Running rich and stalling

I own a 93 grand prix. I cannot keep this car running when the engine is at idle speed. The car has some wicked black smoke coming from the tail pipes and the fuel economy has just gone to crap. I checked the codes and the only thing that came up was (12) RPM pulse not registered, which too my knowledge means nothing is wrong. I already went ahead and replaced the throttle positioning sensor and the MAP sensor on some advice from a mechanic I was talking to. This however did not change the problem. I'm at a point right now where I can't afford to take it in to have a diagnostic performed and i'm starting to think it may be the computer itself. If anyone can give me some insight I'd really appretiate it.

Thanks

Adrian

Have you tried cleaning the injectors and replacing the fuel filter?