The O2 sensor, besides not coming into consideration by the computer until
warm, relatively, only makes minor adjustments to the fuel mixture.  Running
extremely rich is due to something else, eg temp sender, leaking injectors
including cold start, misfire.  Retrace your steps.  If it was ok before you
started, most times the problem lies in where you've been.  (Knocked off
wires, hoses, etc)  Been there too many times.  It could be something new, but
look at your work first, keep smiling, keep cool.  Remember, vanagons are
generally too heavy to push to the edge of the nearest cliff!   p.