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Any 6.2L experts here? New owner with questions

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1 hour ago, Grumpy Bear said:

FYI DGI helps charge cooling yes but VVT controlling IVC angle is what keeps high compression motors happy on low octane fuel. Cylinder pressure is King. 

 Yep - nailed it. All about valve events. For everyone else - read about Atkinson cycle. 


VVT in a single cam OHV setup (our engines) cannot independently control the exhaust valve events though - which is rumored to be a significant contributor to combustion chamber deposits. I digress though. 

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1 hour ago, diyer2 said:

Well my memory isn't what it used to be. Back in the 70's when I was racing 1/4 mile we adjusted timing for better performance. We also changed distributor weights to change timing curve. I do remember spark knock and blown engines. Everything has a limit.

I don't own a big block but you can bet I wouldn't cheap out on gas for it. I believe the engineers are smarter than me. We put a man on the moon with a slide rule I believe. The movie Hidden Figures come to mind. 


my dad calls those the good ole days :) does the same thing every time, predictable. you guys would freak if you saw all the different tables that affect spark and fueling in these ECMs. even humidity plays a factor! That's why some people swear these vehicles have multiple personalities - because it really will run different in the morning compared to the afternoon. 

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On 7/22/2021 at 10:15 AM, whistleblower said:

Recently purchased a 2020 Yukon XL with 60k miles with the 6.2L.  Fleet car and was told maintenance was is up-to-date but who the heck knows.  


My questions after ~1,000k miles of driving split between city and highway:


1 - I feel light surging at various RPMs and various speeds.  Its hard to recreate but I'll be driving, will forget about it and then will feel it again.  Typically happens on acceleration but I noticed it the other day at ~70-75.  Any ideas?  Seems to happen mostly at low rpms but I have not been able to define the pattern.  

2 - Rough idle.  I'll be sitting at a stop light, not thinking about it, and will suddenly feel a shake, almost like a miss.  Same characteristics when starting cold and when warm.  Almost more apparent when warm.  

3 - Less of a concern but just want to confirm that this is normal.  After the engine is warm, my oil pressure drops to ~18-20 when at a stop light, then jumps up to ~40 when underway.  Oil is full.  Confirm this is normal?  


Regarding 1 and 2, I'd love input from people who know these motors.  I've done a bunch of reading and have an appointment with a dealership to see if they can find anything [which, honestly, I'd love to cancel - terrified of the potential bill].  I hate to self diagnose as I'm not a mechanic but have found posts referencing new plugs/wires as a fix as well as removing carbon from the intakes.  


Any ideas?  If its as simple as new plugs/wires, I'm going to cancel the dealership visit and find a local mechanic to do that.  


For those with higher miles on your 6.2, how often are you cleaning the carbon?  


The power of this engine is awesome but honestly I've been a little disappointed with the overall driving experience.  My previous car to this, 20+ year daily driver, a 2000 F150 with the "terrible" 5.4 triton @ 200k miles, is much smoother than my shiny new(ish) Yukon.... which makes me think its something that can hopefully be corrected!


Thanks in advance



1:  Sounds like 8 speed shudder.  You can try your nearest GM dealer and have them do an 8 speed fluid swap using the Mobil 1 Synthetic LV HP ATF Blue Label fluid.  That might take care of that.


2:  Somewhat normal.  If you don't run 91 or 93 in it, I would suggest switching to it.  6.2 runs the best and most correct on 93 octane.  Could also be a failing motor mount but seems too low in the mileage for that.  Being it was a fleet ran truck they were probably cheap and ran 87.  There will be plenty on here that say 87 is fine, its not.   


3:  100% normal.  Dual stage pressure oil pump that allows for low pressure when demands are low and high pressures above 3500rpm.  



On 7/22/2021 at 3:48 PM, Black02Silverado said:

The first part might be DFM,  Dynamic Fuel Management.  Something that you are not use to and it could be what you feel.  Other than that not sure.



OP grabbed a 2020 Yukon which would still be AFM.  DFM is in the 2021 new body.

Edited by newdude
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Being it was a fleet ran truck they were probably cheap and ran 87.  There will be plenty on here that say 87 is fine, its not

I drove a fleet truck (K2500-3500 4x4 suburbans mostly) for 30 yrs as DOT GPS Surveyor.  It stated on our Credit Card, regular fuel ONLY, Premium fuel not allowed.  Nor could we charge synthetic oil on the CC.  None of our vehicles required premium. 

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Thanks everyone for the thoughts.  Pardon delay on my end, I was on vacation last week where I rented a 2021 Silverado 1500 with the 5.3L and put about 800 miles of highway / backroads driving.  The 2021 Silverado had ~40k miles on it and ~1400 hours.  A couple thoughts related to responses to my original post as well as the comparison between the 2021 Silverado and my 2020 Yukon XL:


On 7/22/2021 at 1:56 PM, JimCost2014 said:

1st symptom might actually be your torque converter on it's way to failure, are you feeling any vibration/rumble strip effect when it surges?


Hm.  When I think "rumble strip effect", I think very noticeable vibration, which I don't have.  Mine is more a subtle surging, like the vehicle wants to go! but I only have a small amount of gas pedal applied. 


Feels like a pulse, almost as if I was pulsing the gas pedal but I'm not.  



On 7/24/2021 at 9:37 PM, NWI Denali said:

The vibration your feeling and at the speed you're saying sounds similar to the torque converter shuddering the 8L90 gets but you have the 10 speed so thats strange. The rough idle is normal to an extent- how much do your rpms drop? The ecu is programmed to idle these engines so low to save fuel economy that some "stumbling" is normal. They idle at like 500 rpm in gear 

On the idle, its not very much.  50-100 rpm shudder / stumble.  Def feels a little lean.  



On 7/25/2021 at 8:34 AM, swathdiver said:

More importantly, what kind of fuel are you using?  These are high compression performance engines, they are not intended to burn anything less than 91 octane gasoline.  


 I have only used 93 octane.  That said, I just filled up for the 4th time so not a ton of miles with 93 octane.  As others have said, I think its safe to assume that the cheap 87 was run through the vehicle prior to my ownership.  


Regarding my experience with the 2021 Silverado w/ 40k miles - this truck seemed much "smoother", generally, than my 2020 6.2L.  While it didn't have the same power, the acceleration, the idle, everything about it is what I'd expect in a new-ish vehicle.  I'm not sure what transmission this truck had, maybe the 10 speed?  Regardless, the acceleration, low rpm cruise, idle was all superior to my truck.  


I'm going to keep my date with the dealer (lord save me) and try and find a 6.2L on their lot to drive around while I'm there for comparison purposes.  I'll report back with what the dealer finds.  I have no doubt that SOMETHING will be found that I will need to pay for...


Thanks all for the thoughts.    

Edited by whistleblower
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