Following a recent tuneup (plugs, rotor, cap) and O2 sensor change my
mileage has dropped almost 3mpg (from ~18 to ~15). I just replaced parts and
did not change the timing or CO settings. I'm using the same plugs as before
(W7DTC) and the engine seems to run just fine. The new O2 sensor was the
generic Bosch 3-wire sensor that I spliced onto the old connector. I'm at a
loss to explain this--I've been through 3 tanks of gas so it's not just an
anomoly in measuring mpg. Could the new O2 sensor be causing the FI to run
richer somehow? Should I have the CO readjusted? Any other ideas?

Mark McCulley
87 syncro Westfalia