I have an intermittent rough idle on my 01 5.4L that gets worse with engine temp. After a cold start it idles fine, after warming the engine it typically gets worse. However, on hot days or when towing heavy loads it is by far the worst. Sometimes when towing it's so bad that the engine will actually surge as I am rolling to a stop (this has happened twice, now). I have checked for vacuum leaks and found none. To me, the fact that it gets worse with increased engine/ambient temperatures makes me think sticking IAC valve. It makes sense that if the valve sticks a little on cold days, that warmer temps will cause the metal to expand and it will stick even more on hot days. Does that make sense. I planned to remove and clean the IAC this weekend to see if it makes a difference. Am I wasting my time here? Any other suggestions as to what the problem may be?

 

I had a similar problem. Try pop off the IAC valve and clean it out. It's a 15 minute job.