Revving at idle

jb4e

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I recently acquired a 2005 LJR with about 64k miles and the 6-speed manual. Yesterday, on a short trip in town, I noticed some strange behavior at idle. I was at a stoplight in neutral with my foot on the brake, and it felt like the engine was being revved. RPM went from normal idle (around 800) to over 1k, and then back down. It did this a couple times - see the video. It continued to do this on the rest of the drive. About 2 hours later, I went out again, and the behavior had stopped.

Some potentially relevant info:
1. I changed the very worn plugs about 5 days ago, but I have only experienced this behavior 1 time in several drives since.
2. I changed out the OPDA for a crown unit about a week ago.
3. It was a hot day (at least for Seattle). Temps were around 85 and I had the AC turned up relatively high.

Video here:
Is this a normal behavior or is indicative of a problem? If the latter, where should I look?


P.S. I'm really loving this community. One of the main reasons I bought the Jeep was to learn to work on cars. I had never done so much as an oil change and with all the great content on this site, I'm learning quickly.
 
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Look at the photos of my AIC Air Intake Control Valve that is connected to the throttle body. If yours is dirty it could be the culprit. I took off my entire throttle body and cleaned it along with the AIC.

https://wranglertjforum.com/threads...-at-idle-or-low-speed-only.64102/post-1173465

AM: The part is actually called a IAC; Idle Air Control valve (solenoid).

JB4E: Remove the air intake tube from the throttlebode and spray some SeaFoam or Mopar Combustion Chamber Cleaner into the throat without moving the thottle. This will direct the cleaner into the idle air port cleaning the port and IAC solenoid disc.
 
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The two most likely causes of a high idle are 1) a vacuum leak or 2) your IAC (idle air control) which is what provides all of the engine's air at idle rpms. The PCM regulates the IAC and how much air it allows into the engine at idle rpms but if the IAC is dirty, it can clog and get stuck in the open position letting in too much air which will cause a high idle.

I'd first clean the IAC which is simple. Go buy an aerosol can of throttle body cleaner. Then remove the air intake tube from the top of the throttle body and start the engine. While the engine is at idle rpms, spray the cleaner into the throttle body and the IAC will suck the cleaner in through itself and its in/out air passages. The cleaner will flood the IAC so the engine will die, that's ok just restart it and do it again. Don't raise the engine rpms to stop the engine from dying, that would cause the air flowing into the engine to bypass the IAC. Keep spraying it at idle rpms until you've put somewhere around 1/3 of the can through the IAC. Then you can use the rest of the can to clean the rest of the throttle body's interior, spray it while revving the engine a bit and clean the valve plate at the very top of the throttle body too. Throttle body cleaner with a rag is good for that. This method, spraying the cleaner into the engine while it's running, does a more thorough job of cleaning the IAC's entire air intake including its in and out air passageways.

The Idle Air Control Passage Inlet, below, is where the air passes through the opening in the throttle body and into the IAC.
TB & TPS Sensor Locations.jpg


If that doesn't help it's likely a vacuum leak which can be in the engine compartment or in the HVAC system which is vacuum controlled.

This is the vacuum system, item #8 is a plastic vacuum reservoir which is located below the battery mount. It's what provides a limited amount of reserve vacuum when the engine isn't running. Primarily to give you power braking if the engine stalls while you're driving.

Vacuum Leak Potential Sites.JPG
 
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Colojeep, yes:
3. It was a hot day (at least for Seattle). Temps were around 85 and I had the AC turned up relatively high.

Could this cause the behavior in the video?

I'll clean out the throttle body/IAC regardless - seems like a good maintenance step regardless.
 
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Colojeep, yes:


Could this cause the behavior in the video?

I'll clean out the throttle body/IAC regardless - seems like a good maintenance step regardless.

Turn off the A/C and watch the Tach at idle. The rpms can vary a little as the A/C compressor cycles, if the idle is steady with the A/C turned off.
Don't worry about it.
 
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From my own experience:
I had similar problem, RPMs not being stable at idle.
The AC is not an issue, because even when the compressor runs it does not create any heavy load on 4.0 engine, it is not Toyota corolla.
In my case it was bad IAC.
Mopar one costs like 130$. Chinese crap from Amazon is 9$. I have 2 Chinese ones, installed one and everything works as charm now. The second one is in the glovebox, if I need to replace it. It requires dealing with 4 screws only, so it is easy change
 
From my own experience:
I had similar problem, RPMs not being stable at idle.
The AC is not an issue, because even when the compressor runs it does not create any heavy load on 4.0 engine, it is not Toyota corolla.
In my case it was bad IAC.
Mopar one costs like 130$. Chinese crap from Amazon is 9$. I have 2 Chinese ones, installed one and everything works as charm now. The second one is in the glovebox, if I need to replace it. It requires dealing with 4 screws only, so it is easy change
Did you try cleaning the IAC first before replacing it? They get too dirty to operate properly more commonly than they fail.
 
Did you try cleaning the IAC first before replacing it? They get too dirty to operate properly more commonly than they fail.

I did clean the IAC, the entire throttle body and the IACs pass. It started to work better, but still idle RPM was not really stable. After IAC replacement, idle RPM is solid as rock, as long the engine temperature is under 220F.
 
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