Rough idle on cold start, not as bad after warmed up

[NonHyphenated American]

Ive posted before about a rough idle on cold start and after sitting for alittle and then in smooths out after driving. Right now Im at 35,700 some miles on a 2021 Sport S with 3.6 gas and auto trans. I have replaced the pcv, no change. Cleaned the throttle body and mass airflow sensor, no change.

Today I replaced the EGR valve and EGR temp sensor, not as bad but still there. I disconnected the battery for just alittle over half an hour, not as rough idle. However, after I left the gym and sat in the parking lot the idle was kind of smoothish.

Right before my factory warranty ran out I took it to the dealer and they said nothing was wrong especially with no check engine light and no codes showing.

Only things left would be plugs and/or coil packs and maybe injectors. Its weird it only does it worse after cold start up and smooth out after driving. If it were a plug, coil pack, or injector shouldnt it idle rough all the time and run like crap? My last trip I got 21mpg at some points going 75. I get decent fuel mileage for the mostly city driving and I use injector cleaner now and then and manualy shift the auto to high rpms to blow out the crud. All hoses appear well connected. Im at wits end.

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[R CA JT]

Not knowing this engine very good, I wont be much help but I have the same issue in my 2020 3.6 sitting around 71K.
First thing in the morning and then after sitting for the day, 4-5 hours or more, it just seems to stumble a little for 20 or 30 seconds before smoothing out.
I have just written it off as something is dirty and it takes a moment for the air and fuel to balance to the right mixture or something along those lines.
Otherwise it is running great and quiet with more pep than I expect from it, especially with large tires and the weight of the rack and tent etc sitting on it.
I plan to do a similar list of cleaning and replacing like you did and hope it gets better. It usually smooths out before I take off but it does give me that moment of concern I'd rather not get.
I used to get a horrible clunking and jolt from the trans because I had a habit of pushing the start button and immediately popping it into drive with a single motion. Luckily, I've broke that habit and found some patience and no more clunk.

 

[Jteakus]

With no codes it could be a leaking injector.

 

[Jteakus]

You need a scanner that reads data or it's all a guess

 

[R CA JT]

Ya, no lights on dash
I do have a scanner that provides live data, I just need to know what the numbers should be before I can tell which ones are off. To be honest it is so slight I haven't put that much effort into it. Figuring a tune up and cleaning would fix it. If not, then I'd start getting technical. Life has just been crazy busy.

 

[bgott]

Do you have to add coolant? You might have a seeping head gasket. Pull the plugs and run a borescope down the holes and look at the tops of the pistons. You can get a borescope from Amazon these days for cheap.

 

[NC_Overland]

My 2020 has done that since new and under warranty and they always said it’s “normal” since no codes. It runs smooth as soon as you put it in gear or it warms up for a few minutes. Dealership was useless. I just kinda got used to it.

 

[drrags]

Here are some things to check, in no particular order:

You didn't mention what gasoline you use. My goto is Shell and Chevron. Every third tank or so I'll fill up with premium to get their special additives. My engine runs smooth enough that I have to look at the tach at a stop to make sure it didn't shut off. I make trips to Texas and back and fill up with whatever is available and I can tell the difference for sure.

If you have JScan or whatever, you can see if you have a P030x code. The x being the cylinder that's not up to speed. It's not specific enough to tell you if it's a plug, injector, sticking valve, cam shaft, or head gasket leak, but it will get you started. At your mileage, plugs and injectors are the least likely.

Do you hear any ticking? Put a stethoscope on the cam phasers. They can wear for various reasons.

You can go into Live Data in JScan and read different attributes of all 6 cylinders at the same time. If 3 cylinders on a bank show different reading from the other bank, for example, go into the oxygen sensor readings. There are two per side; an upstream and downstream. Oxygen sensors are relevant because they are heated when cold and change as temps increase. Maybe you have a weak heating element. Everything works when all four are evenly hot.

Anyway, Live Data on JScan is very informational as you can read so many parameters. You don't need to know what a value is supposed to be, just compare that all 6 are evenly matched.

Good luck.

 

[R CA JT]

Thanks rags, that's good info. I'll have to hook it up and see what my scanner shows, I didn't think it does quite that in depths but it does o2 sensor values, pressures and %'s and etc.. I'm not a tech just have to do my own work mostly because I can't afford to pay someone, I learn what I need for a specific task then seem to lose all that knowledge the moment I've fixed it.. I turn 50 in a couple weeks so I can start blaming my age for it...haha

NC_, that's where I am at with the issue. Unless it gets worse, it must be normal.

Oh and 99% of the time I'm running low octane Costco gas...I know, but I throw an injection cleaner in the tank once every.....not often enough.

 

[R CA JT]

I bought a scope from temu just for the hell of it like a year ago.
The thing is awesome, it has a light, voom, flip image. It's great and only like $25 or something ridiculous like that.

 

[drrags]

I'm glad to hear that. Costco gas is top-tier, up there with Chevron, Exxon, and Shell. Sam's Club, on the other hand is bottom-tier.

 

[ShadowsPapa]

Oh and 99% of the time I'm running low octane Costco gas...I know, but I throw an injection cleaner in the tank once every.....not often enough.

Yes, Costco is good gas, although I've never had a lick of issues with other gas in any of our Jeeps. Try some seafoam...........
I do injector cleaner at the first sign of a bit of roughness although never had it happen on a 3.6 - they seem to run on whatever I put into them.

Setup JSCAN to show you cylinder misfires and see what happens -

 

[Gabby53E]

Ive posted before about a rough idle on cold start and after sitting for alittle and then in smooths out after driving. Right now Im at 35,700 some miles on a 2021 Sport S with 3.6 gas and auto trans. I have replaced the pcv, no change. Cleaned the throttle body and mass airflow sensor, no change.

Today I replaced the EGR valve and EGR temp sensor, not as bad but still there. I disconnected the battery for just alittle over half an hour, not as rough idle. However, after I left the gym and sat in the parking lot the idle was kind of smoothish.

Right before my factory warranty ran out I took it to the dealer and they said nothing was wrong especially with no check engine light and no codes showing.

Only things left would be plugs and/or coil packs and maybe injectors. Its weird it only does it worse after cold start up and smooth out after driving. If it were a plug, coil pack, or injector shouldnt it idle rough all the time and run like crap? My last trip I got 21mpg at some points going 75. I get decent fuel mileage for the mostly city driving and I use injector cleaner now and then and manualy shift the auto to high rpms to blow out the crud. All hoses appear well connected. Im at wits end.

Our 2018 JLU is in the shop with the same issue. It turns out coolant has been leaking in the cylinders. The symptoms make sense, as it was a slow leak overnight, which caused the rough start. Once the coolant burns off, the engine runs smoothly the rest of the day. The next morning the process starts all over again. Only way they found it was borescoping in the morning at the shop. Our jeep was showing misfire codes with the check engine light though.

 

[ShadowsPapa]

Our 2018 JLU is in the shop with the same issue. It turns out coolant has been leaking in the cylinders. The symptoms make sense, as it was a slow leak overnight, which caused the rough start. Once the coolant burns off, the engine runs smoothly the rest of the day. The next morning the process starts all over again. Only way they found it was borescoping in the morning at the shop. Our jeep was showing misfire codes with the check engine light though.

There are other tests and checks for that as well. Scoping the cylinders under the intake means taking a lot of time and removing a lot of parts -perhaps to find nothing.
Apparently, though, that's their favorite method and it will make them more money if it's not a warranty thing and they take it all apart, only to find no leaks into the cylinders.

It's not really a common issue - but symptoms do fit. You should also note coolant loss over time (there should be almost no loss even after a year).

 

[Pickle_Bill]

Our 2018 JLU is in the shop with the same issue. It turns out coolant has been leaking in the cylinders. The symptoms make sense, as it was a slow leak overnight, which caused the rough start. Once the coolant burns off, the engine runs smoothly the rest of the day. The next morning the process starts all over again. Only way they found it was borescoping in the morning at the shop. Our jeep was showing misfire codes with the check engine light though.

My sons 2018 JLU had the same issue. Seems "kinda" common for that year (under torqued head bolts passenger side). We replaced the head gasket, and now it runs better than ever. Big job though...gotta remove the timing cover and everything in front of it.
On that note, it seems very uncommon for JT's (as well as later model JLU's).

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