Rough idle when cold starting

[Wyorebel]

I have a 2020 ram rebel with a 5.7 in it, last week I went to start my truck and it idled extremely rough, to the point it almost sounded like it was trying to backfire.
Turned it off, restarted it and it purrs like a kitten.
Took it to the dealer Monday, they ran a diagnostic on it, no codes showing on it and there clueless about what’s going on or how to fix it.
Anyone had a similar issue, if you did what was culprit
Thanks all

 

[Wild one]

Did you happen to just fill it up,if so try another gas station and fill it up with 91 or 93,and see if anything changes

 

[Wyorebel]

Always run 91 with no ethanol, when it started that was first thought was bad gas or water in it, I dumped sea foam and heat in it nothing changed
Temperature seems to effect it though if it’s fairly warm it won’t do it
Told the dealership that it may be a sensor but since there’s no codes being thrown they won’t check them

 

[Curmudgeon]

While ethanol-free gas has been mentioned, here's a question I've been thinking about lately - does E-free gas have any additives? I have very serious doubts that this would have any bearing on the OP's issue, I was just wondering.

@Wyorebel good luck. Intermittent problems can be a real bugger to resolve.

 

[Gero]

Hmm. Interesting that it did not show any codes. Could be bad fuel but even that would throw codes. I was thinking pcv valve but normally you would noticed other symptoms like poor fuel mileage. I've ran 87 since day 1 and have never had any issues.

 

[Monthey]

I have a 2020 Rebal that will hardly start when cold, the rest of the day it runs great. I use only 91 octane ethanol free gas like the owner's manual recommends. It started doing it a couple of weeks ago. Gillette Ram dealership said to try mid-grade gas instead of premium, if you can believe that. Talked with a mechanic friend who runs a mechanic shop, and he said it was the fuel pump going bad, so I had him install a new one. Still doing it and getting worse. No codes, 90K miles, regular oil changes. Runs fine the rest of the time, just first start of the day when it happens.

 

[Jeepwalker]

I had something like that happen a couple times over the yrs. My WIN module is kind of Iffy (need to clean contacts again). It seemed almost like it was associated with that. Just correlation..but I can't prove anything. Not sure if that is a related cause in your truck.

I guess if it were me, I would verify the battery connections are real clean. Make sure the throttle bore is real clean (not a gum-ring in there), wiggle the bundle of wires under the fuse box (make sure the fuses are nice and pushed down in their spot) ....and monitor the situation. See if it happens again.

 

[Sherman Bird]

I have a 2020 ram rebel with a 5.7 in it, last week I went to start my truck and it idled extremely rough, to the point it almost sounded like it was trying to backfire.
Turned it off, restarted it and it purrs like a kitten.
Took it to the dealer Monday, they ran a diagnostic on it, no codes showing on it and there clueless about what’s going on or how to fix it.
Anyone had a similar issue, if you did what was culprit
Thanks all

Sadly, today's "junior league" of technicians are VERY lacking in diagnostic skills. A scanner is the equivalent to telling your primary doctor your symptoms and he/she does a precursory exam. A VAST majority of maladies would escape this primary level of looky-see from the doctor. Further, specialized diagnostics are called for if, they really know what they're doing germane to a preliminary scan. (Did they drill down on Mode6 data, for instance?)

Just because the "dealer" scanned it means very little. FROM my perspective as a long-toothed professional automotive technician, Most guy are egregiously undertrained.

With ZERO data from you, going by a SPECIFIC glimpse of a symptom of your description, I'd suspect AND eliminate a fuel delivery problem FIRST.
Secondarily, I'd do a relative compression rest with a 600 amp inductive probe and my DSO. (digital storage oscilloscope) to eliminate a mechanical issue, such as a lifter bleed down. If indicated, I'd do a cylinder wave form analysis on any indicated errant cylinders.

You MIGHT get lucky here on the forum if someone happens to hit on a pattern failure issue germane to the brand/model.