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Unread 07-01-2010, 04:50 AM   #1
nforcer
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Intermittent Idle Fluctuation

OK, I have read some similar threads but I have not found exactly what is going on with my Jeep. I bought this Jeep (04 LJ auto) with only about 3800 miles on it. I have put about a 1000 miles on it and I am getting an intermittent idle fluctuation. It starts right up in the morning and, once warm, runs at about 500-600 RPMs. On my drive to work I run a stretch that I run at about 70-75 MPH. Now, sometimes after I run at a higher speed and I transition to city driving, my RPMs will sit at around 250-300 RPMs. It does not do this all of the time. If I have the air on it does not do this and it sits right at 700 RPMs. In addition, the needle, when the air is not on, bounces a little bit.

Also, and I don't know if this is related or not, but this morning I backed into my parking space at work, at full lock, and when I shifted it back into drive, still at full lock, it stalled out.

Real quick last night I pulled the hose off of the throttle body and it looks like new inside. Any ideas?

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Unread 07-01-2010, 06:05 AM   #2
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My first thoughts would be vacuum-related, so my initial reaction would be to check your vacuum lines and see what you've got. If that doesn't turn up anything, then start looking at the TPS and IAC.
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Unread 07-01-2010, 06:56 AM   #3
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Quote:
Originally Posted by Sundowner View Post
My first thoughts would be vacuum-related, so my initial reaction would be to check your vacuum lines and see what you've got. If that doesn't turn up anything, then start looking at the TPS and IAC.
I'm thinking that it is not a vacuum problem because it is not doing it all of the time and it is not doing it at all with the air on. My thought was also the IAC but the throttle body looks really clean and the miles are low. I wonder how dirty it can really be? I am also wondering if a plug change would help since the Jeep has sat for most of its life?
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Unread 07-01-2010, 07:10 AM   #4
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I had a Ford truck that did EXACTLY that, and it turned out to be a pinhole in a vacuum line...thus, my initial reaction. It could be a stuck piston in the IAC, or something with the TPS. I'd certainly check the plugs if you haven't already.
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Unread 07-01-2010, 08:24 AM   #5
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OK, I will check all of those this weekend. I think I am going to have a hard time pinpointing what the problem is because it is not happening all of the time. What are the most common areas for a vacuum leak? I never had one in any of my 4.0s...but I had plenty on my last YJ!!
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Unread 07-01-2010, 08:53 AM   #6
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I've found most of mine right around the fittings to which the hoses connect...that's an area of stress and stretching, and that's where they seem to break...at least for me.

Before you crack into sensors, check the connections for your IAC and TPS...an intermittent problem could easily come from one of those not making good contact with the sensor plug.

I suppose it could also be O2 sensor related, now that I think of it. Those can cause all manner of problems. But check the other stuff first.
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Unread 07-02-2010, 06:40 AM   #7
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OK, last night I put in new plugs in (Champion 7034) and it ran terrible (I think I may have gotten a bad plug so I am going to try and take them back). It felt like it was missing and the threw a code. I put the old plugs back in and disconnected the battery to reset the computer (I don't have a scan tool). Started it up and everything was back to normal. I could be my imagination, but it feels like it was running better after all of that. The idle was sitting at around 600 and the pickup felt better. The tach needle is still jumping a little bit and I feel like the engine is still not running perfectly smooth. I checked the TPS and the IAC are both clean. I found another old thread on here where they sound like they are talking about my problem (not the lugging part) but it is an OLD thread.

http://www.jeepforum.com/forum/f9/id...eading-102358/

So now I am beginning to think that the slightly uneven idle is normal?
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Unread 07-02-2010, 06:44 AM   #8
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I suppose it could also be O2 sensor related, now that I think of it. Those can cause all manner of problems. But check the other stuff first.
Wouldn't this throw a code?
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Unread 07-02-2010, 07:11 AM   #9
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Wouldn't this throw a code?
It should, but it's not unheard-of to not have a code.

You changed to a 7034 and it was running badly? Now, that is pretty unheard-of...did you check the plug gaps? Was your coil rail reinstalled properly? What code was it showing? If you could post that, it would help a lot.

Also, why did you change the plugs? Were the old ones in bad shape? What was their condition?
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Last edited by Sundowner; 07-02-2010 at 07:30 AM..
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Unread 07-02-2010, 07:22 AM   #10
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Yeah, I know it is weird. I don't have a scan tool so I don't know what the code was. I thought maybe the coil rail was not seated properly, so I took it off and put it back on. Same thing. Plugs are pregapped at .035. I am going to see if they will give me another set and do it again...
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Unread 07-02-2010, 09:20 AM   #11
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I ran it this morning after leaving the battery disconnected overnight. It ran fine, with the tack moving just a little bit as usual, but when I got home it dropped down to around 300-350 RPMs. If I put in park and then back into drive the RPMs return to normal (around 600 RPMs). I'm stumped! Any ideas?
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Unread 07-02-2010, 09:26 AM   #12
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Quote:
Originally Posted by Sundowner View Post
Also, why did you change the plugs? Were the old ones in bad shape? What was their condition?
Sorry, did not answer this. I changed them just to troubleshoot. The Jeep has been sitting most of it's life and I figured it would not hurt to change the plugs (even though they don't really go bad). The plugs looked good, nice tan color.
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Unread 07-02-2010, 09:55 AM   #13
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Not sure I would have swapped them just for troubleshooting...but since you did, it's worth checking the gap on everything. Sure would be nice to have that code that came up when you swapped them out, though...

Well, I have three possible answers at this point, and after that, I'm tapped out without knowing what codes it might be throwing. From what you've said, it's not vacuum-related and it's not RPM related, but rather, heat related...and somehow, park-to-drive related. Therefore:

1. O2 sensor beginning to die.
2. Temperature sensor...which may or may not be the case, depending on how much impact it has on the air/fuel mixture once warm (we need someone more educated than I to answer this).
3. Something else...sensor connector or the like...

Wish I could help you more!
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Unread 07-03-2010, 05:46 AM   #14
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Sundowner, thanks for you help...since you are the only one who posted. I was going to work on it yesterday, but I had to do a full brake job on my wife's van so it would pass our awesome PA inspection. I talked to my brother yesterday who was a trained mechanic. He lives out of state, but he said he would bet it is IAC or throttle body related. Even though everything looks clean, I am going to take it all apart and clean it. I will let you know if that makes a difference...
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Unread 07-03-2010, 11:15 PM   #15
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Good luck with that. Hope it works out for you.
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