Auto Stop while in motion

[TReed]

Hello everyone,
First time posting on this or any other forum.

I recently purchased (my wife’s vehicle) a 2021 GMC Yukon Denali with the 6.2 V8. It was a little sluggish for a 6.2 and so I wanted to free it up a little. My daily driver is a 2019 Silverado with the 6.2. So, I have an idea how much more power it should have. Yesterday 7/25/23 I took it to a local muffler shop and had a Borla ProXS (40359) muffler welded in, the resonators and cats and everything else are still there. (NOT a full cat back exhaust). At the same time my CAI (K&N 63 series FIPK 63-3110) arrived and I installed that as well. I’ve done this to almost every truck or SUV I’ve owned. Including my current Silverado. Side note it sounds awesome and is much more responsive, unlike all the 5.3s I’ve had where I’ve hardly noticed a difference in performance…

What’s strange is after test driving it, it seems like when I come up to a red light or stop sign and let off the gas to start to slow down (from about 20-30mph) the engine goes completely quiet (as though the auto stop feature has engaged but, that’s not supposed to happen until you come to a full stop). The tachometer still shows around a 1,000 rpm. Now, it could be that it’s in v4 mode and it’s just idling so low that it sounds like it’s not running but, it sure sounds eerily quiet. I mean I can’t hear any engine noise, at all. As soon as I touch the gas I can instantly hear the exhaust note again. I can’t tell if it’s starting or if it’s just reengaging the additional 4 cylinders. Since the vehicle is moving, the wind noise and/or road noise is loud enough that I can’t tell if the engine is starting again or if it’s running the whole time. Like how you can hear it start when you let off the brake after being completely stopped. I pushed the button to disengage the Auto Stop feature and no change, the engine is still so quiet (while coasting/braking to a stop) I can’t hear if it’s running or not.

Has anyone else experienced this? Am I just imagining this? Is there some reason why the Auto Stop feature would engage while the vehicle is still in motion now that I’ve altered the exhaust and intake? Or is it actually just running so quietly that I really just can’t hear it at all (immediately after installing a much louder muffler)?

 

[Joseph Garcia]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

What does your tech show, while you are going through this driving sequence. IMO, if the tach does not go to zero, the motor is still running. Other Forum members more familiar with your model year will chime in.

 

[TReed]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

What does your tech show, while you are going through this driving sequence. IMO, if the tach does not go to zero, the motor is still running. Other Forum members more familiar with your model year will chime in.

Hi. Thanks.

I don’t have any pictures right now. I’ll try to upload those later.

The tach does show 600-1,000 rpm when this happens though. It’s so quiet though.

I think I’m going to have to put someone in the cargo area with the rear glass open and try to recreate the issue in an empty parking lot.

 

[B-train]

Try shifting to neutral and coast to see if the tach drops to zero. Also, it has DFM, which means it can run on a little as 1 cylinder. My guess is that you are not hearing anything while coasting because it's just barely hitting on any cylinders.

 

[TReed]

Try shifting to neutral and coast to see if the tach drops to zero. Also, it has DFM, which means it can run on a little as 1 cylinder. My guess is that you are not hearing anything while coasting because it's just barely hitting on any cylinders.

I tried shifting to neutral while in motion, yesterday. If I remember correctly it reengaged all the cylinders. Just like if I had touched the throttle.

I guess I need to read up more often. I knew about the AFM but, I didn’t realize that it was different than DFM. I assumed GM just wanted to call it something different again. So, I didn’t know that it could disable more than 4 cylinders. That could easily explain why I didn’t have a loss in RPM but, there was no discernible sound. Although, it seems like I would have noticed this kind of quiet before the muffler and CAI install. Maybe I was paying closer attention because I was test driving it for that purpose and now I’m conscious of it now. Or maybe the increase in airflow and decrease in back pressure is allowing it to go to 1 cylinder in situations where it didn’t before.

Thanks for the info.

 

[Marky Dissod]

Your '19 has V4 mode (still?)
Your '21 has V8 mode, V7 mode, V6 mode, V5 mode, V4 mode, V3 mode, AND V2 mode.
(Not sure about 1-cylinder mode; the single cylinder would need to change each time to balance crank stresses.)

GM designed the OE exhaust to minimize audible detection of the mode changes.
Your exhaust is likely making it more audible.

If your mods released more power, then yes, that might allow even more use of cylinder deactivations.

I do not believe the ECM allows use of DFM in neutral, but I could be wrong.

In any case, are you considering the disabling of the AFM / DFM subroutines in your vehicles?
How long do you want to keep your '21 Denali and your '19 Silverado?

 

[GMCnewbee]

Our 2022 Denali is very quiet at idle. I was watching my wife back up the driveway and noticed that when she picked up the throttle just a little you hear the motor but with no throttle you basically hear nothing. I happen to like that because I like a nice quiet idle. It does make the great V8 sound that we all like under power, but I like the stock setup better than a louder performance exhaust.

 

[Chad G 1979]

As these rigs coast to a stop, even from higher speeds, they basically shut off the cylinders and it is dead quiet until you slow down to around 15-18 mph or you press the gas pedal and then you can hear the cylinders start up again. Mine has done this since nee so i assume itz part of their fuel saving strategy

 

[Marky Dissod]

As these rigs coast to a stop, even from higher speeds, they basically shut off the cylinders and it is dead quiet until you slow down to around 15-18MpH or you press the gas pedal and then you can hear the cylinders start up again. Mine has done this since nee so i assume it's part of their fuel saving strategy.

It's formally known as
Deceleration Fuel Cut Off - DFCO.
If the throttle drops quickly enough to 0%, the ECU closes the fuel injectors.
While DFCO is in effect, the road spins the engine, not spark and/or fuel.
DFCO is unavailable under 18MpH (AFAIK).

 

[ZKWBQD]

Hello everyone,
First time posting on this or any other forum.

I recently purchased (my wife’s vehicle) a 2021 GMC Yukon Denali with the 6.2 V8. It was a little sluggish for a 6.2 and so I wanted to free it up a little. My daily driver is a 2019 Silverado with the 6.2. So, I have an idea how much more power it should have. Yesterday 7/25/23 I took it to a local muffler shop and had a Borla ProXS (40359) muffler welded in, the resonators and cats and everything else are still there. (NOT a full cat back exhaust). At the same time my CAI (K&N 63 series FIPK 63-3110) arrived and I installed that as well. I’ve done this to almost every truck or SUV I’ve owned. Including my current Silverado. Side note it sounds awesome and is much more responsive, unlike all the 5.3s I’ve had where I’ve hardly noticed a difference in performance…

What’s strange is after test driving it, it seems like when I come up to a red light or stop sign and let off the gas to start to slow down (from about 20-30mph) the engine goes completely quiet (as though the auto stop feature has engaged but, that’s not supposed to happen until you come to a full stop). The tachometer still shows around a 1,000 rpm. Now, it could be that it’s in v4 mode and it’s just idling so low that it sounds like it’s not running but, it sure sounds eerily quiet. I mean I can’t hear any engine noise, at all. As soon as I touch the gas I can instantly hear the exhaust note again. I can’t tell if it’s starting or if it’s just reengaging the additional 4 cylinders. Since the vehicle is moving, the wind noise and/or road noise is loud enough that I can’t tell if the engine is starting again or if it’s running the whole time. Like how you can hear it start when you let off the brake after being completely stopped. I pushed the button to disengage the Auto Stop feature and no change, the engine is still so quiet (while coasting/braking to a stop) I can’t hear if it’s running or not.

Has anyone else experienced this? Am I just imagining this? Is there some reason why the Auto Stop feature would engage while the vehicle is still in motion now that I’ve altered the exhaust and intake? Or is it actually just running so quietly that I really just can’t hear it at all (immediately after installing a much louder muffler)?

Most people I know will PERMANENTLY disable the auto stop-start function. There are several devices on the market that do this. There's one that mounts under the hood that completely takes it away. My preference is another product that mounts behind the dash. It is super easy to install once you pop off part of the dash. A YouTube video shows you how to do this. On my GM vehicle it will permanently remember how I've pushed the override button for stop-start. Every time I start my car, I instantly get a display showing that stop-start has been disabled. Remember, this technology does NOTHING to improve gas mileage. It only helps with the unrealistic tests done by the EPA stop-start technology will wear out your engine faster, the sooner you get rid of it the better.