1995 Isuzu Rodeo blowing black smoke!!!!!

I have a 1995 Rodeo. It is a 3.2L, fuel injected, with 163706 mi's on it.

It frequently blows black smoke. "Sometimes" the check engine light comes on when this happens and then sometimes it does not. There is a noticable loss of power, even when I put the pedal to floor. What I have noticed is that by "feathering" the gas pedal I can maintain speed, however the gas gauge drops like someone cut a hole in the tank!

I have taken it to a repair shop, and they replaced the throtle body assembly, and "cleaned" all sensors.
They put it on a computer to check for any codes and said that no codes came up.

I personally think that it is the fuel mixture sensor that is causing the problem, because I believe that black smoke means the fuel miixture is too rich? I would like to know where the sensor is and if I can replace on my own without much trouble.

Has anyone else had this problem? If so please help, I am at wits end. My wife drives this vehicle and the last thing I need is for her to break down on the road.

Thanks and God Bless,

Bill Robertson