I have an 89 4.0 that came from the previous owner with only one ground left on it. So I added one from the firewall to the engine with 4awg wire and another from the battery to the fender with 4awg and bang! No more electrical issues but now it idles horribly but accelerates fine. It idled very well before these grounds. Can anyone help please?

 

first would see if you knocked something loose but the computer just might have to relearn