Wavering idle after warm up

[FernsMJ]

Hello all! 
 

I’ve often lurked in these threads just looking for advice but I’d figure I’d just ask this time, as me and my Dad can’t seem to figure it out easily. 

 

My 4.0 92 MJ has a wavering idle once warm. After about 15 to 20 minutes of idle and messing with the throttle, the idle will begin to idle up to what sounds like 3k and drop down sounding like it’s about to stall. I can stop the fluctuations in the idle by unplugging the IAC valve but it continues once it’s plugged back in. 

I’ve checked my fuel pressure as I replaced it with an aftermarket rail and regulator and it sits at ~30 PSI and doesn’t change with the idle ideations.
I’ve replaced the IAC valve and the O2 sensor thinking they just “went bad” but I’ve had no change in the idle after warm up happened :/ 

I replaced the vacuum line from the intake manifold to the MAP sensor and my next theory is a bad MAP or a vacuum leak somewhere else. 
I have changed the intake manifold to the newer style and changed the Power steering pump to match, I’ve also replaced the throttle body to a machined aluminum and larger bore diameter. I’ve made other upgrades in other places like suspension but those are the only ones that feel relevant.

 

The symptoms initially began going down the road at highway speeds, it began to buck and almost cut out so I coasted to a turn out and shut it off 

 

I’d appreciate any recommendations or suggestions as I miss being able to take my beloved truck more than down the road and back. I’m stationed in Alaska so it might take a sec to implement anything that needs to get shipped.

 

Thank y’all for your time,

 

Fern and his MJ 

[FernsMJ]

 

 

[FernsMJ]

I've replaced the MAP and TPS and have had no change :/

Idles great for 10-15 minutes but returns to that wavering idle.

[89 MJ]

I would check for vacuum leaks around the intake manifold, especially where the manifold bolts to the head. 

[FernsMJ]

I had a bit of a leak in the IAC valve housing on an aftermarket TB I've been using but I sealed it with a new gasket and fixing some stripped threads.

I attempted to tighten the manifold bolts to the head and they budged a little, but no obvious "loose-ness", I'm gonna pick up a torque wrench and give them a more proper once over. No noticeable change after those and it still has that faltering idle after about fifteen minutes after a cold start. When driving it bucks and surges after some normal street and highway use. Next I am going to change the CPS with one I have on hand to see if there is any change.

[Eagle_SX4]

The issue could be the throttle body itself. I had a similar issue trying to run a Renix IAC valve on a HO throttle body. The pintle size is not the same. Maybe your aftermarket throttle body was made incorrectly and the IAC portion was machined or cast wrong. 

[FernsMJ]

It has been running fine with it for about a year but it is definitely worth finding one from a local scrap yard since you say you have a similar issue, I have an extra Fel Pro gasket I'll replace with it as well.

[Eagle_SX4]

Double check prices before you pull one out the local junkyard. My local junkyard charged me $60 for a throttle body last month. The throttle bodies can be found on eBay for about $30. Also the later 96+ 4.0 throttle bodies are the same and will work just fine on the 92 engine. The only difference is the map sensor was moved on to the throttle body. Just plug the port or run a hose to your existing map sensor.