odd rpm issue

2003 Renegade with 164xxx miles.

Noticed an odd issue with the libby's rpm today. My wife was going to see some friends in Asheville and I went outside to start the jeep up for her and let it get warm.(Been having a heat issue where the pass side is hot and the driver side is lukewarm at best and then it will get hot eventually (figure this is blend door related) I did do a coolant drain and flush on it Thursday. I wanted to see if it was possibly the heater core.)

Back to the issue I noticed, it was close to being fully warmed up and I revved it up to about 2000-2200 rpm and held it there for a few seconds and let off. The rpm dropped to around 400 and the jeep shuddered a little bit and then it went back to normal idle. It only dropped low for about a second or two. I drove it around the block and drove it up some good hills by my house and it never messed up. Idled fine at stop signs never surged when I got on it. Got back to the house and went to back in the drive way and when I went from drive to reverse when it hit neutral it did it the low idle for a second and then was fine.

I pulled the codes, its the P0420 code which has been on there for a few weeks now, but no new codes. The other day when I did the coolant flush this wasn't happening because i was revving it up to 2500 rpm and holding it there for 30 seconds and then letting off and it would drop back to normal idle.

We are experiencing some really cold weather here that we don't normally see, so I was wondering if that could be a possible factor, it wasn't do this Thursday or Friday when it was warmer out. Anyone experienced anything like this before? Am I being overly cautious or do I need to look for something that could be wrong?

Thanks,
Andrew

I'd figure out the PO420 ASAP but my 02 did this for years. Never made sense and not always when it was cold but every once in a while. Never had the pO420 and didn't cause any major issues in my case. Just gave me the occasional WTF moment.

I am going to look into the P0420 code. It was there and then went away for several weeks and came back recently. I am going to inspect the plugs and wires on the O2 sensor and if it looks good I may replace the downstream O2 sensor and see what that does before replacing catalytic converters.

This was the first time experiencing that odd idle issue. I am thinking its the temperature in all honesty cause it drove great and idled fine as it was warming up and didn't do it a few days ago when it was warmer temps, it was just revving it up and letting off the gas quickly today in cold weather that I noticed it.

I doubt the P0420 is related. The cat would have to be really plugged up for it to impact performance. Mine has never done that and it was -14 this morning, but every KJ has its character lol.

My 04 has done a little stutter at idle since I bought it whether it's 40c in the summer or -3c in the winter always when it sits idling for a few minutes. Did new could and plugs still does it never throws codes.

2004 3.7 limited

Since you just did a flush trapped air may be causing an issue, but blend door is a possibility too. Top rad hose engine side, has a hex air bleed screw, I only had luke warm until the air was gone.. I used paper towels around when hot & opened it slightly, then closed a few days in a row. Check when cold coolant reservoir to be at the cold line and I hope u used HOAT or Zerex-G05 coolant only, otherwise..bad news.

idle=Remove and clean the IAC. It controls the idle. Things get corboned up & sticky.
Jump to post #10 link: http://www.jeepkj.com/forum/f196/idle-air-control-no-screws-44826/

To clarify, the heat was acting up before the flush, I did the flush to verify if there was trash in my system or if it was pointing more towards blend door. HOAT was used.

I will check that link out. I cleaned the IAC beginning of this year, but it probably wont hurt to do it again or just replace it.

Thanks,
Andrew

You got me wondering what mine would do. Mine is like yours only worse. With the engine warmed up having run it for about 30 minutes, in Park, I blipped the throttle up to about 2000 rpms and when it dropped, it went all the way down and died. I was able to restart it immediately. When it settled at 600 rpms again, I blipped it again with the same result. It died. I restarted again just to make sure that I could and then shut it down. It was about 17 degrees outside. I wouldn't think that would have much to do with it, but I never tried blipping the throttle when it was warmer out. It runs fine so I guess I'll just stop blipping the throttle But I would like to know what's causing that.