Re: 99 4.0 Check engine light
Get your codes checked immediately and write them down, all of them. Having a CEL on for one reason, will not let you know you have another problem. I just had my transmission worked on and my CEL has been on for 8 years. But, with it on, I never got the new transmission related codes.
As for idling rough, the Idle Air Controller is a simple, $35 part at Advance Auto Parts and changes with 2 botls and a quick connect electrical plug. In looking for vac leaks, I had my upper intake smoke tested and no kidding smoke from the IAC, so I obviously changed it.
Still no joy on the CEL though. Same codes P0171 & P0174.