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Old 11-11-2015, 02:03 PM
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Random rough idle/wanting to stall after exhaust manifold change

Hey, the other day I swapped in my new exhaust manifold and now I am having a little bit of an issue. I am posting a new thread because everything I've searched seems to only answer a high/rough idle.
--98 5speed 4.0L Sport -- 110k miles

What is happening seems somewhat random. The jeep starts up perfectly fine and idles at first perfectly. Oil pressure is better then before i swapped the manifold too, and no CEL codes. IAC was also replaced about 2 weeks before i did this and was working fine (was a mopar part too)

On start up, if I just let it sit and idle, after probably 30+ seconds it'll sound a little rough and rpms will be a little lower, sounds like a little rattle at the back of the motor cant tell where exactly if that helps and after 2/3 mins it will idle normally and the rattle will go away.

After starting to drive, the Jeep will sometimes act like it wont accelerate (after I'm already rolling, usually in 2nd or 3rd gear by the time it happens), if i floor it and keep it floored (the peddle) after like 2-3 seconds it'll then register/acting like i did floor it, and then go back to normal. This sometimes will happen by the time i get to the end of my road or 3-4 roads away. This also sometimes happens only once each time i drive, or once each time I stop/go. Also the idle is sometimes dropping as if it wants to stall but not always. This all seems to happen mostly when the jeep isn't fully warmed up but still sometimes will happen when warm.

I have one of those bluetooth OBD readers, and it shows vacuum & throttle reading the same as before all of this. Also when i did this swap, one of my injectors came loose and after putting back in, after starting poured gas out, but i replaced all of the injector O-rings and there are no more gas leaks (doesnt smell either)...also 4 of the injectors i can spin freely while they are in place...could this be bad?

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Old 11-11-2015, 05:27 PM   #2
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Temp sensor?


If you mess with plug to sensor do it with engine off as it is too close to back of fan to have hand on it with engine running

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Old 11-12-2015, 12:43 AM
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i realized the more I'm in stop/go the idling gets worse..I was leaving the parking garage today at school, stop/go all the way to the bottom, the idle got worse as i kept going. Tomorrow after work I'm going to just go under and check everything hoses/connectors and all to make sure i didn't just knock something loose.
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Old 11-12-2015, 12:29 PM
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I checked connection..hose and electrical, looking at my Bluetooth obd it is a vacuum issue, pressure drops almost in 1/2 when the rough idle kicks in....didn't think it was a vacuum issue since what I read usually said that would keep idle pretty high not low...but I guess I'll try possibly changing the manifold gasket, probably some dressing/sealant on it instead of just the gasket this time and see if that works
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Old 11-12-2015, 01:24 PM   #5
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Vacuum Loss may be result of rough idle rather than cause
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Old 11-14-2015, 11:52 AM
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Originally Posted by Digger84 View Post
Vacuum Loss may be result of rough idle rather than cause
that makes sense since the more i watch the reading of the pressure its very random....the more i play with it while driving it seems it will only really happen when accelerating, but if i ride the clutch for like 2 seconds when it starts to drop power (hurts me to do that) it then kicks it back to working fine.

Do you think this could be something with the throttle body? I've reused the gasket a few times, maybe it got warped a little bit with being on/off so many times? Also the pipe from the cat to manifold isn't seated properly, so maybe that's causing random times of a leak, but when my manifold was cracked it threw a code and no code now... the pipe isnt seated correctly since the bracket with the bolt holes was rusted off..my buddy's mom is scrapping her Cherokee so I'm going to go tomorrow and see if that exhaust will fit...it is the 4.0
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Old 11-14-2015, 12:09 PM   #7
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If air comes into exhaust before the upstream oxygen sensor that sensor detects the extra oxygen and it is read as as over lean so injector pulse is increased giving you a false over rich making engine stumble

Most noticeable at low rpm when decreased flow in exhaust so easier for outside air to get in and any that does is a greater portion of total flow
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Old 11-14-2015, 03:26 PM
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If air comes into exhaust before the upstream oxygen sensor that sensor detects the extra oxygen and it is read as as over lean so injector pulse is increased giving you a false over rich making engine stumble

Most noticeable at low rpm when decreased flow in exhaust so easier for outside air to get in and any that does is a greater portion of total flow
ah so that does sound like a good idea of where the issue could be from, since when accelerating its a little under 2k rpm when it gets past that it seems to be fine, ill post tomorrow if getting this other exhaust stops the issue (if that one isn't already rusted out too)
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Old 11-16-2015, 04:02 PM   #9
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how do you check for this? I'm ignorant on the o2 sensor system

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