Thick black smoke is a rich condition. You do NOT need to rebuild the engine. It is likely the engine temperature sensor, or a bad connection at the sensor. If the sensor is bad, or disconnected, the ECU thinks the engine is extremely cold and richens the mixture. Other possible causes of a rich mixture: leaky injectors or a leaky cold start valve, possibly the air flow meter, but not likely.
By the way, black smoke is a rich mixture, blue smoke is burning oil, and white smoke (steam) is vaporized coolant (i.e. blown head gasket).
Good luck,
Dave