According to Bentley's and the VW Digijet Protraining manual for the
1.9l engine, if the Throttle Idle Position Switch is closed (foot off
gas) AND if the engine is at more than 1500 rpm, fuel to the engine will
be cut off.

I know the switch is working and that it is connected to the ECU because
I have an LED which I can see while driving temporarily wired it. The
LED goes dark when my size 10 is off the pedal, indicating that the
switch is shorting ECU pin 4 to ground, and since that voltage comes
from the ECU, the ECU "sees" the switch.

I know the ECU is seeing engine rpms because it would just shut off fuel
if it didn't think the engine was running.

So it appears that the ECU is getting all the input it needs: throttle
idle position switch and rpm.

But when going downhill in gear, foot off gas as indicated by dark LED,
and with the transmission revving the engine above 1500 rpm, the O2
sensor goes full off-scale rich.

The O2 sensor is new, and I'm monitoring it on my Ken Lewis-0-Matic O2
meter. During normal driving and idling, it bounces around in quite a
lively fashion.

Based on this, it appears that the engine is NOT cutting off fuel as the
manual says, and I am puzzled. I've tried a second ECU with the same
result. What, oh what, could I be overlooking?

--
Mike "Rocket J Squirrel" Elliott
71 Type 2: the Wonderbus
84 Westfalia: Mellow Yellow ("The Electrical Banana")
74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano
KG6RCR