>
>The story so far:  about two months ago a sudden increase (or should it be a
>decrease?) in mileage from 27 mpg to 10 mpg. About a month ago, increase of
>about
>ten thousand percent in black smoke emission.  (Sure discourages tail-gaters!) 
>Lots of excellent advice and suggestions from you folk, change of ECU and Temp.
>sensor 2, adjustment of AFM.  Minimal improvement.
>
>Friday, finally took it to a mechanic.  It took him three hours to figure out
>what was causing the problem and I think it was a great piece of detective
work.
>
>Wire coming from the O2 sensor on the 84 is a single lead that plugs into
>another
>lead coming out of the wiring harness.  That harness lead goes into the
multipin
>connector.  For reasons known only to themselves, VW engineers made that
harness
>lead a co-axial cable with the shield grounded.  Somewhere along that lead,
>inside the harness, the insulation around the inner wire broke and allowed the
>signal to ground on the shield.  
>
>The mechanic, Niel, took endless trouble to explain the operation to me.  Seems
>at 600 degrees the O2 sensor generates a 0.5 volt signal to the computer.  Over
>that voltage indicates a rich mixture and the ECU decreases the fuel flow.
>Under
>0.5 volts indicates a weak mixture and the ECU feeds in more gas.  A grounded
>wire will produce a 0.0 voltage so guess what the ECU does?!!.  Niel simply
took
>apart the multipin connector and checked the voltage at the pin, then cut
>off the
>faulty lead and replaced it with a single wire lead.  Now ole Smoky purrs like
>a kitten and I am looking forward to 27mpg. again.
>
>
>
>