I used to have a thread here on rough idle which was over 50 pages long (or something close to that). My 2018 Yukon had rough idle when I bought it with 7K miles on odometer, spent endless hours and over $2800 of personal money to fix the rough idle and it was all wasted. Finally found a local dealer who was very aggressive and traded good-optioned Yukon for bare-bones 2020 Tahoe. I was in 26 2018+ Tahoe/Yukons by now. That 2020 truck had a smooth idle and still has it 70% of the time but I found the following (purely subjective but I am just trying to pay back to the users of this great forum):
1) These 2018+ 5.3 engines are incredibly sensitive to gasoline brand. Shell, BP, Costco seem to produce the smoothest idle in North Carolina. Shell is universally hated here and on BTOG but again I am just sharing my experience with 5.3 trucks that I drove extensively for months. I do see poorer mileage on Shell. It got Chevron twice when we were on South Carolina and it was by far and I mean by far the smoothest gas and superior mileage.
2) Contrary to the manual, these trucks do not like 87, they retard so much or even detonate on 87 (not always audible but according to logs it is there). However, they seem to idle best on 87. When I put 93 it seems to give it rougher idle. 89 in also a problem as this is the least selling octane and according to two gas station owners they told me they sell <10% in 89 so that gas is not the freshest (assume it has more sediment too). These are high-compression engines and they should prefer 89 or 93 and they probably do but idle is not the smoothest with 93.
3) I see almost no MPG gain with 89 nor with 93 unless it is Shell. With Shell 89 and 93 the mileage is worse than BP or Costco. With Chevron it is like magic, the truck idles perfect and gas mileage is just 10% better (measure on Fuelly).
4) Exxon gives me rough idle, no matter what octane (again dozens here prefer Exxon and for good reasons, but on my two trucks here in NC Exxon is not it).
4) Gas is not gas and no Federal or State is not tightly regulating the additives and do not monitor every station weekly.
I like the ride that these trucks are producing it is very comfortable but be prepared to live with rough idle. 2020s are also abysmally bad with squeaks and rattles in the cabin my dealer is trying to fix them but really cannot. He has tried 7 times now and it keeps rattling in the dashboard, driver door, center console. The truck is 5 months old and has 11K miles now.
Good luck to us all with GM.
