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Old 08-20-2010, 07:17 PM   #1
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High Idle ONLY after warm up!!

I pretty positive I have a small crack in my exhaust manifold. After warm up and only after warm up the idle will sit consistantly at around 1800 rpms. It idles fine when cold, but starts increasing pretty quick after running for a few minutes. I was thinking that if my IAC is the problem that it would do it all the time and not only when the Jeep is at running temp. Figured if its the TPS that I would have a low idle. I've checked all my vacuum lines and haven't found any of them to show signs of wear or looseness. I'm just trying to get some advise before I go out and just start buying all kinds of parts.

Edit: I searched on this site, as well as quite a few others before posting this. I found a lot of talk about high idle when cold and low idles but nothing exactly like the problems I'm having.
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Old 08-21-2010, 11:57 AM   #2
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I just hit a *bump* in the road

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Old 08-31-2010, 01:40 AM   #3
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Just replaced the IAC and still idles high after warm up. The air charge sensor (screws into the top of the intake manifold) looks bent on the inside and was caked up with black oily crap when I unscrewed it and took it out. If that sensor isn't reading right would that cause the high idle ONLY after the Jeep is warmed up? Literally, the RPMs will slowly creep up as the Jeep warms up. It maxes out at around 1800 RPMs at idle after full warm up. Any advice at all?? I could go purchase a new TPS, MAP and Intake Temp Sensor and just go from there but those things aren't returnable after they've been installed already so I'm hoping to narrow this problem down before forking out $$ I don't have. Any help is greatly appreciated. Oh I also went over all the vacuum hoses and they all seem to be seated down pretty good.
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Old 08-31-2010, 02:36 AM   #4
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I have a small crack in my exhaust manifold but that wouldn't cause a vacuum leak or high idle after warm up would it?
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Old 08-31-2010, 03:39 AM   #5
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The first thing I would consider is that the EGR valve is not closing completely when it gets hot. So I would remove and thoroughly clean the deposits out of the EGR valve. After that, try manually opening and releasing the valve; it should close easily and completely every time. If it does not close consistently, the EGR valve should be replaced.

After having been driven that many miles, it is also likely that fit of the valve stems in the cylinder head valve guides has become loose. When that happens; air can be sucked in to the intake manifold through the valve guides. Loose valve guides will in that way cause the air fuel mixture to become diluted. Loose guides will also result in inconsistent valve sealing, which causes the cylinder compression to drop. And both of those things can create increased or unstable idle speeds.

There is one other possible cause for this problem: Today's gasolines contain chemicals which produce heavy carbon deposits on valve stems. This can prevent valves from closing consistently or completely. So it might be worthwhile to add a full bottle of Chevron Techron fuel injection and combustion chamber cleaner to the fuel tank just before filling it with fuel. This product often takes 50 to 75 miles of driving to do its work. But the results are better than other products of that type.

I tried all the things suggested and the problem persisted on my 91 Jeep Cherokee. I was about to give up on it and just beginning to really get used to driving that way.
I then notice about half the exhaust manifold bolts were loose and tightened them to the proper torque settings which somehow cured the high idle problem.
I have no idea how this could be connected, but it is something others can try if the problem cannot be resolved in the proper manner.



hey i found this on another forum it's a cherokee but i hope it helps good luck
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Old 08-31-2010, 03:42 AM   #6
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oh btw the OP said they replaced the IAC also w/o effect
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Old 08-31-2010, 10:01 AM   #7
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Thanks for the response....Any idea where the EGR valve can be found on my 98 2.5L?
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Old 08-31-2010, 04:15 PM   #8
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Quote:
Originally Posted by chrisb0108 View Post
Just replaced the IAC and still idles high after warm up. The air charge sensor (screws into the top of the intake manifold) looks bent on the inside and was caked up with black oily crap when I unscrewed it and took it out. If that sensor isn't reading right would that cause the high idle ONLY after the Jeep is warmed up? Literally, the RPMs will slowly creep up as the Jeep warms up. It maxes out at around 1800 RPMs at idle after full warm up. Any advice at all?? I could go purchase a new TPS, MAP and Intake Temp Sensor and just go from there but those things aren't returnable after they've been installed already so I'm hoping to narrow this problem down before forking out $$ I don't have. Any help is greatly appreciated. Oh I also went over all the vacuum hoses and they all seem to be seated down pretty good.
Quote:
Originally Posted by chrisb0108 View Post
I have a small crack in my exhaust manifold but that wouldn't cause a vacuum leak or high idle after warm up would it?
Quote:
Originally Posted by chrisb0108 View Post
Thanks for the response....Any idea where the EGR valve can be found on my 98 2.5L?
Bump for more advice...
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Old 08-31-2010, 06:33 PM   #9
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For search result purposes.....the high idle ended up being a result of loose manifold bolts. The far bolt near the firewall was so loose I could turn it with my finger and all but one of the other bolts were really loose. Idling at a nice and quiet 700 RPMs now!!!
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Old 08-31-2010, 11:17 PM   #10
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Quote:
Originally Posted by chrisb0108 View Post
For search result purposes.....the high idle ended up being a result of loose manifold bolts. The far bolt near the firewall was so loose I could turn it with my finger and all but one of the other bolts were really loose. Idling at a nice and quiet 700 RPMs now!!!
right on bro glad to see it
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Old 08-09-2015, 05:01 PM   #11
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I know this is a very old thread, but I was recently having similar issues and it ended up being from loose header/manifold bolts also. Thanks to this thread, I saved a lot of time and money chasing this issue. Thanks to all who contribute !!!
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Old 12-01-2016, 09:39 AM   #12
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If anyone is still on this thread... How much stuff had to be removed to get to the manifold bolts on the 6 cylinder. Just to tighten them up not pulling the manifold or anything
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Old 12-01-2016, 12:14 PM   #13
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Quote:
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If anyone is still on this thread...
I think this guy is still looking for his EGR valve:

Quote:
Originally Posted by M.r.G.5150 View Post
The first thing I would consider is that the EGR valve is not closing completely when it gets hot. So I would remove and thoroughly clean the deposits out of the EGR valve.
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Old 12-11-2016, 08:15 PM   #14
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After replacing 5 different sensors. I finally tightened the manifold bolts....Two in the back were finger loose. It idles perfectly now. Right around 600-750.
Happy days...

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