Chevrolet Repair: 94 Chevy S-10 4.3 Vortec V6, gas milage, oxygen sensor

Question
Changed CPI unit a month ago. Now running good, but gas milage is down to 10-11 mpg. Blowing black smoke when I rev up the motor. Only code I get is an O2 sensor. Book shows I should have 2 sensors and one heater, can only locate one sensor on right side exhaust pipe. Sensor tests good. Any suggestions?

Thank you.

Answer
Hi Doug,
If there is black smoke, chances are, the oxygen sensor code is a rich reading, and it is working correctly.
I would probably run some injector cleaner thru it, and probably check the fuel pressure.
Is there any possibility one of the hoses popped off one of the injector poppet nozzles?

Van