My jeep (98 4.0) was idling poorly as soon as it reached operating temp. The jeep also would choke and just about die while driving. If I push the clutch in I can rev the engine with no load.

I thought it may be fuel pressure but that holds well even during the rough running.
I unplugged the IAC during the rough idle and it began to idle high, 1500 rpm or so. When I did this everything off idle the jeep runs OK. It doesn't choke or stumble but it won't idle.
I replaced the IAC and then it wouldn't run properly again while driving. Unplug it again and all is well.
I repeated that test several times with the same result.

Before I fire the pars cannon at this thing, any ideas? TPS maybe? Why does the IAC seem to have an effect on the way it runs higher in the RPM range?

Thanks. I couldn't find these specific symptoms anywhere that made them seem related.

Have you removed and cleaned the throttle body?

Yes I should have mentioned that. Throttle body was removed as was the IAC from the throttle body and both were thoroughly cleaned.

To add to the list of things I have tried, I also have a 99 that runs fine, so I took the entire throttle body assembly (with IAC, TPS, and whatever the little unit mounted to the side is) off each jeep and swapped them. The 98 still runs like garbage and the 99 still runs fine.

When you unplugged the IAC, when exactly did you do it? With the engine running or did you shut off the engine first?

The IAC is a stepper motor. The signals it gets from the ECU are just "move in" and "move out". Depending on the time when you unplugged it, the position of the plunger of the IAC might be fixed in a way that the idle is high. The other way round it would mean that your ECM is setting the wrong position for the IAC when it is plugged in.

Possible root causes: ECM or electric gremlin in the wiring harness.

Regards
Johnny

Your first post re the problem is confusing: idles bad but unplug iac and then idles high? Unplug and all runs well?

Good test- so it's not your IAC, TPS, MAP (that's that 'unit') or TB. Or at least not the actual components themselves since they all work fine after swapping to other jeep.

I'd check the wiring to them next and see what voltage readings you get on them




Ps: Do you have a possibility to see live data?
obd2 scanner?

I plugged one of the extra IACs in while it was running just to see if it would react at all. It was moving around so there's something. I can't say how correct the motions were but it was in fact moving.
I tried unplugging it before starting it and while it was running and the results seem to differ, probably based on what you were saying about the exact last location it was in when power was taken away from it. I tried swapping the ECU's but the jeep would only start then die pretty shortly after so nothing really to be learned there.

I am going to swap coils, then check the grounds and compression just to eliminate those issue. I have taken a look at the vacuum lines but haven't dug too deep. Visually I don't see anything. I did notice that as I let it run longer last night and it started to idle more poorly than ever, the small motor that appears to be connected to some of the vacuum lines on the firewall on the passenger side was going crazy. Seemed like an excessive amount of action going on.

Thanks for all the help thus far.

I don't currently but I have a buddy with a laptop with the appropriate software to do so. What should I be checking for exactly?

You can read info like TPS position, open/closed loop, oxygen sensors.
If you're happy that you have a clean and good working IAC and TPS, I'd verify voltages on them next.

Sorry it's been so long since I have had a chance to update this. I was able to borrow a nice snap on scanner from a buddy.

I have good news. I was able to find A problem, that solved 95 percent of my issues. There was a wire in the IAC plug that was backing out atiny bit when I would plug it in. It stayed in the housing but wasn't making good (at times any) contact and throwing everything out. I fixed that plug and now the jeep starts fine and idles fine and for the most part drives fine.

My current issue is that as soon as the Jeep switches from Open Loop to Closed Loop it starts to run like crap, but only for a minute or so. I can rev the engine up over 3K and it will run ok and after doing that a few times it runs fine, then it will continue running fine until I shut it off and go through the process again. I don't see anything on the live data from the O2 that's jumping out at me during that time or the IAC or TPS. It is very consistent, does it every time now and goes away quickly every time.

How many miles are on your oxygen sensors? They have a finite lifespan as they get quite the workout and they can get slow as they age before they actually set a code.

If any doubt, get new NTK oxygen sensors installed. That is the original equipment sensor in your engine. I wouldn't use Bosch as while I like Bosch products generally speaking, some 4.0 engines are a little finicky with their oxygen sensors. Also absolutely none of the no-name "el-cheapo" sensors and anything with universal wiring, they are junk.

Good luck and keep us updated!