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Old 01-11-2014, 04:22 PM   #1
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Rough Idle After Warm-up

Hey Everyone,

First post in a while(Jeeps been running good!) Recently though, my '03 wrangler with a 4.0L has been idling rough, specifically after it comes up to temp. So rough that it seems like it wants to stall, but never does. It just "lobes" between 600 & 700 RPM, and then goes down to about 450 or so. Also, this only happens when I come to a complete stop. I could have it in neutral and be rolling and the engine idles fine. The second I come to a complete stop though it starts to drag and seems like it wants to die. It also has a bit of hesitation under initial acceleration. I have no CEL and I'm a little perplexed. I do have a squealing noise coming from the drive belt, could be a bad pulley.

Within the past year I've changed plugs, plug boots, fuel pump, both upstream O2 sensors, and put a new cat-back exhaust on. I also cleaned the throttle body and IAC. Tomorrow I'm changing my thermostat, thermostat housing, and upper radiator hose because of a small leak in that area. Not sure if that has anything to do with it. Any ideas/input would be appreciated.

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Old 01-11-2014, 04:32 PM   #2
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try cleaning your MAFS

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Old 01-11-2014, 04:40 PM   #3
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I believe my jeep has a MAP sensor rather than a MAFS, but I will clean it tomorrow. Thanks!
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Old 01-11-2014, 05:01 PM   #4
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ya you're right. sounds like you're pretty knowledgeable from the original post but ill say this anyway incase you or someone else didn't know: just make sure when you have the sensor pulled you do not touch the actual sensor as it will probably never work again
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Old 01-17-2014, 12:50 PM   #5
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I've still got a pretty rough idle. Is it worth flashing the pcm? What other steps could I take to diagnose it? Could it be a timing issue? I'm about ready to take it somewhere.

Ps the tstat and housing install went prefect. No more leak.
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Old 01-17-2014, 01:11 PM   #6
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I think it's a sensor somewhere. It runs off preset factory settings until it gets to temp. Then switches to info from sensors to run. I don't know a ton about this. It may apply to all sensors or a select few. It would make perfect since to cause your problem though. Maybe someone more knowing will see this and chime in. Good luck.
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Old 01-17-2014, 01:20 PM   #7
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I think it's a sensor somewhere. It runs off preset factory settings until it gets to temp. Then switches to info from sensors to run. I don't know a ton about this. It may apply to all sensors or a select few. It would make perfect since to cause your problem though. Maybe someone more knowing will see this and chime in. Good luck.
This was the reason i recommended cleaning the MAFS (or MAP sensor) earlier im not sure what other sensors this may apply to maybe o2?
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Old 01-17-2014, 01:27 PM   #8
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I'll check the MAP again. Last week when I plugged my reader in I did have a pending code for one of the o2 sensors, but it never triggered a CEL. And the pending code hasn't returned since. That's why I thought flashing the pcm might help?
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Old 01-17-2014, 06:39 PM   #9
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Well if you have the resources to reflash yourself i dont see why not it can only help but if not why pay a dealer until you have ruled out all other options... I would also check your TPS, although if this was a problem there would probably be other symptoms while you drive. At this point i am going off of knowledge from working on my '07 xterra since my YJ has a little less technology but, maybe try disconnecting your battery for awhile (not sure how long i think i waited an hour?) and then reconnecting it so that your ECM will reset, i fixed an acceleration lag with this trick
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Old 01-17-2014, 06:55 PM   #10
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I do have a hesitation while accelerating so it could be the tps. And the pcm flash is pretty simple so I'll do that first thing in the morning.
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Old 01-17-2014, 11:47 PM   #11
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Bump. Lots of folks on now. Any ideas?
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Old 01-18-2014, 11:22 AM   #12
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Bump. Lots of folks on now. Any ideas?
Guess not. Thanks for trying tho
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Old 01-18-2014, 05:34 PM   #13
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The flash didnt help?
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Old 01-19-2014, 04:51 PM   #14
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No, the flash didn't change anything. Still idling rough. Guess I'll have to start looking into sensors. I changed both upstream 02 sensors about 3 months ago. Should I start by changing the downstream sensors? Could the downstream sensors cause a rough idle? If not, what other sensors should I first check?
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Old 01-19-2014, 05:05 PM   #15
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It would be nice to hear from someone else about this but from my knowledge the downstream sensors are to check the catalytic converter not the air-fuel ratio so probably not... I would start by checking the upstream connections. Maybe even give the contacts a blast of contact cleaner... Other than this could the timing be off? Im about at the ends of my knowledge. Again it would be nice to hear from someone with more experience
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Old 01-19-2014, 06:11 PM   #16
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Thanks for all the input Pitt. Anyone have suggestions on checking the timing? As I understand it, the timing being slightly off could cause what I'm describing.
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Old 01-19-2014, 07:00 PM   #17
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If timing was off wouldn't it be happening when cold as well? I think so. I could be wrong though never had the problem my self. I would think that would cause it to run rough all the time not just when it gets up to temp.
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Old 01-19-2014, 07:21 PM   #18
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The title of your thread leads me to believe that it may have something to do with a warmer engine where something could be expanding and permitting a vacuum leak.
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Old 01-20-2014, 06:27 AM   #19
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The title of your thread leads me to believe that it may have something to do with a warmer engine where something could be expanding and permitting a vacuum leak.
Alright. I've never had to check for a vacuum leak before. Any suggestions? Also do you happen to know the "normal" vacuum pressure in a tj? I've noticed the vacuum pressure drop off just as the engine idle get weak and stumbles. It goes from 19 in/Hg down to 17 in/Hg. Is that normal?
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Old 01-20-2014, 09:46 AM   #20
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My bet is you have a bad upstream 02 sensor, of which your 2003 has two of them.

The engine computer pays no attention to the 02 sensors during warm up but once the engine is warmed up, then it starts relying on the 02 sensors to provide the proper air/fuel mixture.
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Old 01-20-2014, 10:11 AM   #21
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My bet is you have a bad upstream 02 sensor, of which your 2003 has two of them.

The engine computer pays no attention to the 02 sensors during warm up but once the engine is warmed up, then it starts relying on the 02 sensors to provide the proper air/fuel mixture.
Thanks jerry! I replaced both upstream o2 sensors a few months back. I used ntk. How are they with replacing defective units? I bought them from rock auto.
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Old 01-20-2014, 03:50 PM   #22
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Also, is there a reason the issue is only seen at idle?
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Old 01-23-2014, 09:33 AM   #23
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Well I bought 2 new upstream sensors. Will throw them in and update you all.
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Old 01-23-2014, 11:26 AM   #24
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Well I bought 2 new upstream sensors. Will throw them in and update you all.
Cool looking forward to seeing if it works!
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Old 01-23-2014, 11:33 AM   #25
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Cool looking forward to seeing if it works!
x2
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Old 01-26-2014, 09:37 AM   #26
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See below for what the sensors I pulled out looked like. So far the Jeep seems to be running better but the idle is still weak.

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Old 01-26-2014, 01:41 PM   #27
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What exactly do you mean by weak?
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Old 01-26-2014, 01:44 PM   #28
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It'll sit around 625-650 then drop to 500, then surge to 700. It does this over and over when warmed up and sitting at idle.
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Old 01-26-2014, 01:44 PM   #29
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Oh and I got those rpm numbers while using my reader.
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Old 01-26-2014, 01:48 PM   #30
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Keeping an eye here as I have the same issue on my 98. Have real aces several sensors although at the moment I can't remember which ones. But mine doesn't happen all the time. It's intermittent. Some says it's amazing and others like yesterday, just seems she is tired. Lol

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