98 TJ high idle / running rich

New sensor on, cleaned throttle body and IAC again, very little change. Still idles pretty high when it reaches temp, when I put in the clutch it’s first instinct is to increase the RPMs but that’s for about a half a second and then it goes down, so that’s a slight improvement. There were a few times I’d be in neutral and the RPMs would be jumping from normal to about 1500 but then it would just go back to idling high. Disconnected both battery terminals and waited to reset the computer, didn’t do much other than clear the check engine. What other ideas do we have?

Have you had any luck resolving this issue? I'm in a very similar boat as you with my Automatic 06 LJ with 157,XXX miles. I have the same high idle after warm up, and am occasionally throwing a P0301 Cylinder 1 misfire code.

So far all I have done the following on my jeep

- Disassembled and thoroughly cleaned the throttle body
- Replace all spark plugs with Autolite Iridium XP's Gapped @ 0.040"
- searched for any signs of a vacuum leak (I couldn't find any)

@Jerry Bransford Do you recommend I also start with the upstream o2 sensors?
 
Have you had any luck resolving this issue? I'm in a very similar boat as you with my Automatic 06 LJ with 157,XXX miles. I have the same high idle after warm up, and am occasionally throwing a P0301 Cylinder 1 misfire code.

So far all I have done the following on my jeep

- Disassembled and thoroughly cleaned the throttle body
- Replace all spark plugs with Autolite Iridium XP's Gapped @ 0.040"
- searched for any signs of a vacuum leak (I couldn't find any)

@Jerry Bransford Do you recommend I also start with the upstream o2 sensors?

Start with the upstream O2 sensors (I believe you have 2 but don’t quote me). Still haven’t resolved my issue but I now know it’s an exhaust leak. From all the wrenching I had to do to take the original O2 sensor out I definitely increased the size of the crack I had and now it is clearly audible. Have the new exhaust manifold, waiting on motor mounts to arrive next week before I install. Hoping that is the fix to my issue, will keep you updated.
 
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Start with the upstream O2 sensors (I believe you have 2 but don’t quote me). Still haven’t resolved my issue but I now know it’s an exhaust leak. From all the wrenching I had to do to take the original O2 sensor out I definitely increased the size of the crack I had and now it is clearly audible. Have the new exhaust manifold, waiting on motor mounts to arrive next week before I install. Hoping that is the fix to my issue, will keep you updated.

Yes I do have 2. Good luck with your exhaust manifold endeavors. Those bolts can be a PITA.
 
New manifold in and problem solved. Every bolt on my manifold was super loose, could almost have gotten them off by hand. So if anyone is having similar issues or is concerned about a possible leak, check those bolts first. The amount of vibration and jostling these rigs take can definitely start to back those out over time, especially since a lot of them are over 150,000 on the dash. I also got new motor mounts that I'm going to bolt on to combat that in the future.

Overall, not a super difficult thing to do but tight and annoying. My biggest suggestion if you're going to replace your manifold: take off the fender. It's annoying and you need to remove some components that sit on it in the engine bay, but once that's off this job could take all of an hour with the extra room it gives you. Didn't do it on mine because I didn't want to go through the "hassle" of taking everything off, instead spent several hours cursing at bolts that didn't want to thread and contorting my body in various ways to reach spaces that you literally cannot see. Take the damn fender off.

Thanks again for all the help and advice, and if anyone has any questions please feel free to ask.
 
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Did those sensors do the trick?

I'm going to be changing some things out later tonight and I'll will update this thread tomorrow. Fingers crossed! I ended up purchasing the following from RockAuto.

NTK upstream o2 sensor bank 1, sensor 1. PN# 23018
NTK upstream o2 sensor bank 2, sensor 1. PN# 23160
Standard Motor Products IAC Valve. PN# AC482
 
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I'm going to be changing some things out later tonight and I'll will update this thread tomorrow. Fingers crossed! I ended up purchasing the following from RockAuto.

NTK upstream o2 sensor bank 1, sensor 1. PN# 23018
NTK upstream o2 sensor bank 2, sensor 1. PN# 23160
Standard Motor Products IAC Valve. PN# AC482
Once the IAC is in start the engine and while it's idling spray TB cleaner into the throttle body. That'll clean the IAC's air passages leading to and away from the IAC. Make sure to keep the engine at idle rpms so the cleaner won't bypass the IAC and its air passages.
 
Once the IAC is in start the engine and while it's idling spray TB cleaner into the throttle body. That'll clean the IAC's air passages leading to and away from the IAC. Make sure to keep the engine at idle rpms so the cleaner won't bypass the IAC and its air passages.

@Jerry Bransford @The Name’s Suze

Well last night I swapped out both upper o2 sensors as well as the IAC sensor. While I had the throttle body off the intake I thoroughly cleaned every part of it with throttle body cleaner and it looks much cleaner now. During my drive to work this morning the high idle was still there. I tried spraying starting fluid around the vacuum line harness to find a leak but I cant find anything . Does anyone else here have any suggestions as to what to look for? I have not tried tightening the intake & exhaust manifold bolts yet but I will look at those also.

Its seems like a strange scenario to me. It only seems to happen once the engine is up to operating temp. The picture attached is this morning on my work commute after coming to a stop sign with the transmission in neutral. Its an automatic so the higher idle is very noticeable when braking to come to a stop.

High Idle.jpg
 
Lots of other places for vacuum leaks...you've got the T heading into the cab for inside HVAC... behind the battery...and then it drops down to a vacuum reservoir inside the fender.

-Mac
 
TPS fixed 2 of my rigs with similar symptoms of high idle. One was a '87 XJ with Renix system and may not count, other is a '05 TJ both 4.0L.
The '05 did set a CEL. Thought I would mention it.
 
TPS fixed 2 of my rigs with similar symptoms of high idle. One was a '87 XJ with Renix system and may not count, other is a '05 TJ both 4.0L.
The '05 did set a CEL. Thought I would mention it.

Ok thanks for the heads up. In the past I had a random P0301 cylinder 1 misfire code occasionally pop up but I haven't seen that code in many months nor have I noticed any other strange symptoms besides the high idle.
 
I had a similar problem with my 99 4.0 TJ except the idle only ran a little high and I kept getting the getting the running to rich fuel code. ( If I remember correctly the code was P0172) I replaced my Computer , IAC , MAP and cleaned my throttle body. Replaced both front and rear O2 sensors with NTK , replaced my exhaust verified my manifold did not have a leak and still had the problem. Brought it to 2 shops waisted 500 bucks. I never thought about my newly replaced fuel injectors being a problem. ( I bought Amazon replacements bad idea). Jerry let me know I should only Mopar fuel injectors on my Jeep. I found the injectors at Rock auto for 1/2 the price of the dealer, still expensive but it fixed my problem. I hope you get your issue fixed soon good luck.
 
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If you find out you the ECM is the problem I got a rebuilt one from Flagship back east. I believe it was 165 dollars and guaranteed for life. I will have to send it back on my
own dime if I have a problem but it has worked great for 2 years.
 
I had a similar problem with my 99 4.0 TJ except the idle only ran a little high and I kept getting the getting the running to rich fuel code. ( If I remember correctly the code was P0172) I replaced my Computer , IAC , MAP and cleaned my throttle body. Replaced both front and rear O2 sensors with NTK , replaced my exhaust verified my manifold did not have a leak and still had the problem. Brought it to 2 shops waisted 500 bucks. I never thought about my newly replaced fuel injectors being a problem. ( I bought Amazon replacements bad idea). Jerry let me know I should only Mopar fuel injectors on my Jeep. I found the injectors at Rock auto for 1/2 the price of the dealer, still expensive but it fixed my problem. I hope you get your issue fixed soon good luck.

Hmmmm. Very interesting , thank you for your input.

I have noticed the "harder" I'm driving (eg. taking off quickly, near wide open throttle driving characteristic, etc) the higher the rpm's will be at idle after coming to a stop and putting the transmission in Park or Neutral.

When I drive very light and conservative the lower the rpm's will be at idle after coming to a stop and putting the transmission in Park or Neutral.

I wonder if there could be any correlation here that could help diagnose.
 
I believe you said you replaced your 02 sensor per Jerry’s info I know he is the guy who helped me diagnose my issue when about everything was replaced and reputable shops failed. It makes me wonder if the computer has an issue. I use a blue driver diagnostic tool I bought from amazon for 125 bucks it shows the O2 sensors etc you can watch it when you are running the engine to see if the voltage is good to the computer. I am sure their are other diagnostic tools that do the same or more. If you don't have one it might help to diagnose if it’s the computer. I bought my Jeep figuring I would have to replace everything which I have I so replaced the computer but it was not the issue. Again good luck
 
I believe you said you replaced your 02 sensor per Jerry’s info I know he is the guy who helped me diagnose my issue when about everything was replaced and reputable shops failed. It makes me wonder if the computer has an issue. I use a blue driver diagnostic tool I bought from amazon for 125 bucks it shows the O2 sensors etc you can watch it when you are running the engine to see if the voltage is good to the computer. I am sure their are other diagnostic tools that do the same or more. If you don't have one it might help to diagnose if it’s the computer. I bought my Jeep figuring I would have to replace everything which I have I so replaced the computer but it was not the issue. Again good luck

that is the joys of a 17 year old vehicle I presume. @Jerry Bransford is always super helpful. What is the typical rpm level after the engine is hot and in park vs drive?