Irregular idle while coasting in neutral - JeepForum.com
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post #1 of 10 Old 01-04-2019, 08:54 AM Thread Starter
Rbeguelin
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Irregular idle while coasting in neutral

My son and I just relaced his Bank 2 number 1 O2 sensor which had been throwing a bad code. (He read the O2 replacement thread here but did not report back that we should have replaced all four! Gen Z er) After the install, the code went away however, the engine started an irregular idle when coasting such as up to a stop sign or red light. His Jeep is a 2005 Unlimited with the six cylinder 4.0 . When the engine is first started the idle is fine, driving appears to be fine, but when coasting with the clutch or in neutral the engine seems to be searching for the correct idle first too low then it bounces to a bit too high then back again. If I give it gas it will settle back to its correct idle. No codes are currently being shown. He is planning on driving to New York today
Any ideas???

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post #2 of 10 Old 01-04-2019, 09:17 AM
tworley
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If it holds the RPM its caused by the speed sensor and an open/closed throttle position. When slowing, your throttle position is closed but the speed sensor is still sending a signal, the PCM adjusts the IAC to a specific pressure. Once you stop, the PCM then sets the IAC to the normal idle speed. It is perfectly normal.

If it is actually fluctuating though (800-1200 for example), I would start by giving the IAC a real good clean.
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post #3 of 10 Old 01-04-2019, 01:08 PM
mukluk
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To put it a bit more clearly, the engine idle/IAC setting is determined two ways:
- VSS signal at rest (the Jeep isn't moving) and closed TPS signal, the IAC is moved to attain a specific engine rpm range
- VSS signal above zero (the Jeep is moving) and closed TPS signal, the IAC is moved to attain a specific manifold pressure

In addition to the advice to clean the IAC and its passage in the throttle body, it'd be a good idea to also check for leaks at the vacuum lines or the MAP sensor's rubber fitting.

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post #4 of 10 Old 01-04-2019, 03:01 PM Thread Starter
Rbeguelin
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Thanks for the replies and info. Following a u tube video we took off the IAC and cleaned it. It is the type of IAC that has only one torx screw and is not a plunger shape but hollow. We carefully cleaned it and where it fits, in with carb cleaner and a rag. My son took off for NYC and the jeep is still searching for the correct rpms when coasting such as pulling into a toll lane. Two other thoughts we have. The first is a vacuum hose which attaches to a nipple on the intake manifold near the IAC. It was not on when I first saw it but we may have dislodged it working on the IAC, however when I put it back on the niple it was very loose, I cannot imagine it was creating a very good vaccum. Secondly I have read other threads here suggesting the MAP sensor may be faulty. Rather than throwing parts at the problem is there a way to test the MAP sensor? I did read where one pulled up to the auto parts store and just started swapping out parts. Our NAPA here in Rockville MD is very good BUT I think they might not let us try that method
Other thoughts
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post #5 of 10 Old 01-04-2019, 03:49 PM
fracturedapple
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To test the map you'll need a multimeter.

You say it only does this when you are coasting to a stop?

What if it's a garbage in garbage out kind of thing were at slow speeds the VSS is putting out a poor signal and the pcm thinks the jeep is stopping then moving, stopping then moving and so on and it's adjusting the idle as the condition calls.



Also to test the VSS you need a multimeter too.

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post #6 of 10 Old 01-04-2019, 03:52 PM
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As mentioned, the PCM tries to hold a specific manifold pressure range via the IAC when the throttle is closed and the vehicle is still moving, so having a loose vacuum fitting on the intake manifold can most definitely cause the hunting idle issue your son is having. The number one priority should be to fix the loose vacuum fitting.

See the attached excerpt from the FSM.
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post #7 of 10 Old 01-04-2019, 04:48 PM Thread Starter
Rbeguelin
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Got it...How best to make the vacuum connection tight. My son thought about wrapping a small zip tie around the connection...but those can have flat spots and too much tension could rip through the rubber 90 degree angle boot. I know from other experience with vacuums it doesnt need a lot of pressure to create a proper seal - what have others done?
I have a muti meter - a good one -- just the operator needs help! How can I check the IAC and the MAP with it?
Thanks
Rob
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post #8 of 10 Old 01-04-2019, 08:53 PM
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If it's the fitting itself that's loose, it may just need to be snugged down or some tape wrapped on the threads and screwed back in. For a loose line to fitting problem, a small zip tie would likely work just fine, being careful not to overtighten it as you noted. If the loose line is a typical rubber vacuum hose, trimming a bit off the end to get back to a section that isn't stretched or cracked is an option. Another idea would be to smear a small dab of rubber cement or RTV on the fitting then slipping the hose/tube on and allowing the sealant to dry, just be careful not to get any sealant where it might clog up the line or fitting.

Testing the MAP sensor:
- you should have a good ground at pin 2 (DB/DG), and with the key on there should be between 4.5 to 5.2v at pin 3 (YL/PK)
- with the engine running, snap the throttle -- the sensor signal (pin 1 VT/BR or reading on scanner) should go from below 2v at idle to above 3.5v at WOT
- likewise, slowly depressing the throttle should have the signal go from roughly 0.8v at idle to above 3.5v at WOT with a smooth transition

Without any codes being thrown or access to a DRBIII scan tool, about the only thing you can do to check the IAC motor is to measure the resistance across its two pins with the plug disconnected, the reading should be 9.7 +/- 1.0 ohms.
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post #9 of 10 Old 01-05-2019, 11:25 AM
fracturedapple
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To add their is one thing you can do with your meter and the iac.

It's duty cycle controlled. Most meters have a frequency setting.

If you back probe the VT/GY wire and koer you should be able to see the frequency at which the pcm is turning off and on that circuit.

Any decent change in frequency will tell you that the pcm is changing the idle speed using the iac.

If you are hearing and seeing the idle change and the frequency is staying the same that means something like a vaccum leak is causing the idle to change.


Skewed inputs from the map, tps or VSS can trick the pcm into thinking it needs to change the desired rpm. During decel.

Alternatively if you had an occiliscope you could do the same in greater detail while looking at the wave form.

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post #10 of 10 Old 01-07-2019, 07:35 AM Thread Starter
Rbeguelin
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Thanks for the ifo all. I have relayed the posts to my son who is back in Utica NY. He will try the tape on the nipple to see if it helps - after that he is off to a local garage. If I find out what the results are I will post them here!
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