Oh, another thought on this old thread..... something that should always
be checked if it's not your car: if you're seeing O2 sensor readings that
are super-rich, especially if they are consistently rich on all sensors,
make sure the engine grounds are good. Missing engine ground cables and/or
severe rust can cause sensors to wind up with a poor ground reference and
then you get bad sensor values.

The original poster's engine didn't sound like that sort of issue but it's
a good idea to spend a minute doing a visual check of grounds no matter what
you're looking at.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."