Rough idle at start-up and during idle for a few minutes

BillNyeIsHigh

BillNyeIsHigh

TJ Enthusiast
Original poster
Joined
May 10, 2018
Messages
105
Location
Charleston, SC, USA
03 sport w/ 4.0

Been having a rough idle at start-up when engines hot, seems to be missing. Usually can give it some gas and things pop back to normal. Lately, giving it some gas doesn't seem to do anything, rough feeling stays, no real change in RPM, still seems to be missing. Turning off and on doesn't change this. Eventually it stops.

Today, sitting at idle for a few minutes after commute home, engine drops to that weird, half idle and seems to be missing. Turn jeep off, start it. Runs fine. Idles for a few minutes and does the same thing. Turn it off again, starts up fine again no problem. No codes present.

I don't think its related to the IAC, its been replaced and also the issue seems to go away when giving it gas. I also had a hanging idle when pushing the clutch in which is why I replaced the IAC. It seems to be better now, but not perfect. Was not able to find any apparent vacuum leaks by spraying carb cleaner on the vacuum lines.
Any ideas?
 
Will check into it. I can definitely smell unburnt fuel it seems when the issue comes up. I also have noticed a slight decrease in MPG.

I am going to replace my plugs first and go from there.

If any other ideas out there, feel free to send them my way.
 
Could it be CPS? I have an XJ friend who just had very similar symptoms in his 4.0, stuttering and dying when hot, misfires. Heat soak fixes didn't work for him since it was happening when driving/at stop lights not just startup. Replaced the CPS and he's good as new.
 
Last edited:
BillNyeIsHigh said:
03 sport w/ 4.0

Been having a rough idle at start-up when engines hot, seems to be missing. Usually can give it some gas and things pop back to normal. Lately, giving it some gas doesn't seem to do anything, rough feeling stays, no real change in RPM, still seems to be missing. Turning off and on doesn't change this. Eventually it stops.

Today, sitting at idle for a few minutes after commute home, engine drops to that weird, half idle and seems to be missing. Turn jeep off, start it. Runs fine. Idles for a few minutes and does the same thing. Turn it off again, starts up fine again no problem. No codes present.

I don't think its related to the IAC, its been replaced and also the issue seems to go away when giving it gas. I also had a hanging idle when pushing the clutch in which is why I replaced the IAC. It seems to be better now, but not perfect. Was not able to find any apparent vacuum leaks by spraying carb cleaner on the vacuum lines.
Any ideas?
Click to expand...
My 2002 has been doing it only on short stops,it is heat soak on one of the injectors,let it cool and starts up fine right,take the injectors out and replace or have them cleaned,
 
BillNyeIsHigh said:
Are you expecting upstream or downstream to be the main culprit here?
Click to expand...
If it’s been 80-100K since they were replaced, go ahead and do them all. Upstream causes the drivability issues, but the computer uses both signals to dial in fuel/air mixture and old O2s become worn and send bad data.

There’s also this as a possibility:

Thread 'TSB: 18-031-03 (4.0L Rough Engine Idle After Restart Following A Hot Soak)'
https://wranglertjforum.com/threads...idle-after-restart-following-a-hot-soak.2345/
 
Thank you for all the replies, been busy and haven’t been able to come back to this.

I do have injector #3 wrapped with the mopar insulation. I also do have the silver heat shield on top of the manifold but it does look like it’s in rough shape. A friend and knowledgeable tj guy in my club mentioned fuel boiling/vaporizing in the fuel rail. I’m guessing this ties with the heat soak issue.

On my way home from work, it started dropping idle and then coming back up while I was sitting g there. If I sit and idle for a couple minutes it will start to do it.

I have some screen caps of my O2 bank 1 and bank 2 sensor data. Something tells me that b1 is having issues. Is that indicative of the the idle drop and recovery?

7E5F1605-D5E7-4C09-A522-4D0467B9C969.png


6125F128-66D4-4880-BFF5-10F6F165568E.png


C9ACC50B-25D2-412F-B6E4-76E34E052C4D.png
 

Attachments

  • IMG_3022.MOV
    7.9 MB
  • IMG_3021.MOV
    31.7 MB
Last edited:
Pull your fuel rail and prime the system a couple times and see if any injectors leak. I had the same problem I changed plugs three times. My fuel trims were rich. Ended having 3 bad injectors. My fuel rail was also full of crap from bad gas I assume.
 
Wtrask said:
Pull your fuel rail and prime the system a couple times and see if any injectors leak. I had the same problem I changed plugs three times. My fuel trims were rich. Ended having 3 bad injectors. My fuel rail was also full of crap from bad gas I assume.
Click to expand...

I’m ignorant here. If I pull the fuel rail
How do I prime the injectors? I was thinking the fuel rail was priming the injectors.
 
Pull the fuel rail out with injectors attached. The injectors have o rings that seal them into the intake. To prime them all you need to do is cycle the key and make the fuel pump turn on. Make sure when you reinstall it you use some grease or Vaseline on the on the o rings.
 
The fuel pump creates the pressure. The fuel rail just distributes it to the injectors. The fuel pump and regulator is all done in the pump.
 

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