Found yet another cause for rotten gas mileage today.  Last week, the
header pipe from cylinders 1/3 lost the bolts where it joins the
collector. (1.9 wasserboxer) When the bolts fell out, the gasket
disappeared.  Not having a spare, I decided to just put up with the
small leak and just bolted it back together without the gasket.

Fast forward to Monday.  Head to school. (100 mile one-way commute)
3/4s of a tank????  Filled up and headed home.  3/4s of a tank?????
Only difference was the silly gasket.  Replaced the gasket and the
mileage went from 13 to 22.

Hypothesis:  The exhaust leak not only allowed exhaust to escape, it
also allowed fresh air in.  BEFORE the O2 sensor.  O2 sensor reads lean
mixture from lessened and diluted exhaust and tells the ECU to richen
it up to compensate.  Hence 13 mpg.

The upside is that now I *know* that my O2 sensor is working.

Ya learn sumptin gnu every day...

Jim