You don't give much info here, I presume its misfiring,is it intermittently, is it permanent, is there smoke, what colour is the smoke, did you perhaps put bad fuel in just before this misfire started. You see there is plenty you are not telling us, but the most common fault I came across if all the basics are correct is the injector harness. Basics are, good fuel, good fuel pressure, cooling system good, induction system good, compression good.