1994 2.3 liter bad gas milage help !!!!

I have a 94 2.3 quad 4 that has been getting terrible gas milage 15 mpg it started around 2 months ago with a check engine light the code came up as o2 sensor that was changed but no fix. It runs fine at idle and around 45 mph the service engine light comes on and black smoke ( to much fuel ) comes out the back. any ideas on what this could be and what I can do to fix this
thanks

im having teh same exact problem with my 94 v-6. from what ive gathered to fix this you should clean or replace teh mass air flow sensor, the MAP sensor, hte IAc (idle air control i think), and you will need to change the plugs if youve been running rich because tehy will be carboned up. also you may need to replace the o2 sensor again because it also will be carboned up if youve still been running rich. have the ignition coils checked too. another possibility is that you could have a bad cat converter, if it is plugged up i guess it can cause these problems. Finally ive heard taht it can be bad vaccuum lines that are causing the fuel pressure to be off which may cause rich running. all i know that i am better off taking it to an experienced mechaninc because i have dumped about $200 into fixing this myself and have had little to no success. you might want to consider just taking it somewhere before you wind up wasting your money too, it is such a long list of things that can cause pretty much the same problem that it is very difficult to narrow pinpoint and fix these problems