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Old 08-15-2013, 04:38 PM   #1
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Little rpm bobbing when coming to a stop

New motor, tranny, clutch, header, slave, master cylinder- all installed within the past month- less than 100 miles. Itake professionally cleaned in a tank
As i come to a stop, i notice the rpms bobbing up and down, sometimes goes below idle rpms, but not enough to stall out- at idle it does not run rough at all-
I think clogged cat as its the stock exhaust at the body has 155,800 miles on it...ordered new cat from quadratec today-
Is there anything else that i should check? Almost everything is brand new on this engine- runs great when driving- no issues at all- just when stopping

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Old 08-15-2013, 06:38 PM   #2
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And i also ordered new o2 sensors as well- just to be safe.

Thoughts?

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Old 08-21-2013, 04:33 PM   #3
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New o2 sensor installed but i had to cut out the old rusted exhaust

Also the old cat looked great- honeycomb inside it was perfect...oh well

Waiting for magnaflow exhaust toget here before i drive it...moved it into the garage and it was soooooo loud
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Old 08-21-2013, 04:37 PM   #4
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And i also ordered new o2 sensors as well- just to be safe.

Thoughts?

Hopefully NTK and not some no name brand or Bosch.

Fuel pump for idle issues maybe?
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Old 08-21-2013, 04:44 PM   #5
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Ntk lol
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Old 08-21-2013, 04:45 PM   #6
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Ntk lol
Good call!
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Old 08-21-2013, 04:56 PM   #7
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My LJ has a slightly unstable idle for a short period of time after I exit the interstate at speed and come to a fairly quick stop. I don't see it under any other operating conditions. It doesn't affect me enough to look into it, though.
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Old 08-21-2013, 05:09 PM   #8
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Yea same here only see. When i push in the clutch and go to nuetral when stopping at red lights- weird...
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Old 08-21-2013, 06:15 PM   #9
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My LJ has a slightly unstable idle for a short period of time after I exit the interstate at speed and come to a fairly quick stop. I don't see it under any other operating conditions. It doesn't affect me enough to look into it, though.
My 2005 LJ does this also. It will drop down so low I think its going to stall and then it picks up RPM's and idles fine. Seems to run fine otherwise.
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Old 08-21-2013, 06:31 PM   #10
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Gents sounds like your idle air control is dirty or sticking. That's what controls idle rpm. They can get carbon-ed up and not properly control idle air flow at low rpms. They can be cleaned with carb cleaner or replaced. New IACs run the gamunt on prices from $30 to $250. I've never replaced, always cleaned and not had any problems.

A link to a YouTube showing cleaning is

.

If the link doesn't work just Google " jeep 4.0 liter idle air motor" and it should be 1st one up.
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Old 08-21-2013, 06:39 PM   #11
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Had the same kind of problem, replaced idle air control and throttle position sensor and it runs great now
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Old 08-22-2013, 02:09 PM   #12
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Sounds good and im burning it up right now to get rid of the new smell!!!
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Old 08-23-2013, 12:51 PM   #13
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Clean the throttle body, it's probably has a lot of black gunk (carbon) sticking to it. Dirty TB or sensor causes the fluctuation at idle
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Old 08-23-2013, 12:56 PM   #14
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Tb was already cleaned while i was swapping motors- didnt think of the iac and cleaned it yesterday- let it run for a while and i didnt have any issues- cant drive too much as i have no hood lol
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Old 08-23-2013, 02:28 PM   #15
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Tb was already cleaned while i was swapping motors- didnt think of the iac and cleaned it yesterday- let it run for a while and i didnt have any issues- cant drive too much as i have no hood lol
Let us know if its ok when you can drive it around a bit.
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Old 08-23-2013, 02:51 PM   #16
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Gonna drive it around the neighborhood in a few!
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Old 08-30-2013, 11:15 AM   #17
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Cleaned iac real good but it still bons when i push in the clutch to go to neutral at a stop...
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Old 08-30-2013, 03:21 PM   #18
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Cleaned iac real good but it still bons when i push in the clutch to go to neutral at a stop...
Hmmm, the only other thing I can suggest (other than computer diagnosis of the idle parameters) would be to check for vacuum leaks. A vac leak allows excess air into the intake that the computer isn't reading so you get a momentary over lean fuel mix as the throttle plate closes. When rpm drops below minimums, fuel flow and air flow will be raised to compensate. Problem is vacuum leaks are hard to track... brake boosters leak, your hvac dampers can leak, all sorts of stuff can leak without a telltale hiss. They only way I have been able to isolate is to temporarily disconnect and cap each vacuum port one at a time and see if the problem ceases.

Otherwise find a shop that can plug into the diagnostic port and read the real-time parameters as you come down to idle. You should see the drop in rpm and should see what appears to be the trigger event.

Its a tough one.
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Old 08-31-2013, 05:45 AM   #19
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Gonna pick up a new iac. While it was cleaned it still wasnt in perfect shape...
Thought about vacuum leaks but it idles perfectly when not driving it- only have the issues when coming to a stop.
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Old 08-31-2013, 01:58 PM   #20
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New iac and tps...

And the rpm bobbing when coming to a stop continues- but not when idling....idles fine around 500-600.

Yay....
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Old 08-31-2013, 07:26 PM   #21
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Reset the computer.

Mine was a vacuum leak from a hose that popped off, fixed it then reset computer and good as new.
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Old 08-31-2013, 08:35 PM   #22
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Yea i just gotta find the leaky hose now lol
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Old 09-02-2013, 09:55 AM   #23
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Alright so i checked this morning for a vacuum leak with my mechanic neighbor-has his own business and is really good at this stuff-
No leaks found at all. We sprayed each hose with a lil bit of throttle body cleaner- since it evaporates over time- and not once did the engine ever idle roughly.
As i said before it only bobs when im coming to a stop at a light or stop sign. Sitting in the yard at idle it is perfectly fine...
Stumped.....
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Old 11-07-2013, 04:20 PM   #24
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Rear o2 sensor fixed it!!! Yay

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