Another JR

I don’t understand why auto 4wd is a concern. Can you explain?

The “off road suspension” is simply the Rancho shocks - no other suspension changes. Read the owner’s manual on line for a more complete description of how hill descent control works and its speed ranges in high and low transfer case ratios. I have it and have tried it, but I doubt I’ll use it much.

I don’t see engine driven accessories listed in the powertrain warranty coverage listing on GMC’s web site. I assume Chevy’s is identical. https://www.gmc.com/owners/warranty-details

Dunn - Do you have 20 inch wheels? When I was test driving very similar trucks the ones with 20s definitely had a stiffer ride and transmitted road bumps more than the trucks with 18s. I chose the 18s for that reason and also for more sidewall when airing down off pavement.

You had me going there for a few seconds - until the headlight thing.

Thanks for the recommendation - I’ll remember that for the future. I hate the semi-monopoly safelite has with the insurance companies and like to patronize a good private glass place. We had one in Renton for many years but they succumbed. Auburn is definitely close enough for me to use. (By the way, the Auburn Line-x place is great, too. ) I may have created some confusion. My windshield is fine. I was commenting to the OP who said they finally replaced the windshield when it cracked all the way across, and then also pointed out the owner’s manual advice.

I realize this is off topic, but a cracked windshield is not just a visibility concern - it also is an energy absorption concern if something large hits your windshield. Certain things that would be stopped by an intact windshield will come through a significantly cracked one. The manual says to have the windshield replaced through the dealer to ensure compatibility with the HUD and camera systems.

The 2021 owners manual capacities page 404 says the double cab diesel with standard box has a 29.4 gallon tank.

Thanks for the help, gemarsh. Use of tire pressure to aim headlights on a truck was a new one for me, too.

I know how to properly aim headlights - I’ve been doing it for 40 years. I just was wondering if GMC has made the adjustment method impossible for a DIY person after reading the manual. My truck is out in the barn and it’s cold, rainy, and windy out, so I was asking here rather than looking myself. The camper stays on almost all the time, so I want to adjust them for that load. I have no problem with having the headlights aimed a little low for the small amount of unloaded operation I do. The camper weighs 1800 lb and the payload is 4054 lb, so it will be a small adjustment. However, I’ve been flashed enough times that I want to be courteous to other drivers and fix it.

2021 GMC 3500HD SLT (LED headlights)

The owner’s manual says see the dealer for headlight adjustment. I’m interested in adjusting mine down a little to compensate for my camper lowering the rear a couple of inches. Does anyone know if the adjusting means is easily accessible?

That’s what I like to hear. While I only have one of these trucks, no failures and no limp modes when I’m on highway trips or remote camping was my top priority beyond basic capability. The gas engine and 6 speed were ideal for me.

Personally I’m glad if GM has included a feature to protect from overheat damage. I’d just like to understand it and know how to avoid getting the truck into that condition in the first place. That TFL video showing them starting and continuing a long, high altitude steep climb in warm conditions (or any weather for that matter) with a maximum trailer load in terms of both weight and aerodynamic drag by just holding the pedal to the floor is not how anyone who cares about their equipment would drive. (Yes, i understand their intent was to provide a comparison. ) Personally, I would find a speed that could be held with the engine between 3000 and 4000 rpm on the grade, and I would climb at that speed if it was safe for the road conditions. I’m curious to know if the truck would have a problem with that technique on that Ike climb, and that’s the test that would have been meaningful to me (what speed can it hold on that climb) when deciding on a truck. However, even that test would not be critical to me because I don’t ever expect to tow with more than 10k and have no reason to tow a heavy boat over the Rockies. I understand it may be important to those with other trailering needs.

I have a 2021 3500hd SLT CCLB SRW gas with x31 and 18 inch wheels with stock Michelin tires. My understanding is there is no suspension difference between my truck and a Denali. I measured the high point of the cab at 78 and 3/4 inches just now, but that is with a popup camper installed. When I load the camper, it drops the rear fender well height by 2 inches and drops the front less than 1/4 inch (I measured when I first installed it). Since the high point if the cab is midway between the axles, it would be 1 inch higher without the camper, or 79 and 3/4 inches. Based in this you should be fine even if you have the 20 inch wheels and stock tires, which would make the truck 1/2 inch taller.

So can it be seen from coolant temp or transmission temp when the ecm will prevent downshifts?

Me too. You always have the option to force it to 4th or lower by shifting to L. At least that’s what the manual says.

Mine’s a 3500 and the recommended rear pressure for maximum load capability is 80 psi.

I had said above that I was running the front at 55 and the rear at 60 with no TP warnings, but that wasn’t exactly accurate. My rears actually have been at 63 when cold for the last few weeks, and temps have been dropping slowly here in Seattle. Last night I received a tire pressure warning for the rears, and the indicated pressure was at 62. They both alarmed at the same time while i was driving. So I guess I found out the warning set point for the rear tires on my truck is 62 psi.

Regarding oil change interval discussed above, it’s a good idea to do some research to understand the intake coking issues that some fear will be caused by the direct injection system eliminating fuel washing of the intake areas of the cylinder heads, and the resultant potential for oil deposits to form from the crankcase ventilation system introducing oil vapor and oil condensation in those areas. Other direct injection engines have had problems from that. Part of your defense against that is the additive set in the Dexos approved oils. Those additives break down over time, and you should consider that in deciding on your oil change interval. See the threads on direct injection and on catch cans for discussions of the coking issue.

I don’t have enough experience with this system to be much help, but I would suggest disconnecting and reconnecting the trailer electrical connector with the truck running if you haven’t already. That might reset something.

I have no idea if GM has some aerodynamic strategy here, but I suspect the primary benefit of shutters is intended to be the same as that of a winter partial grill cover.

The owner’s manuals for all the trucks are available on line. The procedure for programming for a new fob are in the manual. https://www.chevrolet.com/support/vehicle/manuals-guides?compIndex=1&year=2020&make=Chevrolet&model=Silverado+HD

Is the water temp normal when this happens? I’m pretty sure the fan clutch is a simple viscous clutch based on looking at the parts catalog, so no connection to any external sensor or computer.

I have had my gas 3500hd slt for 8 months and 3500 miles, with no such issues. On your fogging issue, have you verified that when it fogs up and you turn on the defroster that the a/c compressor is running? Your cooling fan coming on after they replaced the heater core makes me wonder if they failed to fill and bleed the air out of the cooling system properly. A bad temperature sensor telling the computer the engine is excessively hot might cause both the fan to come on and the a/c compressor to be locked out. That’s just a guess though - I have no idea if such a sensor or logic actually exists in the system.