The air bypass or mixture adjusting screw will not bypass enough air for the
AFM vane to stay closed. If it could than the air cooled engines would have
a problem as the AFM vane has to open to run the fuel pump. It is just a
trim adjustment. It must be noted that when trying to adjust this you need
to disconnect the O2 sensor and restart the engine after the disconnect to
not have any O2 sensor compensation. Adjust fir ~.5 volt at the sensor with
it disconnected. Then when you connect it you should see the system react
and change the sensor readings. Normal fluctuation should be ~.2 to .8 volt.

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
neil n
Sent: Friday, July 06, 2012 4:52 PM
To: vanagon@GERRY.VANAGON.COM
Subject: At Idle: AFM Flap, CO Screw Relationship. Explanation Found

Hi all.

Was curious about the relationahip between the AFM air bypass and AFM flap.
Maybe someone else is too.

http://www.race.nangreaves.com/golf/FU01/ch7.6.html

From that page (Digifant II as used in other VW's but very similar to the
2.1 WBX it seems):

"The idle mixture (% CO) is adjusted by turning the idle mixture screw,
changing the amount of air allowed to bypass the air flow sensor flap. This
changes the opening force acting on the flap, which changes the
potentiometer signal to the control unit. Since the injector opening time
(and therefore the amount of fuel delivered) is influenced by the
potentiometer signal, the idle mixture is changed."

Through my various stages of trouble shooting and learning about Digifant on
the 2.1 WBX, I had figured that the air bypass, at idle, would allow the AFM
to stay closed. e.g. that air bypassed this flap and that the mixture was
controlled by adjustment of air only, via the CO screw. Not true it seems.

Neil.

--
Neil n

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