Well, I haven't tried EVERYTHING... At least not yet. Alright, first the symptoms: upon startup (most startups hot OR cold) jeep will start and idle to 1000. Then after 10-60 seconds will drop to 700, which is ok. But it will then drop into the 200-400 range and will actually rev itself back up to prevent it from stalling. If I don't let it idle (say I take it on a drive after starting), it will seem as though the power is restricted and sounds like it wants to die out until I give it more gas. Once I do it will make a popping noise and the rpms will shoot up and I will have to brake. It will do this a few times and I have to go above 30mph to get rid of it. But once its gone, its home till I start the engine again, it won't do it at stop signs or anything. Ok, now what I have done to try and get rid of it: new cap, rotor, wires, plugs (champion copper), new high flow cat, cleaned iac, replaced tps, seafoamed vac lines, gas tank, oil, took tb off and cleaned it. I do regular oil changes and I have no overheating issues. I guess the only things left to check is: map sensor, cps, and.....? It is throwing a code for the nss and one that does not show up on their computer. Please help what could be wrong with my green bean?!

 

what era XJ?

 

99 sir

 

The NSS code can probably be cleared by cleaning the NSS and conector. I'm curious what the other code might be. Their scanner won't identify the Pxxx code?
I'm also suspicious that your IAC is sticking or not responding correctly despite being cleaned. Once you hit 30 mph it's fine until you shut down again? The problem comes back even if you re-start immediately?

 

It depends. Sometimes, such as today, yes it will start right after shutdown. Other times no. Sometimes it will not be very severe, other times once it revs up when driving it will be VERY loud exhaust wise and will boost me up about 10mph so I have to slam on my brakes. It makes for quite the sight in my neighborhood. And I will have the codes read again and post what it says. A buddy of mine is parting out a Cherokee so maybe I can grab the iac outta there and throw it in and see if it goes away. If so I will buy a new one.

 

Was it before or after the champion copper plugs?

 

Before. Everything that I have listed that I have done had been done as a result of this. It has progressively gotten worse. It started in the spring.

 

Your cat doesn't rattle when you kick it by chance, does it?

 

I replaced the cat this weekend with a magnaflow

 

Oh come on...you're taking away all the easy answers, lol.

 

I know lol that's why I am so confused

 

A thing that you havent addressed yet is the CPS.
Your 99 will have it fail at some point during its life, as with all 4.0 's and the stalling/popping sound, then picking up again could very well be this.
Replace this next. The NSS has nothing to do with this and the rough idle can be other things as welll, but you addressed a lot of them already, try the CPS replacing first. Get a quality one, because cheap once are notorious for being faulty, leaving you thinking you addressed the CPS already, but not solving the problem.
Let us know how you go.

 

I'll check the cps when I figure out how to. Any more suggestions?

 

just wondering why your not trying other places to read the codes ?

 

Fix the vacuum leak. Then reset the computer by disconnecting the battery for 8 hours. Other internet bull will not reset it.