My 240 has a pre-cat O2 sensor, which hasn't been active since I used the sensor wire to tie the exhaust to the underbody after donating the bulk of the system to an interstate highway.

I wired what I believe to be a working sensor up a few days ago, and my fuel economy has dropped from ~26mpg to 22 and then 20 mpg (and this is all highway driving).

I have been meaning to get around to replacing the exhaust manifold to front pipe gasket, as the former gasket doesn't seem to be in there anymore and it's pretty noisy up front - so I wonder if this isn't my problem (O2 sensor isn't getting an accurate reading, so pushing more fuel at highway speeds); thought, though, that it would have the opposite effect (O2 sensor would be happy there wasn't much in the exhaust because I'm actually losing some before it gets to the sensor).

Have disconnected the sensor again for now to confirm it as the culprit, and at 130mi per day I should know pretty quickly. Thoughts?

 

Fix the exhaust leaks and then install a new O2 sensor...

 

Exactly. Exhaust leaks make the O2 sensor run lean, so the ECU dumps more fuel in. Bad input means bad output.

Regards, Andrew.