Rough Idle when the engine is cold.

I have an 1988 S10 pickup 2 wheel drive, manual tranny, 2.5 litre Vtech engine. The vehicle has 290,000 miles with about 100,000 on the second motor. The engine starts right up when its cold, but immediately begins to run rough and puffs of dark exhaust and a very rich smell comes from the exhaust. Even though the engine runs rough I can put it in gear and drive it and it accelerates just fine. As the engine gets warm the idle gets better. At operating temp everthing is fine. No codes are coming up. I have replaced, plugs, wires, cap, rotor, water temp sensor and high energy ignition module. I recently took it to a mechanic and he thought the MAP sensor was on the edge of being bad. Enough to cause a problem but not enough to cause a code. He checked the fuel pressure also, it is good. The only items left are the injector and fuel pressure regulator. Could the O2 sensor be bad? I know I have replaced it, however, I am not sure how long ago it was. That is all I can remember that I have done and that I have tried to get this problem corrected. If anyone has any ideas, please, let me know. Thank you.