My 91 240 gets really poor mileage. It used to get ~ 29/21 mpg on the road/city. That was in May and now it gets 22/14. When you accelerate, there is light black smoke out the exhaust and fouled plugs.

I checked the airmass, OK, the O2 sensor works fine, at least the output changes with RPM and then goes down. I replaced the back pressure controller.

This seems like it must be something else, otherwise why would the O2 sensor show a change; it should control the mixture.

Thanks for the help, in advance.