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Low idle while coming to complete stop?

MickeyS

NAXJA Forum User
Location
Pasadena
96 XJ runs great otherwise. I have low-ish idle while coming to a dead stop on the brakes (can dip down to 690) but once stopped, goes back up again to 720-750 idling in drive. idle is normal 750-800 in park. coming to a stop you can hear the rpms drop and feel it a bit. gets a little better when the engines good and hot.

obd2 absolute manifold pressure reads 10-13 in hg depending on temp but usually between 11-12 which the engine at 210. checked for leaks but can't find any on the manifold but maybe i'm missing something.
 
Clean the Idle Air Controller. If the symptoms persist, replace the IAC. Genuine Jeep parts cost more, but work better and last longer.
 
My 96 does the exact same thing but it isn’t consistent — sometimes it’s just fine

I do have a cracked exhaust manifold — figured no sense in chasing the low idle till I change out the manifold
 
My 96 does the exact same thing but it isn’t consistent — sometimes it’s just fine

I do have a cracked exhaust manifold — figured no sense in chasing the low idle till I change out the manifold

Totally unrelated, but do what you gotta do.

That cracked exhaust manifold is not causing your low idle.

Cleaning throttle bodies and IACs is considered a maintenance item............
 
i have cleaned the iac but the problem persists.

i tried replacing it with an aftermarket one, but immediately got iac codes and a 2000 rpm idle so i figured that was a chinese dud and returned it. i guess the next step is trying a mopar sensor?
 
Still having this issue. To bring it up to speed:
New iac
Cleaned throttle body
Recent battery/alternator replacements
Recently replaced fuel filter with wix

Would a crack on the manifold cause this? Coming to a stop engine can dip to 690 before recovering in a few seconds where it idles between 720-750. 720 a bit low.
 
Genuine Jeep IAC ? A cracked exhaust manifold should have no effect on the idle. If the engine does not stall, I wouldn't worry about it.
 
It feels like something's amiss but yeah, it's not throwing codes. I figured wait until it gets worse and then it would be easier to diagnose, but I don't drive it enough. I'm going to put a pack of $5 injector o rings on this week and triple check vacuums going to throttle unit, see if it helps at all.

I just don't want it to cause a bigger issue down the line, as it seems to be running a bit lean and lower mpgs. (13 town)
 
If it is an automatic transmission, the clutch in the torque converter could be staying locked.
 
Mine is cured but I don't know exactly what did it -- Noticed the problem went away after a can of Bk 44 injector cleaner was run through the tank

Here is what I did since the last post in this thread by me:

Replaced cracked exhaust manifold
Total cleaning of intake manifold and IAC while it was off
Added DEI injector insulation kit -- New Injector O rings installed with the kit
built and installed a heat shield for under the intake manifold
Replaced motor mounts
Ran a can of BK44 injector cleaner thru the system

At this point low idle issues are gone
 
that's almost exactly my attack plan once the weather cools down here. get that damn manifold off swap the mounts and super deep clean the fuel/intake system. I will order a can of that bk44. looks like good stuff the injectors are 180k miles old, but I have run chevron and seafood through it before.
 
that's almost exactly my attack plan once the weather cools down here. get that damn manifold off swap the mounts and super deep clean the fuel/intake system. I will order a can of that bk44. looks like good stuff the injectors are 180k miles old, but I have run chevron and seafood through it before.

That is your problem... they are not made for seafood, they are made for gas.
 
If it is an automatic transmission, the clutch in the torque converter could be staying locked.

How would I figure out if this is happening? I'm a bit dense and don't know much about the transmission. It happens only when coming to a dead stop and then idles up to 740ish in D, and nicely in park 750-800.

I confirmed the maf vacuum elbow was good today and spent a good ten minutes spraying carb cleaner all around the throttle body/injector area and whatever other vacuum lines I could see in the engine bay, and found no leaks whatsoever.
 
A quick check of the t/c unlock would be to drive along steady at highway speeds, and lightly hit the brake with your left foot. You should see the converter unlock and the rpm jump up a bit. Pretty rare to have it not unlocking though, and the usual problem is that it's not locking in the higher gears due to a bad solenoid or bad switch on the brake pedal, or sometimes a bad TPS.
 
:spam:
A quick check of the t/c unlock would be to drive along steady at highway speeds, and lightly hit the brake with your left foot. You should see the converter unlock and the rpm jump up a bit. Pretty rare to have it not unlocking though, and the usual problem is that it's not locking in the higher gears due to a bad solenoid or bad switch on the brake pedal, or sometimes a bad TPS.

I understand now how it should work. Just took it for a jaunt on the freeway and the converter unlocked properly. I still have a suspicion this is a fuel/injector/wiring/voltage issue. at night with lights on, cigarette accessories going, voltage can dip to 13.4-13.5 where the rpms can go as low as 650.
 
I am still wondering if you installed a Cheap crappy Chinese IAC or a genuine Jeep IAC ?


...... voltage can dip to 13.4-13.5 where the rpms can go as low as 650.

That is normal. The alternator typically does not achieve full voltage until about 1500 rpm.
 
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