Steeper descents dangerous behavior

XT Carol

XT Carol

New member
I've been a member here for a while and enjoy learning from others' experience. I'm a senior, but have Jeeped all my life, been in an active Jeep club when I was younger, etc. I got my 2018 Jeep JLU Rubicon, 3.6V6, 8-speed auto transmissiion, as my retirement dream vehicle. I RV and tow my Jeep behind a large motorhome. Not to waste your time, but this gives background.

I've been exploring an area of Utah lately that has 11,000 ft.+ mountains, miles of nice trails, and mining history to see and photograph. My Jeep has proven to be perfect for these conditions, but with one glaring, and dangerous, exception:

I'm used to gearing down appropriately, whether in high or low range, to let the engine hold back the speed without braking. The safe way to descend and pretty standard. But my Jeep reaches a certain point, and it's like there is an invisible foot on the throttle. It speeds up, from 1,000 rpm to over 3,200 rpm (no input from me), and the only way to slow it back down to a safe speed is use the brakes. And it requires quite a bit of brake force, because I am working against whatever is making the engine throttle up.

I've noticed it from when I first had the Jeep (March, 2020), but in the terrain I'm in lately, it has become more than just annoying. It's dangerous, hard on the Jeep, and nerve-wracking as a driver. I even stop every few minutes to let the brakes cool off, because on these trails brake fade could be fatal. I'll add that it happens in 2HI, 4HI, and 4LO. I'd say 4LO is the most exciting. When I'm easing down a rocky trail in 3rd/LO and the engine throttles up to over 3,200 rpm as I try to slow it down.

Before this Jeep I had a 2005 Liberty diesel w/5-speed auto transmission that I did lots of 4-wheeling in as well. It could about descend anything without needing brakes, wonderful control. Better even than earlier CJ Jeeps I've owned. But this Rubicon has something bad going on with it that I don't understand. I think the factory has to have programmed it to do this, why I would have no idea as it's so dangerous.

I do have a Tazer Mini in the Jeep, to have more control over annoying or useful features. But I've completely removed the Tazer, and the problem is still the same. I didn't think it would be the Tazer, but wanted to be sure.

I've searched around on different forums, asked at dealerships, without finding anyone saying they have a similar problem, but I can't be the only one. It has to be the way the main engine computer is programmed, and if so there should be a recall about it. I'm fed up enough with it I'd even pay someone to reprogram the computer, but I don't know if that can even be done. It's also something that's impossible to demonstrate at a dealership.

There's got to be some solution! The Rubicon is a wonderful vehicle overall that I love driving, except for this. Has anyone out there ever experienced this?

Carol -- 2018 Jeep JLU Rubicon, 3.6V6, 8-speed auto transmission, 2-inch Mopar suspension lift (otherwise completely stock)
 
No, I haven't. Mainly because I read that mode is only available in 4LO/1st gear. So I'd be crawling. I think top speed in Hill Descent mode is up to 4mph? That's just a guess since 1st is so low. The downgrades I'm running are [guessing] 10-15%, rocky, washboarded, with switchback curves. 3rd gear in LO range is nice, with occasional better stretches in 4th. 1st gear HI range is too fast. My old Liberty was great on this stuff. But without this glaring flaw, the Rubi should be perfect on these trails. The gear ratios are great and I love the 8-speed.
 
You sure its throttling up? Not just engaging the gear? 3k rpms sounds about right for 4 low and 3rd gear. Jeeps weigh a lot, gravity is pretty strong, drivelines are designed now to offer extremely little resistance for greatest mpg especially an auto, comparing a gas to a diesel engine for engine braking isnt a fair comparison. It sounds to me like its just getting pulled down. However your brakes really shouldn't need to work that hard that they need to be cooled after every couple of minutes, unless you're just doing that for peace of mind and they aren't that hot.
 
The reason I know it's throttling up the engine is that this Jeep has the new digital gauges that give so much information. I put it on the display that shows instantaneous mpg. I find if I drive by that I get better mileage, but that's on the pavement. So as I begin descending the 'mpg' gauge always shows "99mpg", which is a fully closed throttle. I see that commonly on descents on the highway, but that's where it can coast like that without a problem. So I'm descending and watching this gauge. I find once rpm reaches about 1,800-2,000 (still at 99mpg), you can actually feel the vehicle accelerate without input from me. Along with that feeling, the gauge begins displaying anywhere from 12-20mpg. That shows to me the engine is throttling up. It will run along up to about 3,200 rpm, still showing low mpg, until I use the brakes forcibly enough to overcome the engine's force and slow everything down. At 3,200 rpm it becomes a dangerous speed on the hills I'm on. Once I brake it down to about 1,000 rpm the process begins all over again. When the gauge reads 99 mpg, you can feel the strong holdback the engine is providing. Just like a normal vehicle with a manual transmission. In reality a diesel engine provides less holdback than a gas engine on descents, but using 4LO in the Liberty I could almost idle down even steep hills. This 3.6 engine feels like it could easily hold back the weight of the Jeep, but not with the computer telling it to accelerate. I can only do the above for maybe 8-10 minutes without feeling I should let the brakes cool off. If the Jeep did NOT do this, the gears and engine feel they could easily slow it on steep grades. Especially since I'm in LO range doing this, and the Rubi has some really low gears. Even 3rd LO range is slow. I think it's just got to be a programming issue. But I'm surprised I don't see more others saying they've experienced it. I have only seen 2018 owners say they've seen it. Makes me curious if they fixed it in 2019 Jeeps.
 
As a comparison, my Liberty weighed around 5K as this JLU does. It's engine was a 2.8L 4-cylinder diesel. So quite a bit less engine for braking.
 
I will try and test the HDC (Hill Descent Control) tomorrow. I originally saw a YT video stating HDC could only be used in 1st gear LO range. But a snippet from a Jeep manual I just read online tonight says not so, that it will take over shifting of the transmission and braking to hold the vehicle at the selected speed. Perhaps this is what Jeep engineered into these vehicles for steep downhills, instead of just letting the driver decide the appropriate gear for the hill. I'll post back tomorrow on how HDC works on these old mining roads.
 
XT Carol said:
I will try and test the HDC (Hill Descent Control) tomorrow. I originally saw a YT video stating HDC could only be used in 1st gear LO range. But a snippet from a Jeep manual I just read online tonight says not so, that it will take over shifting of the transmission and braking to hold the vehicle at the selected speed. Perhaps this is what Jeep engineered into these vehicles for steep downhills, instead of just letting the driver decide the appropriate gear for the hill. I'll post back tomorrow on how HDC works on these old mining roads.
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Well you will just have to stop more often and cool the brakes. :rolleyes: Your speedo technique is a little strange I'd have to say.
 
XT Carol said:
Perhaps this is what Jeep engineered into these vehicles for steep downhills, instead of just letting the driver decide the appropriate gear for the hill. I'll post back tomorrow on how HDC works on these old mining roads.
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Hmmm...sounds like a asshole pucker moment for sure. Trying to understand exactly what you are saying and experiencing.

I have a 2018 JLUR and have not experienced what I think you are saying? If descending a grade which is steep, IMO you should always be in 4LO. More control if anything.
Are you positively stating that in 4LO descending you cannot manually shift into 1st?
I have been on steep enough grade where in 4LO and in 1st gear, weight of vehicle still wants to pull vehicle down and RPM goes up. In that event, where trail is that steep and sketchy, I have used the Descent button. But I have only needed to do this once. I do not recall ever experiencing the Jeep taking over in this situation.
 
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