Did you verify the can is sealed well? Remove it and plug one side. You can use a vacuum pump or suck on the other side to see if the seal is leaking. These are not high tech. Just an inlet and outlet but if the canister seal is leaking that’s a problem.
Yeah something isn’t tight if you have never seen above 20. If your a lead foot I can see that but even mild performance I get low 20s. Are you looking at the 25, 50 or what mileage average?
Only had issues in other vehicles with those ultrasonic sensors if bugs or a thick buildup got on them. Not sure if the Sierra is using ultrasonics or radar. Either way if the trucks relatively clean shouldn’t be doing that. Just looked it up. They are radar sensors. Known issue is they sometimes will trigger for guardrails on the highway.
Those 4 BCM screws pull the top fuse block into the lower wiring harness. Just tightening may not fix a poor connection below. I removed mine to install the fog light kit so I got an up close and detailed look at this assembly. I’m surprised GM used such a connection system but they must have their reasons. You should see four green indicator posts near the four screws. If those indicators are not all equally sticking up to indicate engagement then their likely is a bent pin or alignment issue. Check the 4 green indicators carefully. I would not suggest while your vehicle is under warranty removing and reseating the UBM unless your very comfortable doing so.
I had mine in to dealer a week ago to enable the fog light kit. They mentioned a recall for the DEF tank. Think it was just a program update. When I went in my DEF showed one bar low and when I left it showed a half tank. I went home and added 2.5 gallons of DEF and it now shows full. There were reports of the DEF fluid level not being accurate. They may have added some averaging to the sensor reading so it’s a bit more accurate. At least that’s what I suspected. Yours may have gone into limp mode for low DEF. after its reporting more accurate DEF so running more like it should? At least my best guess. Btw - I had no noticeable change in noise or drive characteristics.
With the 3.0L in town depends on number of stop lights! I get around 25 in town if traffic isn’t stop and go. Been as low as 18 but that was a lot of stop and go. I like my trailer mileage pulling a dual axle enclosed trailer about 4000 lbs I can get 18mpg if there isn’t a significant head wind and rather flat ground. I’ve also gotten much better than that following trucks and such but that’s not a given.
The 6.2L is hard to beat since it’s been out there and a known entity. The 3.0L as you said is new. I have a 3.0L and really like it but I rolled the dice. I have no idea how it will do 100K miles from now. Since you seem to be risk averse I’d go with the 6.2.
Yes find a quality shop that uses OEM parts. They can make it like new as long as you don’t have major structural damage. Get a couple quotes to verify.
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