Low Speed Clunking

[PaulRubi]

I’ve been noticing a lot of clunking, and lurching forward during slow speed driving. especially lightly on/off throttle in stop and go traffic, or slowing down to a red light then pressing the gas to accelerate again instead of coming to a complete stop. I just did a road trip and the truck feels great at highway speeds but during the stop and go portions it was unnerving.
I’m on 37s with 4:88 gears, tazered, 20,000 miles. Anyone else notice this?

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[Roctoy]

Automatic or manual transmission?

 

[PaulRubi]

Auto

 

[CJXJJT]

I have also had some issues with my manual. In 1st gear 2wd, and 1st 2nd and third in 4wd low. The engine will rev 400-500rpms higher than where you were when you barely touch the throttle.
You touch it, remove your foot and it continues to rev an additional 500rpms. WTF??

Second scenario,
Also when it's cold if I dont let it run for over 2min From cold. I can start off just touch the brakes ind it will rev up to 2000rpms?? Foot off the throttle.
Any thoughts?

 

[JET_83]

Dang, not even @ShadowsPapa has an answer?

 

[NachoRuby]

I have also had some issues with my manual. In 1st gear 2wd, and 1st 2nd and third in 4wd low. The engine will rev 400-500rpms higher than where you were when you barely touch the throttle.
You touch it, remove your foot and it continues to rev an additional 500rpms. WTF??

Second scenario,
Also when it's cold if I dont let it run for over 2min From cold. I can start off just touch the brakes ind it will rev up to 2000rpms?? Foot off the throttle.
Any thoughts?

Both of those issues are emissions controls. The first one is rev hang, and it's to cut down on NOx emissions from unburned fuel.

The second is kind of like the choke of old days. The engine runs faster on a cold start to warm itself up quickly, and get up to efficient operating temperature, where the cat can operate better. Wouldn't be surprised if these had secondary air injection too, but I've never checked.

 

[PaulRubi]

Both of those issues are emissions controls. The first one is rev hang, and it's to cut down on NOx emissions from unburned fuel.

The second is kind of like the choke of old days. The engine runs faster on a cold start to warm itself up quickly, and get up to efficient operating temperature, where the cat can operate better. Wouldn't be surprised if these had secondary air injection too, but I've never checked.

very interesting. thank you for the info, gonna check out what could be causing and bring it in to the dealer

 

[NachoRuby]

very interesting. thank you for the info, gonna check out what could be causing and bring it in to the dealer

My response was specifically for the issues cxjjt was having at startup and at gear changes. Yours sounds like something else.

 

[CJXJJT]

I found this "rev hang" a pain in the ass as I had to constantly counter with the brakes. It would rev from say 1500 up to 2000rpms. When you're in 4low 2nd or 3rd gear that's substantially faster than what you're asking for.
The choke is exactly correct. It does however really suprise you when you lightly touch the brakes and it takes off on ya'

 

[CJXJJT]

Both of these issues seem like they could be a huge liability for Jeep in a bad situation... ?

 

[ShadowsPapa]

Both of these issues seem like they could be a huge liability for Jeep in a bad situation... ?

No. It's NOT a Jeep thing. Go check the Mustang forums among others.
People call it "rev hang" but honestly, it's been around since the 90s and no one ever bothered complaining about it back then so it's rather amusing to ME anyway to see people just mentioning it as if it's new.
I suppose it's more apparent with a stick.
The 4.0 in my little car - downshift through the gears and then a the end I press the clutch and brakes and stop and as I'm rolling to the stop with the clutch in I see the RPM hang around 1200 or so and when I come to a 100% stop, it drops back down to about 800 RPM.

Referring to what Chad talked about -

Both of those issues are emissions controls. The first one is rev hang, and it's to cut down on NOx emissions from unburned fuel.

The second is kind of like the choke of old days. The engine runs faster on a cold start to warm itself up quickly, and get up to efficient operating temperature, where the cat can operate better. Wouldn't be surprised if these had secondary air injection too, but I've never checked.

Where has everyone been? Seriously - faster idle for a while until the thing warms and goes into closed loop mode, and the thing people are calling "rev hang" - it's not new, it's not Jeep and it's been around for decades. Like I said - the 4.0 in my car does it - it's normal and there's no risk to it! Your brakes can stop you even with the engine on high idle.
How do people thing we got along with carburetors, chokes and automatic transmissions in the 60s and 70s?

I’ve been noticing a lot of clunking, and lurching forward during slow speed driving. especially lightly on/off throttle in stop and go traffic, or slowing down to a red light then pressing the gas to accelerate again instead of coming to a complete stop. I just did a road trip and the truck feels great at highway speeds but during the stop and go portions it was unnerving.
I’m on 37s with 4:88 gears, tazered, 20,000 miles. Anyone else notice this?

In this case, all bets are off because normal troubleshooting doesn't apply because it's been modified.
This could be improper gear setup in the differentials - too much backlash, something loose or improperly set, who knows.
Play in the drive train like lash, loose pinion, loose u-joints are usually more noticeable with a stick.

 

[PaulRubi]

No. It's NOT a Jeep thing. Go check the Mustang forums among others.
People call it "rev hang" but honestly, it's been around since the 90s and no one ever bothered complaining about it back then so it's rather amusing to ME anyway to see people just mentioning it as if it's new.
I suppose it's more apparent with a stick.
The 4.0 in my little car - downshift through the gears and then a the end I press the clutch and brakes and stop and as I'm rolling to the stop with the clutch in I see the RPM hang around 1200 or so and when I come to a 100% stop, it drops back down to about 800 RPM.

Referring to what Chad talked about -


Where has everyone been? Seriously - faster idle for a while until the thing warms and goes into closed loop mode, and the thing people are calling "rev hang" - it's not new, it's not Jeep and it's been around for decades. Like I said - the 4.0 in my car does it - it's normal and there's no risk to it! Your brakes can stop you even with the engine on high idle.
How do people thing we got along with carburetors, chokes and automatic transmissions in the 60s and 70s?



In this case, all bets are off because normal troubleshooting doesn't apply because it's been modified.
This could be improper gear setup in the differentials - too much backlash, something loose or improperly set, who knows.
Play in the drive train like lash, loose pinion, loose u-joints are usually more noticeable with a stick.

10-4. does it make sense that it would pop up 7k or so miles after the regear?

 

[CJXJJT]

No. It's NOT a Jeep thing. Go check the Mustang forums among others.
People call it "rev hang" but honestly, it's been around since the 90s and no one ever bothered complaining about it back then so it's rather amusing to ME anyway to see people just mentioning it as if it's new.
I suppose it's more apparent with a stick.
The 4.0 in my little car - downshift through the gears and then a the end I press the clutch and brakes and stop and as I'm rolling to the stop with the clutch in I see the RPM hang around 1200 or so and when I come to a 100% stop, it drops back down to about 800 RPM.

Referring to what Chad talked about -


Where has everyone been? Seriously - faster idle for a while until the thing warms and goes into closed loop mode, and the thing people are calling "rev hang" - it's not new, it's not Jeep and it's been around for decades. Like I said - the 4.0 in my car does it - it's normal and there's no risk to it! Your brakes can stop you even with the engine on high idle.
How do people thing we got along with carburetors, chokes and automatic transmissions in the 60s and 70s?



In this case, all bets are off because normal troubleshooting doesn't apply because it's been modified.
This could be improper gear setup in the differentials - too much backlash, something loose or improperly set, who knows.
Play in the drive train like lash, loose pinion, loose u-joints are usually more noticeable with a stick.

Regarding the rev hang. I don't think it's really rev hang I'm experiencing. It actually accelerates on it's own.
I completely understand how a choke works, this is a separate issue.
As far as "where have you been," "this problem's been around for decades." Well my most recent vehicle purchases before the 21 JT are a 2012 Toyota and a 2004 Ford. All of my other Jeeps are pre 93. I guess I live in a cave as I don't purchase a new car every other year. Regardless if there's been a turd in the punchbowl for decades, I still want it out of there...

 

[JW Jeep]

Wow !! Just took a trip to Yellowstone and I felt the same thing I think ? When Barely moving just a creeping like your in parade mode. you let off gas then barely touch it. It bangs like back lash ? I have a 2022 overland with 6000 miles completely stock . this really feels like crap and very anoying to say the least.

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