I have a 99 F-150 4x4 with the 5.4... While idling it will stay around the 800 rpm range for awhile then what sounds like a switch clicks and it goes up to 1100 rpm then repeats thee cycle... It does this cycle constantly, hot or cold, and I'm just curious as to what makes it do this?
Do you hear this click with the ac on? if so its the ac clutch engaging and disengaging the pcm will increase rpm to handle load, if it is cycling too much 6-8 times a minute indicates low refrigerant pressure levels, get charged or low pressure switch malfunctioning IMO