I am still having problems with my 93 grand with a 4.0 here is the list of replaced parts Crankshaft sensor, Camshaft sensor, Computer, Timing chain and sprockets, Ignition switch, Map sensor, Throttle position sensor,O2 sensor, Idle air control, Coil,Plugs and wires, Rotor bug and cap, Fuel pressure regulator, fuel and air filters. Timing is dead on compression is all on the high side of limits, fuel pressure is good, all the grounds are good, no trouble codes, no vacuum leaks, and all power levels are where they should be. Yet it has an erratic spark on all cylinders and backfires through the exhaust. Anybody have an idea or the best place to take it to get it figured out.