1999 Jeep Grand Cherokee, Laredo 4.0 litter. I've replaced the plugs with the same plugs that was recommended by the factory and the vehicle ran fine for a short time and then started missing. I brought it back and checked the coil and put a screwdriver to the injectors to see if I heard activity. heard something on all 6 injectors. I know I need to do more test put it seems to be getting fuel at a good pressure. I replaced the old plugs and coil ran the same as it was. replace coil with new plugs ran form second fine then went violently rough and died. wouldn't start. would crank and fire but went nuts with the gauges and the back window rolled down on its own twice. even the fuel pump went on and off over and over. i put the old coil and plugs back in runs fine, same as it ever was? Been resetting, the PMS-ECU. Even tried the old plug,s new coil and it ran put rough. old parts are in it but its not accepting anything new.

 

Hopefully you got a good quality coil... and not one from Autozone.

 

To clarify, when you put the old parts in it ran fine, but the new ones cause it to run rough? Or it's running rough now regardless of which parts you have in?