Hi Dennis --

At 10:31 PM 8/3/2008 -0400, Dennis Haynes wrote:
>Any adjustment of the air flow that makes any difference is proof the o2
>sensor circuit is not working properly. The only time the AFM needs
>adjustment is after some one has tampered with it.

Or when its characteristics have changed...

>That said, your situation with disconnecting the O2 sensor instead of the
>gauge and the resulting response tells me that the gauge is a problem in
>itself.

Only if disconnecting the gauge makes the symptoms disappear...

>Normally, the 1.9 engines will run fine with the O2 sensor disconnected as
>long as it disconnected before the engine is started. The Leakage at the
>O2 sensor input (the .6 bias volts at that op amp or transition input) is
>close enough to what the ECU is try to accomplish that basically no
>adjustment is being made.

More than that, they stay in closed circuit mode and don't refer to
the sensor at all until they see it starting to override the bias signal.

>Now if the O2 sensor meter draws this circuit to ground, it will lower the
>voltage and the ECU will respond by enriching the mixture. Since the
>voltage is not going up as it normally would with the O2 sensor working
>properly, you are going into mixture run away.

True -- although Mike said that the meter stayed centered, suggesting
that the ECU was back in open-loop mode?  Will it return to open-loop
when it's once gone to closed-loop?  But actually in this case it
would have stayed open-loop from start unless the antenna effect of
the long wire was driving the circuit at speeds too high to notice on
the meter.

>Now if this is the case it is likely the meter is also loading down the O2
>sensor. This will cause the engine to always run slightly rich.

The typical input bias on that meter chip (LM3914) is 25 nanoamps, so
I really don't think that's an issue.

David

--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage," '85 GL "Poor Relation"