I have a 98 Expedition 4x4 W/5.4 with 112,000 miles on it. I went on vacation for a week and left it with a full tank of gas. When I got back and took it out for a drive it was idling rough and hesitating when accelerating below 2,000 RPM. All recomended service has been performed on the vehicle, except that I had the spark plugs changed at around 80,000 miles so I'm thinking I could use another change of spark plugs because I had some really bad fuel back in Oct and fuel milage has gone down since then. I'm thinking either bad fuel, bad O2 sensor, or dirty spark plugs. Anybody have a similar experiance with the engine hesitating how I described?

 

Could be your IAC valve. It can get flakey without throwing a code. Eventually, it will throw a code, but by then, you'll be barely able to keep the truck running at idle.

When mine went bad, the gas mileage started to go down too. Have you noticed an increase in fuel consumption? I do a lot of city driving, so I'm more affected by the IAC problem than somebody who drives mostly on the freeway. At least that's what I think is the case.