On the 2.1, the oxygen sensor grounds via pin 19 of the ECU.  From this pin,
a brown wire runs through the main wiring harness and emerges from the same
bundle as the distributor wiring.  It normally connects to the left head but
often has been moved in older vanagons.  Many a vanagon owner has swapped in
parts trying to diagnose poor performance when the cause was a loose or
missing pin 19 ground. If you have a digitool, try disconnecting this wire
and watch the oxygen sensor readings (and performance) go haywire.  I don't
believe a rusty exhaust system will affect the system at all.

Another point to be aware of is that the big green wire that carries the
oxygen sensor current between the sensor and the ECU is coaxial.  It is a
central wire, a layer of insulation, a grounded sheath, then another layer of
insulation.  If the central wire ever makes contact with the grounded sheath,
the circuit is grounded out and the van will run rich forever till the
problem is corrected.  Often a mechanic crimps a new connector onto the end
of this wire, gets the grounded sheath under the connector and then spends
hours trying to figure out why the engine won't run.

Hope this info helps someone.  I learned some of  it the hard way.

Bill
St. Louis
www.geocities.com/wdummitt