Speaking as "Master of the Obvious"... you did a lot of work in the engine
bay, make sure you don't have any dangling electrical or vacuum lines... I
haven't had your particular problem, but I've been amazed (twice) by my van
being able to run (badly) when I forgot to connect something. It's also
possible that all that moving stuff around dislodged something you weren't
even working on.

I think it's unlikely an exhaust leak would cause that much difference, if
you had that big a hole in the exhaust, you'd hear it loud and clear.

At 10:38 AM 7/13/2004 -0700, Craig Oda wrote:
>This past weekend I averaged 5 mpg and there was a lot of unburned
>fuel coming out of the exhaust.  I've got a voltmeter hooked up to the
>O2 sensor and the voltage was measuring consistently lean, despite the
>obvious signs of the engine running rich, such as gas smell and black
>smoke, not to mention high fuel consumption.  There  was also
>drivability problems, including loss of power and some surging.
>
>This morning I disconnected the O2 sensor before I went to work.  The
>engine drove much better.  I'm now thinking that there is an exhaust
>leak that is causing air to leak into the system and causing the O2
>sensor to read the air-fuel mixture incorrectly.  Although this seems
>counter-intuitive since the exhaust is presumably under pressure in
>the system and pushing out of the back, I've heard of a process called
>exhaust scavenging where an exhaust pulse pulls exhaust through the
>system.  Each high pressure exhaust pulse travelling through the
>system results in a bit of pull
>
>I obviously want to reconnect my O2 sensor.  Before I take off the
>exhaust system and change the exhaust gaskets, is there anything else
>I can check?
>
>Stuff I've done in the past week:
>   - changed O2 sensor
>   - changed fuel pressure regulator, fuel pump, fuel filter
>   - took apart entire air intake system, changed the air intake
>distributor box, and most rubber hoses.
>    - changed temp sensor II
>    - adjusted valves
>    - looked for exhaust leaks under the van, but couldn't find any
>obvious ones.  I haven't taken the system off for inspection.... yet.
>
>Also, my exhaust system has a dent in it right after the O2 sensor.
>Could this be causing a problem with the exhaust flow?
>
>I have a spare exhaust system and a newish auxiliary air regulator
>that I can swap in for a test.  Right now, I think it is air leaks
>into the system or maybe that dent.
>
>Any advice before I crawl around under the van and get dirt falling in
>my face, hair and eyes?
>
>-- Craig