My buddy had 99 150 with a 5.4.sitting at idle it idles at 1K rpm or so, but will just rev up to about 1400 or so then back to 1K. Have you guys seen that before?

 

First thing I would do is check the Throttle Body and the IAC valve....if there is a lot of build up (which there probably is) get some Air Intake Cleaner and clean it out.

 

Sounds like the Idle air control valve------------- do a search here first and then clean it

 

it's the idle air control valve. on the driver's side next to the throttle body. If you don't have regular fuel induction service it builds up with carbon and sticks. Remove it spray the inside liberally with throttle body cleaner. Wear eye protection because the spray can deflect back at you

 

my 97 just started to do the same thing yesterday morning. It ran fine, went to the shop and left it idleing everything normal, then i stoped at a stop light about 6 miles dont the road and the rpms droped down to like 600 and acted like it wanted to cut off, so i pulled in to a parking lot and put it in to park and then the rpms jumped to about 1800. it hasnt thrown a code yet and i already cleaned the throttle body so i guess ill try that now, but i dont get how it could start with just start like that

 

Erratic idle is deffinately the IAC. I thought that mine had a vacuum leak due to the fact that it one day was idling at ~1500 rpm. IAC was dirty as crap. The build up is stubborn, and tough to clean. Will be all worth it in the end though. Good Luck.