I had an idling issue on my '89 F150 4.9L; and I asked for help on here early this month or last month. I mentioned how I replaced new spark plugs, spark plug wires, dist cap, TPS, IAC, EGR about three years ago. Not all at once. Recently I checked for vacuum leaks and the idle issue continue. This one particular morning I had to fill up and for ****s and grins I decided to buy premium 92 octane grade. Guess what.......it worked wonders! I usually buy regular grade since its cheaper. I may continue to run premium or mid grade. If any of you should encounter an idling issue like I did, just upgrade to premium. Process of elimination and simple troubleshooting step. Sometimes we have to think outside the box. I hope I resolved this issue permanently.