We have a 2002 Wrangler 4 cylinder. We have an intermittent problem. I hope i can explain it so you can understand. We can be just driving down road at 55 and it starts bucking like you are in wrong gear. It smells like you are burning coal and blows black smoke. We have changed several parts with no success. Parts changed oxygen sensors changed twice, coil, plugs, wires, distributor, we even cut convertor off. No check engine light on. I may have missed a part or two that we have replaced. Please does anyone have any suggestions or ideas on what to do next?