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3.6 knocking under load/low rpm

OddyNuff

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Does anyone else get this? I’ve found a couple threads on it with seemingly no known solution so far.

I’m at 52k miles and power train will be up soon, so I’m debating taking it in. Happens a lot more on anything less than 93 and happened after a road trip, maybe bad gas? But it’s never gone away 100%, but mitigates a lot with 93 fuel.
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benny_rt2

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Hi,

You can check/replace the spark plugs, and clean the mass airflow sensor as a couple of DIY weekend job.

- casual forum lurker
 

mwilk012

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Hi,

You can check/replace the spark plugs, and clean the mass airflow sensor as a couple of DIY weekend job.

- casual forum lurker
There is no mass air flow sensor.
 

Marco#1

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I bought a new 2018 JLU Moab in May of 2019, and as soon as the weather got cold I noticed what sounds something like valve train rattle (but not as fast) or engine detonation (but not as loud). I'm not convinced it it actually either of those things, and I think I've read discussions here about it being transmission noise, but I'm not so sure about that either. I've spent a bunch of time researching what it could be and none of the theories seem very plausible to me. Although some info I've read recently has me leaning toward the predetonation theory.

The noise is definitely more pronounced in cold weather with lower octane fuel. I can duplicate it pretty reliably with under 50% under 10% throttle when RPMs are between 2k and 3k 1800 to 2k. Performance isn't affected at all, and the sound hasn't seemed to get any worse or better, although I can hear it occasionally in the summer now and I don't remember hearing it (noticing it?) the first summer I owned the vehicle. I really don't have much else to offer, but I'd really love to see some progress on this issue and happy to collaborate with anyone willing to discuss some hypotheses and run some tests.


[Edited 11/29 (bold & strikethrough) for more detail / accuracy.]
 
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vanzano

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This is normal, live with it or sell the Jeep. There's a number of threads about this topic all over this forum.
 

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Marco#1

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This is normal, live with it or sell the Jeep. There's a number of threads about this topic all over this forum.
It really doesn’t even bother me anymore, but I am certainly interested to know what it is.


DOWNSHIFT! It's a high compression little v6 pushing around 4-6k lbs. It shouldn't be under load at low rpms.
It typically happens when pulling from a full stop in first gear, so not really an option anyway. Otherwise I’m in second gear and not accelerating hard enough to require a downshift.
 

Marco#1

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It's been a while since I thought much about this, and I wanted to better describe what "not accelerating hard enough to require a downshift" meant (from my last post). I paid close attention to the Jeep when I was driving last night and made some edits to my first post (on Nov 20), which are in bold print.

tl:dr
Some info I've read recently has me leaning toward the predetonation theory.
I can reliably duplicate it with < 10% throttle between 1800 and 2k RPMs.

(Might be even less throttle. It's about as little throttle as you can use without your passenger looking at you to ask if everything is okay, haha.)
 

Zandcwhite

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It's been a while since I thought much about this, and I wanted to better describe what "not accelerating hard enough to require a downshift" meant (from my last post). I paid close attention to the Jeep when I was driving last night and made some edits to my first post (on Nov 20), which are in bold print.

tl:dr
Some info I've read recently has me leaning toward the predetonation theory.
I can reliably duplicate it with < 10% throttle between 1800 and 2k RPMs.

(Might be even less throttle. It's about as little throttle as you can use without your passenger looking at you to ask if everything is okay, haha.)
You're cruising along at 1800rpm and less than 10% throttle, you are by definition lugging the engine. Downshift, give it some more throttle, or just accept the fact that you are causing the issue and keep doing what you're doing. It's fuel injection not magic, it cannot force the engine to run smooth in spite of your efforts to lug it down?
 

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Marco#1

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You're cruising along at 1800rpm and less than 10% throttle, you are by definition lugging the engine. Downshift, give it some more throttle, or just accept the fact that you are causing the issue and keep doing what you're doing. It's fuel injection not magic, it cannot force the engine to run smooth in spite of your efforts to lug it down?
I’m not sure if you’re trolling me or you just don’t understand what I’m saying. I’m not cruising, I’m gently accelerating from a stop. At some point, the engine speed is going to pass through the 1800 to 2000 rpm range, and that’s when I hear the sound. But I only hear it when I’m GENTLY accelerating. If I pressed the gas pedal any less, I would not be accelerating. If I downshifted the Jeep would slow down. Nevermind the fact that my Jeep has an automatic transmission, half the time this occurs when I’m pulling from a stop and I’m already in first gear, the there is nowhere to downshift to. I’m not lugging the engine.

Next time you get in your Jeep, slowly accelerate from a stop. Drive conservatively, like your passenger is holding a cup of hot coffee and on this particular day you don’t want to be an asshole. Right before the needle on the tachometer gets to the 2, that’s when I hear the sound. And I only hear it for one second (or less) because I’m PASSING THROUGH that specific and very narrow rpm range as I’m ACCELERATING from a stop.
 

TheRaven

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I’m not sure if you’re trolling me or you just don’t understand what I’m saying. I’m not cruising, I’m gently accelerating from a stop. At some point, the engine speed is going to pass through the 1800 to 2000 rpm range, and that’s when I hear the sound. But I only hear it when I’m GENTLY accelerating. If I pressed the gas pedal any less, I would not be accelerating. If I downshifted the Jeep would slow down. Nevermind the fact that my Jeep has an automatic transmission, half the time this occurs when I’m pulling from a stop and I’m already in first gear, the there is nowhere to downshift to. I’m not lugging the engine.

Next time you get in your Jeep, slowly accelerate from a stop. Drive conservatively, like your passenger is holding a cup of hot coffee and on this particular day you don’t want to be an asshole. Right before the needle on the tachometer gets to the 2, that’s when I hear the sound. And I only hear it for one second (or less) because I’m PASSING THROUGH that specific and very narrow rpm range as I’m ACCELERATING from a stop.
So I can't say for sure what you are hearing, but if it is detonation, you don't need to worry about it as long as it doesn't stick around or cause loss of power. As has been discussed at length on this forum in our many epic engine debate threads, detonation happens on all modern computer-controlled engines. That's precisely how engine control systems are designed - they push timing to the brink of detonation, and as soon as detonation is detected, they pull back the timing. This is done continuously...the thing is, USUALLY you won't hear it, because the computer catches it before your ears can. But occasionally you will. As long as it is quickly quelled by the engine control system, there's nothing to worry about. If it is prolonged or very loud, or causes loss of power, then you have an issue. Though i've never seen a case with a modern computer-controlled engine where serious knock occurs and you don't get an SES light as a result. No matter how good your ear is, the computer's is better. It knows before you know, every time.

There are so many threads on this site about detonation...but we haven't done a good job of separating the "normal detonation" from "problem detonation". That's very important.
 

Marco#1

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Thank you @TheRaven, this is the best thing I’ve read about the issue in four years, lol. Like I said, I stopped worrying about it a long time ago, but still have been curious.
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