Rough then smooth(ish) idle after startup

[NonHyphenated American]

I have a 2021 3.6 with automatic. On start ups whether the engine is cold or warm when the rpms settle to idle I get a rough idle. My GMRS antenna does a dance to it. After I start driving and come to a stop the idle is either totally smooth I cant even tell the engine is running unless I look down at the tach or there is a slight inconsistent not so smooth idle. Sometimes if I just sit in a parking lot like after the gym the idle kind of smooths out but not totally smooth. I have no check engine light or any codes as far as I know. I have just over 25k miles and my warrant period is almost up and I'm dropping it off at the dealer next week. I use 87 octane like the owners manual says. Most of my driving is in city. Does the computer just act stupid during initial idle because its not sure how to control the engine? I have a funny feeling the dealer will say nothing is wrong and if I bring it in after warranty magically something will be wrong. I swear I have the lifter tick but the last time I brought it in the dealer said since no codes were showing they wouldn't do anything about it.

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[DailyMoparGuy]

Identical to my 2021 with the 3.6L. I first noticed all the things you mentioned around 10k miles.

I’m at 57k miles now, same characteristics but no codes or performance issues. Not saying nothing is wrong, but it certainly hasn’t caused any issues after a couple years and 47k miles.

Edit: mine has a tick but idk if it’s THE tick. I haven’t had a professional listen yet. Again, no engine issues ever.

 

[NonHyphenated American]

Identical to my 2021 with the 3.6L. I first noticed all the things you mentioned around 10k miles.

I’m at 57k miles now, same characteristics but no codes or performance issues. Not saying nothing is wrong, but it certainly hasn’t caused any issues after a couple years and 47k miles

It just all of a sudden started in April of this year after I got back from a decent highway trip. Sometimes the idle is so smooth I have accidentally left the vehicle running while I went shopping because I thought I turned it off. The rough idle is not like a cam lope but my GMRS antenna does a little hula dance. On occasion I put the shifter in manual and run up the rpms to blow out any carbon build up. Haven't seen much of an improvement though.

 

[yoda13]

Mine does this too, and so did my last one. 50k on the last one before a tree killed it and 33K on this one. They did it from the very first for me.

 

[FloridaMan655321]

I have the same 'issue'. I have a Mojave with a manual with 37k miles on it. It started around 25k miles? I don't remember. It's been going on for some time and hasn't gotten worse. Equally no codes or warning lights and once I hit the next stop light it seems to idle perfectly.

 

[bgott]

If you have the ability to monitor scan data you can watch your misfire monitors when it's screwing up.

 

[PuddleJumper]

I have a 2021 3.6 with automatic. On start ups whether the engine is cold or warm when the rpms settle to idle I get a rough idle. My GMRS antenna does a dance to it. After I start driving and come to a stop the idle is either totally smooth I cant even tell the engine is running unless I look down at the tach or there is a slight inconsistent not so smooth idle. Sometimes if I just sit in a parking lot like after the gym the idle kind of smooths out but not totally smooth. I have no check engine light or any codes as far as I know. I have just over 25k miles and my warrant period is almost up and I'm dropping it off at the dealer next week. I use 87 octane like the owners manual says. Most of my driving is in city. Does the computer just act stupid during initial idle because its not sure how to control the engine? I have a funny feeling the dealer will say nothing is wrong and if I bring it in after warranty magically something will be wrong. I swear I have the lifter tick but the last time I brought it in the dealer said since no codes were showing they wouldn't do anything about it.

Hope this helps. This is just a load idle as far as i can tell. The engine is cold but devices immediately demand load like charging batteries, AC compressor, and so on. This is especially more noticeable if the vehicle has been sitting for a few days or more. One would think it would idle up a bit but instead it kinda just lugs through it therefore has a bit of wiggle so to speak. I find turning the AC off for the first minute or so and this issue doesn't seem as bad or goes away altogether. I also daily drive mine so it rarely ever gets too cold so to speak. garage kept and warm.

 

[NonHyphenated American]

Hope this helps. This is just a load idle as far as i can tell. The engine is cold but devices immediately demand load like charging batteries, AC compressor, and so on. This is especially more noticeable if the vehicle has been sitting for a few days or more. One would think it would idle up a bit but instead it kinda just lugs through it therefore has a bit of wiggle so to speak. I find turning the AC off for the first minute or so and this issue doesn't seem as bad or goes away altogether. I also daily drive mine so it rarely ever gets too cold so to speak. garage kept and warm.

When I say cold usually during the days I work it sits for 12 hours over night or for an hour and a half at the gym. I live in the panhandle of FL, and it's been ridiculous hot. The temp has cooled of a little and it doesn't seem as bad lately. Guess I'll find out Monday when I drop it off at the dealer in the morning.

 

[Solar Sparky]

I have this same problem. The dealer said it would likely result in engine failure and that I should drive it hard, but expect nothing to be done about it, until it fails, and I’m stuck on the road somewhere.

 

[ChrisNLA]

I'm not sure that I have ever worried much about a rough idle on start up if a vehicle has been sitting overnight, or all day, or longer. Cold engine under load takes a little time to get its bearings.

Doesn't hurt to let a dealer look it over I though, I reckon.

My 2015 Camaro SS idles pretty rough on a cold start up, but it routinely sits for a week or two at a time. After it has run a few minute it settles in nice and smooth. My 5.3L LS is the same.

Totally different engines though...

 

[WILDHOBO]

If you have the ability to monitor scan data you can watch your misfire monitors when it's screwing up.

It could very well be a misfire, but as I understand it, it won’t throw a code until there are a certain number of misfires in a certain amount of time.

 

[Joe Gladiator]

wow so not only do i have to worry about engine fires i got a high idle on start up some days so this could be a potential misfire.

 

[Solar Sparky]

I'm not sure that I have ever worried much about a rough idle on start up if a vehicle has been sitting overnight, or all day, or longer. Cold engine under load takes a little time to get its bearings.

Doesn't hurt to let a dealer look it over I though, I reckon.

My 2015 Camaro SS idles pretty rough on a cold start up, but it routinely sits for a week or two at a time. After it has run a few minute it settles in nice and smooth. My 5.3L LS is the same.

Totally different engines though...

Agreed, but, it is getting more regular, not just when cold. It was going in for other things, like the fu@#ing auto stop battery failure, and the leaking rear window seal. I’m now tempted to go look at the Mojave X now, and start at mile one again. Lots of gear I have to remove for a trade in I’m keeping the softopper and rack for sure. Probably take the CAI off and go back to stock. The winch can stay on it.

I’m trying to talk myself out of it now

 

[WILDHOBO]

wow so not only do i have to worry about engine fires i got a high idle on start up some days so this could be a potential misfire.

High idle at startup isn’t a misfire. That’s normal startup to circulate oil. I was referring to the rough running after the idle calms down to around 600-700rpms.

 

[ChrisNLA]

Agreed, but, it is getting more regular, not just when cold. It was going in for other things, like the fu@#ing auto stop battery failure, and the leaking rear window seal. I’m now tempted to go look at the Mojave X now, and start at mile one again. Lots of gear I have to remove for a trade in I’m keeping the softopper and rack for sure. Probably take the CAI off and go back to stock. The winch can stay on it.

I’m trying to talk myself out of it now

Hah, I feel ya!

If I ever bail out I'm looking at the new Colorado - won't have any less problems I'll just have more features while I have new problems

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