ring a bell with someone???

Thanks,

Alan
'91 200q20v, 150k