The O2 sensor input is actually an input to a transistor based
amplifier. The leakage at that junction is ~ .6 volt. Without the O2
sensor connected to load it down, the input sees this voltage as the .5
to .6 volt it is trying to maintain. Therefore, no further adjustment is
made. The only time this doesn't work, is if the O2 sensor ground is
floating higher that 0. The leakage voltage on the ground circuit gets
added to the .6 volt and the ECU will keep enriching the mixture until
it sees what it wants. Here comes the rich running and black smoke.
Check those grounds. The same thing will happen if the o2 sensor lead is
grounding through the shielded cable. This can be caused by water,
especially road salt water getting between the layers. The O2 sensor
reference ground is a separate wire in the FI harness. On syncros, it is
usually connected at the base of the oil breather tower. Plastic makes
for a lousy connection.

Most engine running problems come down to the O2 circuit or the temp 2
circuit. An easy check is to quickly turn the ignition off and back on
while the engine is still running. If the problem goes away for 30 to 60
seconds, the problem is the O2 circuit. If not, check the temp 2
circuit. Hard starting , especially after the engine is warm is usually
and bad connection on the temp 2 sensor. It is extremely rare for this
sensor to go bad, but the connection has a lot to be desired. The
problem is worsened because the idle stabilizer uses this same sensor.
Of the 2 wires on this sensor, one goes to the ecu, the other is a
ground. Add another ground here.

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf
Of Robert Smythe
Sent: Wednesday, January 18, 2006 10:11 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: running problems, 2.1

when you disconnect the O2, sensor I figure the voltage output goes to
zero.
How does the Fuel injection treat this output?

Robert
90 MV