I just put in 784s and new spark plugs in my 2000 4.0 to help with the rough idle. It now idles worse than before, but anything over 1k RPMs it runs great. I have reset the ECU, used Solus to check for misfires, ohmed out injector connections, and test each injector individual and connections. Injectors were professionally cleaned prior to install. I am stumped....any suggestions?

 

I haven't done the injector mod yet but it's my understanding that it's going to run funny for a bit while the ECU figures out what the hell you just did to it. Don't quote me on that though.

 

Did you double check the gap on the new plugs?

 

Yes plugs were gapped.....could this be the fuel pump going bad? I have to prime the pump a few times before it will start in the morning. Going to this bigger injector just made the problem worse maybe? Is the check valve part of the pump or do you hve to buy the entire pump assy?

 

Cleaned IAC and still was misfiring...hooked up Solus and was throwing #4 misfire...pulled plug and it was gapped properly and looked great....tested it out of the vehicle and no spark....brand new NGK plug. I took it back and exchanged and it put new plug in and runs better than ever.
First time for that

 

Glad you found the issue. I had same issue with NGK plugs in my wife's suburban.