I'd go for the Standard Motor Products AC176 in the Daily Driver section at 1999 JEEP WRANGLER 4.0L L6 Idle Air Control (IAC) Valve | Shop Now at RockAuto

A good method to find a vacuum leak is to start the engine and while it's idling, start spraying starter fluid etc. at all the various vacuum fittings inside the engine compartment. If the spray gets sucked into the engine through a vacuum leak the rpms will surge. Item 8 is the vacuum reserve tank and it sits underneath the battery tray.

 

It is probably more likely to be a wiring problem with one or more of the four wires between the IAC & the PCM than the IAC itself. One known problem area is where the harness passes behind the engine where is can drop down & rub on a bolt either sticking through the firewall from the cab or on the engine. It is hard to see back there but worth giving it a careful inspection.

 

Standard Motor Products AC176

The Standard Motor Products AC176 is what I have now, good to know that I did buy the correct one in 2022. (I know that I got that info from here back then, but again, search wasn't giving me what I needed.) Thank you!

more likely to be a wiring problem

As I've been driving the Jeep, I'm finding the high idle issue is inconsistent, so a wiring problem makes sense. I will check it out! Thank you!

The hardware store did not have any Throttle Bottle Cleaner. The closest product match was Mass Air Flow Sensor Cleaner. Would a Carburetor Cleaner would work better? Or doesn't it matter in this case?
In 2022 all I could find to purchase was a package that had two similar products; one was Throttle Bottle Cleaner, and I cannot remember the other, but I do recall the instructions emphatically stated not to spray one of the two on some sensor.

 

A carburetor cleaner will work just as effectively as throttle body cleaner for this.

Make sure you're spraying only when the engine is running at idle rpms. Don't rev the engine! If you rev the engine to prevent it from stalling while spraying the cleaner the cleaner will bypass the IAC. So yes it's normal stall when the cleaner floods the IAC which is actually good since it gives the cleaner more time inside the IAC. Just restart the engine and do it repeatedly until you have at least 1/3 of the can through the IAC.

And don't remove the IAC to clean it, spraying into the throttle body while the engine is idling causes the cleaner to also clean the IAC's air passages inside the throttle body.

 

A carburetor cleaner will work just as effectively as throttle body cleaner for this.

Make sure you're spraying only when the engine is running at idle rpms. Don't rev the engine! If you rev the engine to prevent it from stalling while spraying the cleaner the cleaner will bypass the IAC. So yes it's normal stall when the cleaner floods the IAC which is actually good since it gives the cleaner more time inside the IAC. Just restart the engine and do it repeatedly until you have at least 1/3 of the can through the IAC.

And don't remove the IAC to clean it, spraying into the throttle body while the engine is idling causes the cleaner to also clean the IAC's air passages inside the throttle body.

Thank you again, I thought I had to take the IAC off.
Awesome. Wire check and some spray, too easy!

 

A leak in the check valve to intake connection can cause a high idle. The CCV at the back of the valve cover.

 

So. Sunday I did another test for a vacuum leaks with the half can of carb cleaner I had left from trying to clean out the IAC, paying extra attention to that CCV @RedundanT mentioned. Nothing. So, I disconnect the negative terminal and swap the IAC. Same results, still a high idle. What the heck?!? I drove it some more, start and stop, checking to see if the issue persists. Notice that if I shut the Jeep off and let it sit for a minute, then restart it, the high idle doesn't happen. And doesn't happen until I start driving.

I drive it to work today and am describing what's happening to our shop mechanic. (He's not allowed to actually work on our POVs.) I'm parked outside the shop, so he steps out to have a quick look and listen. I've already got the hood up, and he's revving the engine with the lever connected to the accelerator (I don't know the proper word for that.) He notices that the throttle plate isn't returning to fully closed.... he gets some Howes multi-purpose lubricator and penetrating oil and douses the spring on that lever on the throttle body, revving a few more times to work the oil into the spring, and that is it. Fixed. I'm glad it was a simple fix... but still.... I left the Jeep sit waiting on the IAC to arrive and I could have been driving it! Anyway...

Now I'm wondering, should I put the old IAC back on? It's only a few years old, it looks fine. I did move the plunger in and out several times trying to see if it was froze up. I don't think moving it like that would hurt it.

 

@Jerry Bransford, What's your opinion on putting the three-year-old IAC back on?

 

Keep it as a spare.