Black Smoke coming from the Exhaust on 1995 Dodge Ram 2500

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95 dodge ram 2500 is spewing black smoke. I'm only getting about 3 miles to the gallon and it's pretty consistant wihile driving at a steady pace. Fumes smell really bad and I failed emissions testing. Check engin light comes on from time to time
(2) Answers
You have an extreme "rich" fuel condition. If yu continue to operate the engine, you will, if not already, permenantly damage the Catalytic Converter. Proper diagnostics will have to be performed to determine the cause of the over rich condition. Vacuum leaks, excessive fuel pressure, leaking injectors, defective O2 Sensors, MAP sensors are primary causes. An experienced technician can "read the Adaptive Fuel Strategy" in the Powertrain Control Module (PCM) and isolate the cause.