My 1996 XJ starts fine. It idles high at about 1000 rpm. When I push the accelerator at around 1500 rpm it begins to run rough and back fires. Any ideas on where to start looking? I have new plugs,wires Distributor and cap. I also put in a new fuel pump. I bought the jeep used and just wanted to give it a tune up. It started doing this before replacing above parts. Also no codes on code reader.