I have a 99 4.0 4x4 with ac. All stock (motor wise). Truck has some idle issues. It basically surges up and down from stall to 1k rpms. Trick is its not very consistant. Seems to do it when truck is hot, but not every time. It will work fine on my way to work in the morning, unless I shut it off (at store) then it surges and sometime stalls. After I run it for the rest of the trip to work it will be idling fine when I get there. Thought it was the IAC but wouldnt that be consistant? Cleaned maf and iac with no luck. No obvious leaks in vacuum (tested with carb cleaner). Last year I replaced the front hubs with AVM's and never capped the vacuum lines, but doesnt appear to be the issue. Only at idle or coming down from higher rpm. Occasionally can correct it by reving it up. Unplugging maf stalls the truck almost instantly. Just dont want to throw down the $70 on an IAC unless it sounds reasonable that it could be it (even being intermitent). TPS? doesnt seem likely correct? The rest of the ride is fine. Seems like a needle valve sticking in a carbed truck.

 

4.0 OHV engines are known for vacuum leaks at the intake manifold gaskets. Even if you did try it with some carb cleaner or whatever you might not catch it. I know I never caught mine with any kind of spray I just ended up replacing them because I was tired of it, and they were torn.

 

If you didn't plug the hub vac. lines I would start there. Then look for other vacuum leaks. It does sound like a IAC problem though.