Weird low Idle when warm

[jazz-n-tj]

I have a strange issue with a low idle when warm that I cannot quite figure out where to start. I don't want to just throw parts at it and would like to avoid bringing it to a shop if possible. Here's the issue.

Once the TJ is up to temperature (190+) it idles very low only when I am at a stop. It is a 2000 4.0 with the NV3550 manual 5 speed transmission. When I push in the clutch or put it in neutral while coasting it idles great (800 rpm) but the second that the jeep comes to a stop it drops to 2-300 rpm and has stalled out a couple times. The check engine light is NOT on so it is not bad enough to trigger that.

From my experience with cars, if the issue were the IAC valve or MAF it would idle poorly whether coasting or stopped. I am wondering if there is some sensor that tells the jeep to adjust timing or fuel/air mixture when it is idling at a stop? I initially thought maybe my clutch isn't fully disengaging but I don't think that is the case because I don't smell clutch nor does it do anything if I rev the engine and I believe if the clutch were dragging enough to drop the idle significantly it would make the car rock or move if I revved it.

Please let me know if there is anything I can try that is a likely culprit. I figure it is probably something simple because these vehicles are not highly technical, but I don't want to just throw parts at it.

 

[Jerry Bransford]

If it idles normally when cold but not when warm that usually makes me think bad O2 sensor since the PCM only starts using their data to set the air/fuel ratio once the engine has warmed up. If you decide to replace them, go only with NTK or NGK brand O2 sensors... avoid store brands and especially avoid Bosch O2 sensors which are well known to not work properly in Jeeps.

 

[jazz-n-tj]

Thank @Jerry Bransford would that likely be the issue for it only having bad idle when at a stop versus having the clutch in or having it in neutral and idling? It only idles bad once I come to a stop. It is almost instantaneous. Once I come to a stop, the RPMs instantly drop down low.

 

[Jerry Bransford]

Thank @Jerry Bransford would that likely be the issue for it only having bad idle when at a stop versus having the clutch in or having it in neutral and idling? It only idles bad once I come to a stop. It is almost instantaneous. Once I come to a stop, the RPMs instantly drop down low.

That or a dirty IAC but a dirty IAC would cause the problem when the engine is cold too.

 

[TJ Jim]

Grab a Wi-Fi or Bluetooth dongle to put in your OBD port, and then download an app called OBD ll and read your o2 sensors. It’ll give you a graph and give you more info on how you sensors are working.

 

[jazz-n-tj]

@TJ Jim does the app explain if the are working properly? I wouldn't have any idea understanding the graph you attached with out reference of what is within spec.

 

[Jerry Bransford]

@TJ Jim does the app explain if the are working properly? I wouldn't have any idea understanding the graph you attached with out reference of what is within spec.

Same here.

 

[TJ Jim]

@TJ Jim does the app explain if the are working properly? I wouldn't have any idea understanding the graph you attached with out reference of what is within spec.

YouTube will help understand it to some extent and then there are folks on here that will send you down the right path. I posted on here with some odd to me readings and they steered me in the right direction. I now have stored copies of how my Jeep is running great and can go back and compare it if I have future problems

 

[CNYJeeper]

I have a strange issue with a low idle when warm that I cannot quite figure out where to start. I don't want to just throw parts at it and would like to avoid bringing it to a shop if possible. Here's the issue.

Once the TJ is up to temperature (190+) it idles very low only when I am at a stop. It is a 2000 4.0 with the NV3550 manual 5 speed transmission. When I push in the clutch or put it in neutral while coasting it idles great (800 rpm) but the second that the jeep comes to a stop it drops to 2-300 rpm and has stalled out a couple times. The check engine light is NOT on so it is not bad enough to trigger that.

From my experience with cars, if the issue were the IAC valve or MAF it would idle poorly whether coasting or stopped. I am wondering if there is some sensor that tells the jeep to adjust timing or fuel/air mixture when it is idling at a stop? I initially thought maybe my clutch isn't fully disengaging but I don't think that is the case because I don't smell clutch nor does it do anything if I rev the engine and I believe if the clutch were dragging enough to drop the idle significantly it would make the car rock or move if I revved it.

Please let me know if there is anything I can try that is a likely culprit. I figure it is probably something simple because these vehicles are not highly technical, but I don't want to just throw parts at it.

Maybe I missed it, but have you changed or disconnected the battery recently?? I've noticed doing that to both my 97 & 99 has resulted in low idle in both as the computers are trying to re-learn their mixtures. After about 40 miles or so of driving, the low idle issues have gone away.

 

[TJ Jim]

So the little bit I understand if you look at the top graph you’ll see bank one and bank two sensors one on each. You have four sensors two before the cats and two after. The two before send oxygen levels to the computer which in turn tell the injectors more gas or less gas. Enrich or make more lean. The level the sensors see as perfect is .45 volts. They have the ability to detect between 0 to .9 volts.
The graph I put up is from two brand new ntk o2 sensors which is not too different from the original o2 sensors I replaced. I replaced them because b2s1 had a heater fault. @ItchyDong helped me tremendously in understanding the readings which prompted me to replace b1s1.
https://wranglertjforum.com/threads/explain-o2-sensor-readings.59115/

 

[Baylorbear]

I have a strange issue with a low idle when warm that I cannot quite figure out where to start. I don't want to just throw parts at it and would like to avoid bringing it to a shop if possible. Here's the issue.

Once the TJ is up to temperature (190+) it idles very low only when I am at a stop. It is a 2000 4.0 with the NV3550 manual 5 speed transmission. When I push in the clutch or put it in neutral while coasting it idles great (800 rpm) but the second that the jeep comes to a stop it drops to 2-300 rpm and has stalled out a couple times. The check engine light is NOT on so it is not bad enough to trigger that.

From my experience with cars, if the issue were the IAC valve or MAF it would idle poorly whether coasting or stopped. I am wondering if there is some sensor that tells the jeep to adjust timing or fuel/air mixture when it is idling at a stop? I initially thought maybe my clutch isn't fully disengaging but I don't think that is the case because I don't smell clutch nor does it do anything if I rev the engine and I believe if the clutch were dragging enough to drop the idle significantly it would make the car rock or move if I revved it.

Please let me know if there is anything I can try that is a likely culprit. I figure it is probably something simple because these vehicles are not highly technical, but I don't want to just throw parts at it.

What was the solution to your problem? My TJ is experiencing the same low idle rpms now. I replaced the battery when I first bought it, and everything ran great. A few days later my wife drove it, and left the lights on all night. I jumped it off and drove it around to get a good chrage. Now after doing that I'm having the low idle rpm issue after the engine warms up.