I’ve got a ’98 4.0 automatic with 220K miles. The motor was/is running smooth, but recently the idle raises and/or gets stuck at 1100-1300 RPM. You can blip the throttle most of the time and it will correct itself, falling to the 700-800 RPM range. I’ve cleaned and lubed the linkage, but it didn’t fix the issue. I’ve search and read some posting, but none sounded like my specific issue. I also saw people using “IAC” in their postings. What exactly is that? Any suggestion, ideas would be appreciated.

 

TPS typically causes errtic idle, vaccuum line would do it all the time. a tap on the pedal would not correct it.

 

IAC=Idle controll valve. Adjusts the idle speed on the motor on cold start ups....
That is located on the TB directly looks like a little cylinder. can be removed and cleaned.
you could also have a sticking TB which can be cleaned with carb cleaner once the intake boot is removed.

 

run some seafoam through your throttle body

 

clean the IAC and the throttle body, youll have to remove the throttle body but its an easy 4 bolts. they are t30 bolts on my 93 id imagine they are the same