'89 200q idle mixture adjustment oddity and questions...

[JustaxPHX@aol.com]

This weekend, I've been chasing down a cold start problem/lean condition that 
recently appeared on my '89 200q ... mine is one of the later ones, with dual 
knock-sensors, K24 turbo, etc.  The car has 145k on it and so far as I can 
tell, both from examining the service records from the previous owner and the 
car itself, everything on the car (except the fuel filter, of course!) is 
original, including the throttle switch, which appears to be working fine and 
is testing okay.  The fuel pressure also checked out okay as did the control 
pressure at the warm-up regulator and all of the misc. sensors and switches.

I recently replaced the injectors and inserts because they had started to 
leak.  Yesterday, I determined that while the 02 sensor was still working, it 
was responding sluggishly so I replaced that as well.  I then hooked up a DMM 
and went for a test drive while monitoring the O2 output voltage ... to my 
surprise, the car was running *very* lean on closed throttle (less than .1 
volts) and typically reading .035 to .060 volts at idle.  However, the 
voltage reading would increase to .3 to .4 volts immediately then slowly 
settle back to .2 to .3 volts if I blipped the throttle then let it return to 
idle.  If I nailed the throttle, the voltage would jump up as high as .875 
volts like it's supposed to, which suggests the throttle switch, despite its 
age and high mileage, is indeed working properly as my tests show.

I had some other things to do last night so I left it until this morning, 
when I took it out for a drive and checked the frequency valve duty-cycle.  
Aha ... it was reading 60-62%, which is on the lean side, and I figured that 
all I would need to do is give the 3mm screw on the airflow sensor a small 
turn and bring it back into spec.  However, I found  the access hole was 
still plugged -- I told you everything was original! -- and proceeded to 
drill it out.  Beneath it, I found a second small metal plug that was *not* 
made of aluminum like the first and had to fish this out too before the hole 
was clear.  I then stuck my 3mm T-handle wrench through it and tried to turn 
it but felt no resistance.

Damn, I thought ... somebody's stripped out the adjusting screw!  I got my 
flashlight out to take a closer look and to my amazement, discovered there's 
no adjustment screw in there!!!  I can see threads on the lever arm where it 
goes but there's no screw there ... I wonder where it went?  Judging from the 
look of things, it's been missing for a while since there's an oily film on 
the threads ... what's going on here?  I measured the sensor plate height and 
it's right on the money, so I wonder if Audi chose not to install an 
adjustment screw in the first place?

Although I've got other things to do, it looks like I'll have to remove the 
airflow sensor and find out what's up with it ... fortunately, I have a junk 
one that I can steal a screw from, if necessary, but before I go any further, 
I would like to know if anybody else has any BTDTs to share with me.

Thoroughly puzzled in Scottsdale, AZ ...

JG