2021 GMC Sierra Denali idle issue, downshift to stop

tootall4289 · August 11, 2021

I have 3,100 miles on my truck and it seems to run lean while coming to a stop. The engine will struggle and the brake petal will vibrate a touch. It seem like the DFM is kicking in or the auto stop is starting to soon. I took it to the dealer and of course they couldn't replicate the problem. Anyone else run into a similar issue?

Edited August 11, 2021 by tootall4289
grammer

kmeleon · August 11, 2021

39 minutes ago, tootall4289 said:

I have 3,100 miles on my truck and it seems to run lean while coming to a stop. The engine will struggle and the brake petal will vibrate a touch. It seem like the DFM is kicking in or the auto stop is starting to soon. I took it to the dealer and of course they couldn't replicate the problem. Anyone else run into a similar issue?

My truck does the same. I believe it's dfm as well. Try driving the truck in L9 and see if it feels different. It does to me. I'm actually on the phone with gmc as I type filing a complaint about my truck and the flashing SEL lights I've been experiencing. She seems to be taking charge, I sure hope so. Corporate techs are going to contact the dealer.

603Denali · August 12, 2021

I too have the same problem with a 2020 Denali w/6.2. To add to this, if I am sitting somewhere at idle for more then a minute or so, I get intermittent stumbling. I run 93 octane so it shouldn't be the gas.

tootall4289 · August 12, 2021

I have tried 93 octane and still have the same problem. I agree its not the gas.

2016 Sierra Owner · August 12, 2021

19 hours ago, tootall4289 said:

I have 3,100 miles on my truck and it seems to run lean while coming to a stop. The engine will struggle and the brake petal will vibrate a touch. It seem like the DFM is kicking in or the auto stop is starting to soon. I took it to the dealer and of course they couldn't replicate the problem. Anyone else run into a similar issue?

What engine and transmission do you have?

tootall4289 · August 12, 2021

5.3 with the 10 speed

calgator73 · August 13, 2021

11 hours ago, 603Denali said:

I too have the same problem with a 2020 Denali w/6.2. To add to this, if I am sitting somewhere at idle for more then a minute or so, I get intermittent stumbling. I run 93 octane so it shouldn't be the gas.

Stumbling in gear? Happened twice on my trip to Florida... Trip to Oklahoma it actually backfired. Got an appointment Monday.

tootall4289 · August 13, 2021

It is not stumbling into gear rather as it downshifts coming to a stop the RPMs go so low it seems like it is going to stall and the engine struggles, it recovers once I come to a complete stop.

kmeleon · August 14, 2021

On 8/13/2021 at 10:00 AM, tootall4289 said:

It is not stumbling into gear rather as it downshifts coming to a stop the RPMs go so low it seems like it is going to stall and the engine struggles, it recovers once I come to a complete stop.

Did you try L9? Let me know what you think.

redwngr · August 15, 2021

Perhaps trans not downshifting and/or torque converter not unlocking.

603Denali · August 16, 2021

On 8/12/2021 at 9:19 PM, calgator73 said:

Stumbling in gear? Happened twice on my trip to Florida... Trip to Oklahoma it actually backfired. Got an appointment Monday.

No stumbling at idle. Mine has shifting issues as well.

UNSTUCK · August 16, 2021

My 2021 6.2 definitely has a rough idle at a stop light. Almost a little lope to it. I assume it is just DFM doing its thing. Haven't noticed any issues with low RPM while coming to a stop though.

tootall4289 · August 26, 2021

SEL light is flashing now...back to the dealer.

kmeleon · August 27, 2021

6 hours ago, tootall4289 said:

SEL light is flashing now...back to the dealer.

When did the light come on? After a stop as you were pulling away? My trucks probably done this now more than 30 times. It seems if I bring it to a rolling stop soon after starting the truck and don't exceed 25mph it does this Alot. Right as I tip in on the rolling stop the light will start flashing. I've almost mastered how to set the light off.

tootall4289 · August 27, 2021

11 hours ago, kmeleon said:

When did the light come on? After a stop as you were pulling away? My trucks probably done this now more than 30 times. It seems if I bring it to a rolling stop soon after starting the truck and don't exceed 25mph it does this Alot. Right as I tip in on the rolling stop the light will start flashing. I've almost mastered how to set the light off.

It came on as I was going down a slight incline at about 50 mph. Started flashing and then went out.