I have a 91 XJ with a 4.0. It will start and idle fine for a couple seconds then it will start to idle up, then it will idle way down and start to knock. It will try to correct it but it fails and dies. It will backfire out of the exhaust and the intake sometimes, but not every time. If I play with the pedal I can get it to run higher rpms and it will run smooth before sputtering and eventually it dies. It doesn't knock when running in higher rpms. I replaced the fuel pump, filter, plugs, wires, cap, and rotor. I am getting ready to go clean the throttle body and iac right now.