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Old 09-05-2013, 08:37 PM   #1
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Loud Resonating Idle, Stumbles & Stalls, NO CEL

I just bought a 2004 Wrangler Unlimited w/ 70k and was doing some tune-up work on it and noticed what seems like a slight idle issue and a loud resonating bass vibrations in the cab. I have flushed the radiator, changed the front and rear diff oil, tcase fluid, brake fluid & replaced the sparks plugs.

I've never had a wrangler before so I'm not sure what the engine should feel like or what is considered normal but there is an occasional (every few seconds) slight jiggle when the engine is idling and the rpms fluctuate slightly. When you rev the engine a bit it runs very smooth. It also runs smooth while driving.

The other issue:
When idling after starting the engine everything sounds fine. After revving up to 1.5-2kish when the rpms drop back down to idle speed, there is a loud resonating bass vibrations inside the cabin. I'm thinking this is an exhaust issue but I don't understand why it goes away at higher rpms and why it does not vibrate after initial start up.

I have already removed and cleaned the IAC and that did not help. If I tease the gas pedal long enough I can get the engine to stall.... well I've only got it to stall once. Not sure if this is normal as I am intentionally trying to stall it. There is no check engine light on and everything works fine while driving as far as I can tell.

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Old 09-05-2013, 08:41 PM   #2
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Here is a video of the tach while the engine is running. It is hard to hear but you can kind of see the image start to vibrate after I rev the engine and it returns to idle speed.


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Old 09-06-2013, 07:03 AM   #3
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Any advice?
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Old 09-06-2013, 07:20 AM   #4
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How does the exhaust smell? Smell rich or clean during a stumble? I did run across a problem like this once and it was due to a poor contact with one of the plugs. When installing new plugs I always use electrical conduit grease. Also check the distributor rail, make sure the plug that connects it is clean with conduit grease as well, along with all other connections. Last, did you use the correct plugs? You should be running an APP985 or equivalent. (Not the single P version). Fresh gas in tank? No condensation?
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Old 09-06-2013, 07:35 AM   #5
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It sounds like the exhaust might be doing exactly what you said: resonating. It may just happen to hit the resonant frequency of your cab at those lower RPMs. Unless you hear rattling or it seriously annoys you, I wouldn't worry about it too much. A fix may be a different muffler or cat back system.
As far as the rough idle and stall, it seems like yours isn't as bad as mine was, but I had an O2 sensor go out on me, and the engine would stall when I revved it, and it would fluctuate a lot at stops. The exhaust had actually gotten hot enough to melt through my O2 sensor wires, you may have the beginnings of this happening. I would check along your entire exhaust and see if any wires are touching it and possibly melting.
Hope that helps!
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Old 09-09-2013, 03:51 PM   #6
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Quote:
Originally Posted by NJO View Post
How does the exhaust smell? Smell rich or clean during a stumble? I did run across a problem like this once and it was due to a poor contact with one of the plugs. When installing new plugs I always use electrical conduit grease. Also check the distributor rail, make sure the plug that connects it is clean with conduit grease as well, along with all other connections. Last, did you use the correct plugs? You should be running an APP985 or equivalent. (Not the single P version). Fresh gas in tank? No condensation?
The exhaust smells normal to me. I pulled the plugs just to double check and they all look like they are firing fine. I did use the grease on the plugs and in the boots. I'm using APP985 and I took a 500+ mile road trip after the new plugs so its had plenty of fresh gas through it.
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Old 09-09-2013, 03:54 PM   #7
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Quote:
Originally Posted by TurkTTR View Post
It sounds like the exhaust might be doing exactly what you said: resonating. It may just happen to hit the resonant frequency of your cab at those lower RPMs. Unless you hear rattling or it seriously annoys you, I wouldn't worry about it too much. A fix may be a different muffler or cat back system.
As far as the rough idle and stall, it seems like yours isn't as bad as mine was, but I had an O2 sensor go out on me, and the engine would stall when I revved it, and it would fluctuate a lot at stops. The exhaust had actually gotten hot enough to melt through my O2 sensor wires, you may have the beginnings of this happening. I would check along your entire exhaust and see if any wires are touching it and possibly melting.
Hope that helps!
I don't hear any rattle but I find it annoying that the side mirrors shake while at idle but when I hit the gas it runs nice and smooth. I don't think this is how it should be and that's why it is aggravating me. Would I see a CEL if an o2 sensor went bad?

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