Have a 98 4.6 L. with 55K on it. Engine seams to be cycling at idle, so I checked vacuum. It maxes out at 16-17". Checked all over for vacuum leaks and could not find any. Checked with a hose and couldn't hear any. Isn't this too low and any ideas what to look at next.
Thanks
Kevin

 

Sounds like you may have a sticking IAC. This will cause the idle to fluctuate. I have never checked the vacuum on my 5.4L so I don't know what they run.

 

actually 17" is normal, not sure on the 4.6, but the sensor just after the airfilter in the plastic bell, if it is bad or not setting inplace, can cause a miss or rough idle also.