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Idle hunt

asymptonic

NAXJA Member
NAXJA Member
Location
Austin, Texas
I've seen various threads on this, but this one puzzles me.


Every once in a while, Jeep will start with a high (~1100 rpm) idle at cold, and after warming up a bit will start hunting between maybe 500-750 rpm every second or two. I'm aware of some of the causes of this.


I have a new TPS, clean throttle boddy, clean and pretty new IAC.



Here's the weird part. If I shut down and then start back up a couple seconds later, it goes away. That confuses me. Thoughts?
 
I've seen various threads on this, but this one puzzles me.


Every once in a while, Jeep will start with a high (~1100 rpm) idle at cold, and after warming up a bit will start hunting between maybe 500-750 rpm every second or two. I'm aware of some of the causes of this.


I have a new TPS, clean throttle boddy, clean and pretty new IAC.



Here's the weird part. If I shut down and then start back up a couple seconds later, it goes away. That confuses me. Thoughts?

The automatic shutdown relay.
 
.... a new TPS, clean throttle boddy, clean and pretty new IAC.

Genuine Jeep sensors or cheap crappy Chinese clone sensors ? The symptoms would make me suspect the IAC.

Battery newer, lugs and cables clean and tight, and been Load Tested ?

Never hurts to clean associated wire plugs with plastic safe electrical contact cleaner and to wire brush and snug all grounds.
 
Mopar all the way. Learned that lesson long ago... Battery isn't terribly new (3 years? but starts instantly), and the connections are okay. Not been load tested, but the shutdown followed by perfect behavior on immediate restart seems really suspicious for a simple bad connection. Not that much rattle shutting down and restarting.


I went over the grounds a few months ago and got them to within 0.3 ohm, so that's good.
 
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Followup, I think I bought the Standard Motor Products IAC a while back, not a Mopar. Looking for a legit one now, but it's hard to tell if these are really OEM parts at this point (despite the high price). Can anyone confirm this looks like a real Mopar IAC? Part number is right, but...



shopping
 
Followup, I think I bought the Standard Motor Products IAC a while back, not a Mopar. Looking for a legit one now, but it's hard to tell if these are really OEM parts at this point (despite the high price). Can anyone confirm this looks like a real Mopar IAC? Part number is right, but...



shopping

That is hard to tell but as far as I know, the IAC is/was not made by Mopar. I think it is a Chevy/GM product. My XJ is sporting one from a Pic-Your-Part Chevy truck (I do not remember the year) for many years.
Recently, my XJ does the same thing your is doing maybe, once or twice per week at stop lights mostly before it is fully warmed up. I probably will not worry about it until it get worse. And by the way, I suspect it is the IAC. Maybe I should pull it and clean it. Back there do get more than its fair share of carbon. Maybe the electronics in the IAC is getting lazy.
 
Can anyone confirm this looks like a real Mopar IAC? Part number is right, but...

I see no reason to think that is not a genuine Mopar item. I bought a Mopar XJ CPS not long ago and it was made in Mexico.
 
Genuine Mopar and Standard Motor Products are both made in Mexico.

Replaced the sensor last weekend and the SMP sensor seems a solid and good quality product to me. It works just fine.

The housing and tip that closes the gap is a little different from the Mopar sensor.
 
I have 2 genuine Mopar IACs now and the Standard one. The pintle on the Standard spins freely unlike the Mopar, which may or may not matter, and it doesn't have a part number on it. I'm not judging it's quality at all, it probably is fine, but what I had in before the newest one was also a Mopar and the one *before* it was a Standard.


Point is the issue seems not likely to be the IAC at all, but I'm still driving the new one to try and find out. I definitely have a slightly unsteady idle at all times not counting the aggressive hunt that feels like maybe a vacuum leak, but the hunt feels electrical in some way.
 
Didn't seem to need too. It was clear the PCM was discovering the new sensor at start. Idle revved to 1300 then ticked down to normal and has been normal since, with the exception of the slight unsteadiness that's been a problem for a few years. I haven't had the idle hunt or high idle problem yet with this one, but I also have hardly been driving for obvious reasons.
 
Any progress on idle hunt?

I recently cleaned the MAP sensor with special cleaner. Do NOT use carb or brake cleaner!
The engine runs way much smoother on idle now. ☀️👍🏻
Worth to try?
 
Found a vacuum leak on the throttle body where the butterfly pivots. Replaced the throttle body with a remanufactured one. I haven't had the surge or hunt since then, but I still have some unsteadiness at idle.



I'll give the MAP cleaning a shot. Any special technique or things to watch out for?
 
Oh? I'll google that, but why would restarting help it out? It seems like if that was failing I'd have more drastic problems.

Good to hear you fixed it. The reason the ASD relay could cause and is a frequent cause of a high idle at start is all about how the computer and controls work together. As you know the idle air mixture control is used to set the idle speed. As the engine warms up, it has to compensate. The problems is that the IAC is a motor that is moved to a position and stays there until commanded to change. When you shut off the ignition, the ASD relay keeps power to the ECU for a few seconds, allowing time for it to reposition the IAC to the startup position. If it is bad, it is left in the wrong position, causing the high idle. It takes a while for the ECU to correct. If you shut it down, the ASD is exercised again and most likely works this time setting it to the correct setting, allowing the idle to wake up correctly. This is a straight Bosch type cube relay and they are notorious for getting corroded/dirty over time. Exercising them helps but a good maintenance tip is to replace them all every 4-5 years. If you order them online, they run as little as a buck a piece.
 
Found a vacuum leak on the throttle body where the butterfly pivots. Replaced the throttle body with a remanufactured one. I haven't had the surge or hunt since then, but I still have some unsteadiness at idle.



I'll give the MAP cleaning a shot. Any special technique or things to watch out for?


Nice to hear you’re making progress!

For the MAP sensor
I’ve used a similar product like this:

https://www.autozone.com/miscellane...c-mass-air-flow-sensor-cleaner-11oz/36011_0_0

Sprayed a couple of times in the hole where the rubber elbow fits on the MAP sensor and let the MAP sensor leak out the cleaning fluid.
Then I let the sensor dry for half an hour in the sun before I mounted it on the trottle body.

I notecid that my 4.0 was running much smoother at idle and improved gasmilage.

Resetting the pcm is not needed.

Good luck!
 
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