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Lunging/Bucking in 4lo

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Here is a video of what it does. If you listen close you can hear the driveshaft and gears clicking from the surging. When the rpms drop I let off the gas and it’s coasting.

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Here is a video of what it does. If you listen close you can hear the driveshaft and gears clicking from the surging. When the rpms drop I let off the gas and it’s coasting.

Yeah, that’s weird. I’ve experienced some bucking while turning sharp on firm pack, but thinks it’s from the extreme radius difference during the turn between the front and rear. It’s a lot more than a Wrangler.
 
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Yeah, that’s weird. I’ve experienced some bucking while turning sharp on firm pack, but thinks it’s from the extreme radius difference during the turn between the front and rear. It’s a lot more than a Wrangler.
Ya I don’t think it’s normal. I can see if I was doing it because I was on the throttle and it is touchy at that high of gear ratio but for the Jeep to do it on its own doesn’t seem right. Guess I’ll see what the star tech says. I sent the video to my Jeep care specialist.
 

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If you were driving a MT and it bucked, you would shift up. Sounds like you are just trying to do too much. Don’t rev it to 2k. Just let it go with foot off the throttle.
 
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If you were driving a MT and it bucked, you would shift up. Sounds like you are just trying to do too much. Don’t rev it to 2k. Just let it go with foot off the throttle.
It’s not a manual. I had the Jeep in drive and accelerated in 2nd gear to 1500-2000 rpms and lef off the gas completely. It was coasting and surging on its own.
 

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It’s not a manual. I had the Jeep in drive and accelerated in 2nd gear to 1500-2000 rpms and lef off the gas completely. It was coasting and surging on its own.
I got that - I was using the MT as a comparison. I think you need to be in a higher gear or not accelerating - just put it in gear and let it creep along at idle.
 
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I got that - I was using the MT as a comparison. I think you need to be in a higher gear or not accelerating - just put it in gear and let it creep along at idle.
I can do that but, lets be honest here. Thats not the way it should be and the way i should have to drive it to keep it from lunging/surging.
 
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Well here's an update: After multiple phone calls and discussions with the dealer and @JeepCares the issue was "deemed normal operation". I asked for an explanation on how they deem this normal operation but never received anything. All that @JeepCares wanted to do was offer me an 8 year 125k mile warranty that I left in their hands to give me or not. Thinking about it more and more, it is my opinion that the differential, transfer case, transmission and engine are not able to keep up with one another and cause this surging/lunging/bucking issue. You can hear the driveshaft clunking as the differential gears catch with the pinion. To me this means the ring gear spins faster than the pinion gear and once the jeep slows down enough for the pinion gear and ring gear to catch it clunks and surges the jeep forward due to the high torque output of the low range transfer case and 8 speed transmission in 2nd gear. As you get into higher gears the transfer case and transmission can keep up and there is no surging and driveshaft clunk. I explained all this to @JeepCares and there was nothing they could do. After discussions with them and leaving it in their hands to give me the warranty or not out of good faith, they retracted their offer and it was not available. I felt a warranty didn't solve the issue. They only offered the warranty to "gain my confidence back" in the Jeep I purchased. With nothing being fixed how was this going to gain my confidence back. I didn't care about the warranty, i wanted my Jeep fixed. I posted the video on here to get your guys opinions and it seems that you all are on the same page as i am with this not being normal.

From a previous post, i did get the rear end pop fixed. The control arms were bad.
 

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I would not be surprised to learn that the 4:1 case is the root cause as some JL Rubicon owner's experience the same issue. Not that the box itself is bad but as you theorize, there is a bad spot in the drivetrain as a system due to the gearing of the final drive and the engine tuning.

I don't know if max tow does, if they don't there's possibly some different off road tuning on the Rubicon.

Have you tried to see if there's any difference with offroad + on?
 
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I would not be surprised to learn that the 4:1 case is the root cause as some JL Rubicon owner's experience the same issue. Not that the box itself is bad but as you theorize, there is a bad spot in the drivetrain as a system due to the gearing of the final drive and the engine tuning.

I don't know if max tow does, if they don't there's possibly some different off road tuning on the Rubicon.

Have you tried to see if there's any difference with offroad + on?
No i didn't try the offroad+. I'll most likely try it and see what it does or if it helps but I explained my theory to them and there was no further discussions about it other than it was "deemed normal" and thats all they could tell me. It was wierd because everytime the @JeepCares rep called me the first thing she would say was, "this conversation is being recorded". So makes me wonder if there is multiple complaints about it that they can't figure out or are working on or something. If i ever offroad and need to go into 4 low, i'll have to put the jeep in manual mode and start out in 3rd gear. It doesn't do it in that gear and up. I'll have to also try the offroad+ function also.
 

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Any updates to this?? I just recently experienced something like this while in 4lo but I was coasting and engine braking while descending a hill in 'manual' mode (1st/ 2nd gear). The jeep would sort of initially engine brake while coasting but then would surge fwd and increase RPM to around 3000, and then apply some sort of engine braking. Cycle would repeat..

I found this JL thread..
https://www.jlwranglerforums.com/forum/threads/2018-jlu-sudden-acceleration-downhill-in-4l.37338/
 
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Any updates to this?? I just recently experienced something like this while in 4lo but I was coasting and engine braking while descending a hill in 'manual' mode (1st/ 2nd gear). The jeep would sort of initially engine brake while coasting but then would surge fwd and increase RPM to around 3000, and then apply some sort of engine braking. Cycle would repeat..

I found this JL thread..
https://www.jlwranglerforums.com/forum/threads/2018-jlu-sudden-acceleration-downhill-in-4l.37338/
no I had no change. Still does it and I’m just living with it. I have just assumed it’s because of the rubicon low range transfer case. If I can keep it in higher gears and not 1st gear it doesn’t do it. I think it’a gearing is so low that it surges aheadthen catches up and surges again. There’s a lot of torque multiplication when you’re in first gear and 4 lo. Mine I noticed more going up a slight hill. I never tried it going downhill.
 

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no I had no change. Still does it and I’m just living with it. I have just assumed it’s because of the rubicon low range transfer case. If I can keep it in higher gears and not 1st gear it doesn’t do it. I think it’a gearing is so low that it surges aheadthen catches up and surges again. There’s a lot of torque multiplication when you’re in first gear and 4 lo. Mine I noticed more going up a slight hill. I never tried it going downhill.
Oh that sucks that it does it going up hill. Mine is controlled while on flat ground or up hill (no problem). I noticed this yesterday when I was in Moab on a long hill descent. I didn't want to ride the brakes so I put it in 4lo and manual mode. On my old rig, I could just coast in a controlled manner. If it was going above 3000 rpm, I would just shift into a taller gear..

This truck would coast until 2000 rpm, then automatically increases speed/ rpm to around 3000..it would then apply some sort of engine brake and rpm would drop. Cycle would repeat. It definitely seems like it's computer related..

I don't remember it doing it earlier this year..
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