Mixture problem

[Bruce Kaye <bruce.kaye@edrd.dnd.ca>]

When I last wrote I was having problems with the intake valve of number 5
cylinder leaking  (NA engine) in my wife's car.  The question was to do the
head, swap heads with my NF parts car or swap engines with the parts car.
I wimped out and did the head.  Now the (new) problem ... the thing that
lead me to the bad valve was a noise in the exhaust system ... it's still
there and the engine runs as if the mixture is to rich and it is but I (we)
can't get it right. Gas mileage is way down. With the O2 sensor
disconnected the mixture can be set to 1% CO and then when the O2 sensor is
connected it moves to 1.2%.  I've changed the WP,TB,lifters,injectors and
valve seals. All hoses have been checked and doubled checked. The exhaust
manifold was planned and new studs installed.  The O2 sensor has been
replaced twice.  The cat was just change 3 mon ago and the middle muffler
was just changed because of a broken/loose baffle.  I'm looking at changing
the ECU.  I need help...

Thanks 

Bruce 

87 5000CD Quattro Avant
87 5000s Avant (parts)
86 5000s Avant (wife's) 
83 GPZ 1100
75 TR6