Gary Stearns wrote

  I need some basics on the operation of the digifant system.  I too
have had problems with the lumpy idle, stalling etc.

Hi Gary,

Idle quality can be affected by a lot of things, note the following:

All basics must be set to VW specifications including ign. timing, valve
clearance, spark plug type & gap. Also the air flow meter values must be
at the factory calibration point. The idle stabilizer system must be
working (valve vibrating)and responding to a good closed throttle
signal. Fuel rail pressure must be at specs (~2.5B)with the vacuum line
connected and the crankcase oil should be free of gasoline contamination
with no crankcase air leaks.

If all of the above is in order, the O2 sensor should be producing a
~200 to ~800mv "swing" @ hot idle speed. If signal stays around 200mv,
the mixture is lean, if it stays around 800mv it's rich.

The most common lean mixture problem is an unmetered air leak and your
statement about richening the mixture to achieve a better idle quality
points to such an air leak. A through examination of EVERYTHING from the
AFM to the intake ports would be prudent.

The AFM bypass screw is just a "trim" tool to adjust for minor
variations in production tolerences. It has the greatest influence at
idle air volume and less as the throttle opens wide.

The O2 sensor signal is one of several "influences"(i.e. coolant temp,
air flow volume, RPM, etc)that cause the Digifant control module to
alter the milli-second injector opening time. Changing the ign timing or
AFM settings is covering up some other basic basic fault and putting
your diagnostic efforts into a "no man's land" where reliable diagnostic
procedures can't apply. Why not restore EVERYTHING to factory specs and
if you hear hoofbeats--it's probably horses.

Regards,      John