Mark;
One possibility is the old O2 sensor had your system running lean, hence
the good gas milage. Lean running means higher temperatures though. Any
changes in operting temps?
Drive Safely & Good Luck
Ken Lewis<Kernersville,NC>

On Tue, 9 Jun 1998 17:12:34 -0700 Mark McCulley <raven@HALCYON.COM>
writes:
>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
>

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