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Discussion Starter · #1 ·
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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Discussion Starter · #3 ·
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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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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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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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

http://m.youtube.com/watch?v=VwO3k0Up_bw&desktop_uri=/watch?v=VwO3k0Up_bw.

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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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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Discussion Starter · #14 ·
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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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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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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Discussion Starter · #19 ·
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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Discussion Starter · #20 ·
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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