I thought the gm service bulletins are for tourqe converter vibrations, and hard up shifts on older models. And to replace older fluid with the newer type. A 21 comes with the new fluid and I have seen several people on this site say their dealer will not change the fluid on the newer transmission because they don't have the shudder/ upshift problems. Also my truck downshift perfect on cold fluid the only time it will downshift hard is on fully warmed up fluid.
I have a 21 5.3 crew cab standard bed. It has the 8 speed. The only thing it does that's not normal, is every 10 days or so it will downshift into first gear 6 to 7 mph hard and kinda jerk the truck. But the next 9 or so days it's perfect. Last time it did it it was only for half an day. I got 4,300 miles. I would have maybe took to dealer buts it's rare, and I know if I tool it there it would not do it when I brought it there. My 2017 impala did the same thing but not quite are hard. Maybe the rear drive and long drive shift makes it worse. I used to drive ford 150 for work my last two a 2018 and 2015 both did a hard downshift into first with a loud clunk from the drives haft rear end area, was really bad in sport mode. I'm not very worried about it myself.
I own 5.3. I test drove a bunch of trucks. I didn't have anything to complain about the 2.7 turbo motor it seemed to do a good job. I just wanted more option. I couldn't get comfortable with the custom truck, I needed telescoping wheel and power seat couldn't find that in custom, all I found were lt's with 5.3.
I just install GM wheel liners. There was one option the fuzzy ones. Should cut down on road noise, and I have never had any problems cleaning mud of the front ones. That's why I bought the back, tired of seeing red clay on the white paint in the wheel wells, that won't come off with high pressure washer, and soapy brush at car wash.
It seems the 8 speeds have some weird things they do. I'm sure you have read the complaints over the years. Mine has a hard downshift into first gear. Happens all day, then the next 10 days it shifts great. Then it will come back and have a hard downshift all day again. Sometimes the trans will do a learn, maybe yours will start shifting better.
All the warranties I have seen. If they say 8 years 100k. That's 8 years from when you bought it and 100k odometer miles. I have 8 8 year ALLY warranty. So it's 8 years from the day I bought it. I also have 100,000 on the warranty so since the truck had 35 miles when I bought I have 100,035 miles untill the warranty runs out.
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