In an earlier post I asked why I was getting a 2-3 mpg decrease in mileage
after a tuneup and O2 sensor change. Here's an update:

The engine does not seem to be running rich. Tailpipe deposits are dark
gray, not black. Went for emissions testing today, passed with flying
colors. HC was 8ppm at idle and at 2500 rpm, better than my test a year ago.
HC would be higher if van was running rich, right? CO readings were also
lower--at idle CO was 0 ppm. Does this seem odd?

O2 sensor is working and is not grounded or anything. I can attach a
voltmeter and watch it do its thing as the ECU adjusts mixture.

Another variable I neglected to mention is that I installed a new
syncro-style air filter when I did the tuneup. The old air filter was also
the syncro style.

That's all for now...

Mark McCulley
87 syncro Westfalia