At 07:32 PM 8/3/2008 -0400, Allan Streib wrote:
>Wait a minute -- so you are saying that in theory if a US spec Vanagon
>has had the cat removed and is running a straight pipe that it does
>not need an O2 sensor at all?

Not theory.  O2 sensor is strictly emissions-related and enforces a
very narrow range of mixture oscillating back and forth across
stoich, which is what the three-way (nitrogen oxides, carbon
monoxide, hydrocarbons --> nitrogen, CO2, water) CAT requires.  It
prevents tuning the motor for either max economy (leaner) or max
power (richer).

As a *side-effect* it will compensate for other difficulties that
affect the mixture equally across all four cylinders -- thus making
them  impossible to diagnose without disconnecting the sensor.  As a
different side effect, if one cylinder is misfiring or there's an
exhaust leak upstream of the sensor it will drive the mixture way
rich trying to get rid of the excess oxygen.

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