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Another JR

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Another JR last won the day on September 20

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  • Name
    Jon
  • Location
    WA
  • Drives
    04 Yukon XL, 93 K2500, 21 GMC 3500HD SLT Gas

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  1. Yes, it seemed clear that the person in that thread felt they could not give specifics. As you say, the truck does not have excessive coolant or transmission temperatures under those conditions. However, the actual combustion conditions at high altitude on warm days (lowest air density conditions) when attempting to get maximum available horsepower out of the engine lead to high combustion and exhaust gas temperatures. This increases the formation of NO and NO2. It’s not a matter of dissipating or rejecting heat - it’s simply a combustion temperature issue. The same problem exists with jet engine combustion. I’m guessing they run into a temperature limit they’ve set to limit the emissions, the computer reduces the actual throttle plate position to limit power and rpm (despite the pedal position being floored), and truck won’t downshift because you can’t give it enough actual throttle to trip the downshift. Or they just directly prevent the downshift to limit rpm and thereby limit mass air flow, power and temperature. Again, my speculation on what they are actually doing to prevent the downshift. Why didn’t the Ford 7.3 do the same thing? Same load, but almost 10% more rated horsepower, yes. But more importantly, look at the two videos. When the Chevy was tested they were in shirtsleeves and no snow in sight, not even on the nearby 12k mountaintops. Hot day for 11k feet. When the Ford was tested, they were all bundled up and there was snow all along the highway. Huge difference in temperature and air density, which causes a big difference in horsepower. The difference in air density altitude for that maybe 40F temperature difference was about 4000 feet if I’m reading my charts correctly, so the Chevy was working with a very large additional maybe 12% horsepower reduction (using the rough power loss formula used in aviation) simply due the higher density altitude (lower air density) from the temperature difference. So there was a roughly 20% horsepower difference between the Ford and Chevy on their test days, more than half of which was caused by the temperature difference.
  2. This was discussed in another thread to some extent. A person with apparent insider knowledge said it was an engine cooling related issue (not water cooling) and hinted it was emissions related, but they were apparently not at liberty to give details. My speculation that it is NO/NO2 related is just common sense based on emissions knowledge I have from my work. It is just speculation, though.
  3. If you are referring to the “Ike Gauntlet” video, there was an engine thermal management feature (I’m guessing a nitrous oxides emissions related limit on exhaust gas temperature) that prevented downshifting late in the climb.
  4. I meant was the 10 speed an option and you can also order the 6L90, or is the 10 speed the only transmission offered? Thanks.
  5. As an option or are they only offered in 10 speed?
  6. I was simply pointing out to the OP that measuring the same water temperature when the thermostat is modulating flow does not tell you the airflow has not changed, contrary to their claim. I agree it doesn’t matter up to the point where the thermostat is full open.
  7. Yes, the tire calculators say there is about 3/4 inch difference in diameter between the standard 18 and optional 20 inch tires. Your new tire size is maybe another 3/4 bigger. If you unlock your spare and crank it down just to the point where you can shift it around, you’ll be able to move it and judge how much additional room there is.
  8. It would be close. I have the stock Michelin 275/70-18 tires and I’ve had my spare out to do some wiring. As I was restowing it, I remember I was able to shift it around about an inch, meaning there seemed to be room for about an inch more of tire diameter. I don’t remember what was limiting the movement.
  9. Measuring water temperature when it’s operating in the thermostat control range does not tell you whether the airflow has changed.
  10. That’s purely the dealer choosing not to keep their word to try and make a few thousand short term, not caring if they lose a customer for life. I ordered my truck in December 2020 and took delivery in April 2021 using the supplier discount, which was worth about $6k. The dealer said here’s the price with supplier discount based on the current pricing, and the price will only go up if GM’s price to us goes up. I agreed to that price, and it was on paper, but not on an actual purchase contract form. When the truck arrived, there had been no HD trucks on their lot for months. When I arrived to pick it up, the dealer said they’d had to lock their delivery room to keep people away from it. They told me they’d had three people try to make offers on it in the day and a half it was there. They proceeded to close the deal with me for the originally committed price despite the fact they could have sold it for 5-8k more. That’s what a reputable dealer does. They are interested in repeat business and referrals. This is the second vehicle I’ve bought from them, and I’ll go back if I ever need a GMC again. That was Brotherton GMC of Renton, WA. I have no affiliation with them.
  11. Think about how the fluid from the reservoir enters the system as your pads wear and the pistons extend more. When the pedal is fully released, the brake circuits in the master cylinder are connected to the reservoir. That’s why you are able to use a power bleeder. Pull up a diagram of a master cylinder to visualize it. I’m speculating it may be a bubble in the master cylinder and pressing hard to stroke the master cylinder a greater distance shifts the bubble so it works it’s way back to the reservoir. Completely a guess though.
  12. The feel of my brakes was pretty good from the start, but early on I did the procedure several have recommended on here to firm up the pedal feel, and it did improve somewhat (again mine already felt fine, whereas other have complained of a spongy feel. With the truck running and in PARK, push the brake pedal slowly and firmly as far as you can with large but not stupid force. Let it all the way up, then press it the same way again. I don’t know, but I suspect the reason this sometimes improves the feel is that it clears a bubble in the master cylinder left behind due to the factory filling a dry system and using a power bleeder rather than old fashioned pedal stroke bleeding.
  13. Two comments: i wonder if there is an issue with your truck. Mine passes, accelerating from 55 to 70, on the highway very briskly with 2000 lbs of pop up camper in the bed. I’m very comfortable making passes on two lane highways. Your description of the power you get in that situation seems very different from what I experience. One of the times I most appreciate the heavier duty truck is when I have to brake really hard at highway speeds. I’ve had three serious panic stop situations so far in this truck, each time with the camper on, and it stops really fast under complete control.
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