At 06:59 PM 8/29/2008 -0400, pickle vanagon wrote:
>I noticed something funny.  While engine braking down a long hill, all the
>clear air coming through the exhaust system, while obviously hot, is
>apparently not quite as hot as the sensor needs to be, so that it will
>eventually cool down to the point where it no longer provides good output
>and behaves like it does right after startup.  After driving for a little,
>it warms up again and output is normal.

Sure.  If you had a long enough hill the coolant would all cool off,
too.  It's just as though you'd shut the key off.  On the 2.1l
engines, though, the sensor is electrically heated so it would stay hot.

>I'm wondering what the ECU does during this stage.  Does it know to start
>ignoring the oxygen sensor if it thinks its output doesn't make sense?  Or
>does it just keep chugging along, thinking everything is peachy, and
>consequently send too much or too little fuel as the oxygen sensor fails to
>react dramatically enough to the changes in ratios of the exhaust gasses?

Dennis Haynes is the expert on this, but I think it's safe to say
that VW/Bosch set things up to work correctly.  I believe the ECU can
easily tell when it's driving the sensor, vs when the sensor is driving it.

David

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