after driving highway speeds, RPMs dropping out at stop light

harper

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Oct 17, 2016
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Hello!
(2004, 6cyl, auto transmission)
after driving at highway speeds, I come off the highway and stop at the light or stop sign and the rpms drop way down and she starts to idle rough. when I step on the gas to go forward, there is a hesitation.

anybody have any thoughts as to what is happening here? sensor going bad? vacuum leak?

any help on this would be greatly appreciated.

thank you.
 
Vacuum leaks cause higher than normal engine RPMs, not lower RPMs so that's not the likely cause. My bet is the IAC (idle air control) mounted on the throttle body is dirty. The Idle Air Controller which is what passes air into the engine via an alternate source when your foot is not stepping on the accelerator pedal to open the main air valve plate. When it's dirty, it can't provide enough air to the engine at idle RPMs. Fortunately the IAC can usually be restored back to full operation by nothing more than a good cleaning.

My suggestion is to buy an aerosol can of throttle body cleaner and spray it into the throttle body while the engine is running.

The IAC only passes air at idle RPMs so the cleaner will only get sucked into and drawn through the IAC at idle RPMs. That's when you need to spray the cleaner to clean the IAC, at idle rpms. When the IAC receives a good strong dose of the cleaner, the air flow is normally temporarily blocked off so the engine dies. No problem, just start the engine again. I try to get a third of the can through the IAC so the engine will die multiple times during the process which doesn't hurt anything. Actually, the time the engine isn't running lets the cleaner soak into the gunk more so it actually helps the cleaning process.

The IAC sucks its air in through its 'idle air control passage inlet' slot as you can see below which is where the cleaner spray should be aimed when the engine is at idle RPMs. The engine will die when you do that but that's ok, just restart the engine. When you've sprayed enough cleaner through the IAC, I'd empty the rest of the can in through the throttle body. Keep the engine RPMS up with the throttle lever on the throttle body so the engine doesn't die during this part of the cleaning process.

TB & TPS Sensor Locations.jpg
 
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Hello!
(2004, 6cyl, auto transmission)
after driving at highway speeds, I come off the highway and stop at the light or stop sign and the rpms drop way down and she starts to idle rough. when I step on the gas to go forward, there is a hesitation.

anybody have any thoughts as to what is happening here? sensor going bad? vacuum leak?

any help on this would be greatly appreciated.

thank you.
Hi Harper, had a similar thing going on with mine. Cleaned the IAC as mentioned by Chris & Jerry above, completely resolved the issue.
 
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I recommend cleaning the IAC regardless. It's super easy to do, and practically free.
 
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I yanked that little beastie out. dirty as all get out. cleaned it up with a combination of the throttle body spray, paper towels and a light scrubbing with a brush. what a difference it made. thank you one and all for this information and fix!
 
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I yanked that little beastie out. dirty as all get out. cleaned it up with a combination of the throttle body spray, paper towels and a light scrubbing with a brush. what a difference it made. thank you one and all for this information and fix!

I knew it! Those IACs are notorious for getting dirty and causing these sort of issues.

Glad it's running better now.
 
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I yanked that little beastie out. dirty as all get out. cleaned it up with a combination of the throttle body spray, paper towels and a light scrubbing with a brush. what a difference it made. thank you one and all for this information and fix!
Good to read, glad it helped! Hopefully you also cleaned the orifice in the throttle body the IAC fits into. :)
 
These guys helped me with the same issue. It was the IAC. Ran much better afterwards. Definitely try that first.

Sent from my SM-G935V using Tapatalk
 
well, the issue is back. :(
pulled the sensor out and it wasn't dirty like before.
I did clean the entire throttle body while I had the hood up.
I changed the IAC out for a new one. Does the sensor move in and out of the housing? I noticed that the replacement seems to be shorter than the original.
problem is still persistent when coming down off the highway speeds and hitting city streets. rough idle, dropping out. around town it seems to not be an issue.
could there be a heat issue with some part? coil pack? transmission?
I grasping now for anything.
thanks.
 
Are you 100% certain there is no check engine light?

What brand of IAC did you use as a replacement? Hopefully a genuine Mopar part.
 
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I've been having weird things happening after cleaning mine...is there a chance it just needs to be replaced vs cleaned?
I've stalled out once or twice while driving, and I seem to idle pretty low, like 500 rpms sometimes while at a stop light.
 
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I've been having weird things happening after cleaning mine...is there a chance it just needs to be replaced vs cleaned?
I've stalled out once or twice while driving, and I seem to idle pretty low, like 500 rpms sometimes while at a stop light.
Cleaning them should always be tried first as that normally restores them back to 100% good operating condition. When you cleaned the IAC, did you also thoroughly clean the orifice in the throttle body that the IAC's plunger fits into?
 
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I had similar issues with my 2.5. The same post highway driving rpm drop and rumble. My IAC was spotless as was everything on the TB. I got a tip that it could be a leak so I hooked up a blower to the exhaust pipe and squirted soapy water at everything. I was leaking almost the entire way around the exhaust manifold. That was fixed and symptom is gone but now I have an entire different symptom: it starts bucking at 2k rpms in any gear and ONLY at 2k rpms. Still have yet to figure it out.
 
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When you cleaned the IAC, did you also thoroughly clean the orifice in the throttle body that the IAC's plunger fits into?
Oh yeah, I unbolted the whole IAC housing, detached it, cleaned the IAC Valve and the housing separately and got a bunch of stuff out of there. Used an entire 12oz can of Throttle Body Cleaner and brought everything back to bare metal again.

I watched a video on Youtube about cleaning the IAC Valve, and one guy says be very careful of the spring inside there, and not to use any sort of force or pressure on that while cleaning it...I also saw another guy who disassembled the Valve and cleaned every possible area thoroughly. So I gently cleaned mine as to not ruin anything.
 
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I had similar issues with my 2.5. The same post highway driving rpm drop and rumble. My IAC was spotless as was everything on the TB. I got a tip that it could be a leak so I hooked up a blower to the exhaust pipe and squirted soapy water at everything. I was leaking almost the entire way around the exhaust manifold.

X2, a vacuum leak could be another potential issue!
 
Are you 100% certain there is no check engine light?

What brand of IAC did you use as a replacement? Hopefully a genuine Mopar part.
no check engine light. I don't recall the manufacturer of the replacement. it wasn't MOPAR. I'll try that next.
 
If it wasn't a an OE Mopar IAC replacement, that could indeed be a potential cause.

There's widely documented issues with replacement electronic parts not working the way they should on our TJs, so it's worth looking into that!
 
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