While running idles roughly bouncing between 500 and 1000 rpm. Runs good at 1500 rpm but rich. Backfires thru exhaust at 2000 rpm. Have replaced all sensors and pcm/ecm, timing gear set, distributor, fuel pump, oil sending unit, plugs, wires, distributor, you name it. Local Jeep shop has had on 2 different machines and it shows NOTHING wrong. Please someone HELP!!!

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