At 06:35 PM 6/21/2012, neil n wrote:
>foot taken off gas pedal, throttle valve switch and butterfly closes, IF
>very little or no air came through the AFM air bypass, the O2 sensor would
>sniff a rich mix and affect the ECU fuel shut off process.

Nope.  Once it goes into deceleration mode it's done, the engine is
not running.  On the Digifant (2.1l) ECU, engine rpm has to have
exceeded 2500 rpm or so before it will enter decel mode and stay that
way until the throttle is opened or rpm gets down near 1100 or
so.  I'm not certain whether that means it has to get to 2500 once
since the last time the rpm reached the lower threshold point, or
once each time the throttle switch opens.

On the Digijet it's simpler, close the throttle with rpm above ~1500
and you're shut down until the throttle opens or you reach one of the
threshold points.

>  But with your
>comments, I can see that once the fuel shuts off, it shuts off. I know
>there's a test for this in Bentley but it requires a VW tool (resistor?) to
>be inserted in the throttle valve switch plug. Maybe archives has info on
>that.

Simple test is put a voltmeter on the O2 sensor (or one of Ken
Lewis's neat little LM3914 bar graph indicators which unlike a DMM
are very fast reacting and make excellent little monitors to stick in
the panel somewhere like the spare switch cover).  If the decel mode
is working properly the sender will go to ~0 output.  Or use a 'noid
light on an injector.  But for monitoring that switch and general
closed-cycle/O2 sensor behavior from the cockpit, Ken's little dingus
is the way to go.  Personally I'd rig it with a shielded cable like
RG-174, grounded at one end like the wire from the ECU is.  That way
you won't be putting a CB antenna on your oxygen sensor.

The other cockpit on the closed throttle switch on the 2.1l engines
is the idle.  Normally when you close the throttle (vehicle stopped)
the rpm drops to a bit over a thousand for a couple seconds, then
makes a distinct drop to its final rpm.  If the switch doesn't close,
it will stay a little higher than the usual first step and be a bit unstable.

Yrs,
d