Well,
    It's not your O2 sensor.  Go to Ben's  website @
www3.sympatico.ca/huotx/engine1.htm and read the test for the O2 sensor.
Very helpful!  Next take a look at your throttle switch and see if it's
adjusted right.  I bet it isn't.
Cheers-Dave

Matt wrote-

Previously ----

fter my previouse wan problems this is what happened today!!

Today is was DRY so I tried an experiment.  Several times on the way home I
unplugged and replugged the 02 sensor in and out  Id it like every 5-10
minutes.  With 02 sensor slight bucking every once in a while.  Without, the
idle kicked up a little, bucking gone, but acceleration slightly slower.
(Previoius with 02 sensor connectd i was getting 19-20mpg)  Then comes the
weird thing.  A few minutes after the last time I replugged the sensor,
horrible bucking, whuite smoke out exhasut, then black smoke.  Shut off,
disconnected 02 sensor.  Van restarted, ran fine.  ANy Ideas??

SO today i wenot for a longish ride 2+ hours, all was good first hour+, then
The same thing, sputtering, black smoke...  And the 02 sensor is
disconnected???? Any ideas.. The ground wire to the engine is pretty cruddy,
but could that cause my problems?  PS- The 02 sensor wire is the single
spade one right, the big one is the preheat correct???

Matt