During testing of my O2 sensor, I found that the VW Temp II sensor seems
to engage the closed loop operation sooner than an aftermarket type,
which admittedly ran only a minute or so longer in closed loop. So even
thought the O2 sensor must be at operating temperature, it appeared to
me that the Temp II sensor in my case was the slower of the two in a
cold engine start up.

So if you want to experiment put a volt meter on the O2 sensor black
lead and time how long the system is in open loop.  You might also note
the voltages just before the closed loop starts.  If the volts are .7 or
higher. you might need to adjust the AFM spring or or wiper arm.  I
believe Darrell's site contains the instructions.

 The theory being that the rich running engine will be leaned by closed
loop operation sooner.

Sincerely,

Mark Keller
91 Carat