Rpm dropping oddly

alkaspeltzar · February 22, 2017

Curious if anyone else can try this or has noticed this on their 2014-2016 5.3 Silverado.

When the truck is warm, after driving, if I rev the rpms to 2000 or similar in Park, and let my foot off the gas, they will drop suddenly but the then slowly at 1200 rpms down to idle.

ITs like the computer doesn't let the rpms drop evenly from 2000 to idle

Why would GM do this? What is the point?

Is it to prevent the engine and tranny from slamming in gear? OR some system protection to let the rpms fall fast but then slowly to protect the engine.

Thanks for your comments

redwngr · February 22, 2017

Or i it just the tach?

Sierra706 · February 23, 2017

Either the mass of the flywheel or software keeping it from bogging down

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dorkweed · February 23, 2017

Why are you "revving" it in "park"???

alkaspeltzar · February 23, 2017

Well I was getting an idle drop at stop lights that would make truck shake but then it would recover. So I was messing around one day and noticed this behavior while lightly revving it. What I am curious is if it is normal and other see this?

alkaspeltzar · February 23, 2017

Either the mass of the flywheel or software keeping it from bogging down

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Would you be willing to see if yours does similar?

1kwik72 · February 23, 2017

Completley normal and part of the fly by wire Throttle Actuated Control (TAC) programming. They just don't want to slam the throttle blade shut as on these there is no Idle Air Control (IAC) on the TAC equipped vehicles. On older vehicles the throttle blade was connected to the gas pedal via a cable and had a set screw adjusted to just slightly open the butterfly. The idle was controlled by an IAC which was a stepper motor with a pintle (pointy shaped tip) on it that would extend and retract in a corresponding orifice that allowed air to bypass the throttle blade. When you stepped on the gas the IAC would lift off it's seat to "catch" the idle on the way back down as the throttle was released. Because the sizing on the orifice was about as big as your pinky finger it could respond pretty quickly. If the valve was seated all the way in not bypassing any air the engine would idle pretty low usually about 200-300 below what the computer would target, if it was fully open with the throttle blade closed it would idle about somewhere about 2000 or so. That gave the computer a pretty sensitive and far ranging idle control. On TAC vehicles the position of the throttle blade dictates minimum airflow and idle speed. If you think about how the area increases as the throttle blade is opened it changes pretty drastically close to where the blade sits at normal idle speeds. They slow down its closing close to idle to make sure the vehicle doesn't stall. It happens while driving also but not as noticeable as when it is being revved with no load.

Sierra706 · February 24, 2017

Would you be willing to see if yours does similar?

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Sent from my iPhone using Tapatalk

Completley normal and part of the fly by wire Throttle Actuated Control (TAC) programming. They just don't want to slam the throttle blade shut as on these there is no Idle Air Control (IAC) on the TAC equipped vehicles. On older vehicles the throttle blade was connected to the gas pedal via a cable and had a set screw adjusted to just slightly open the butterfly. The idle was controlled by an IAC which was a stepper motor with a pintle (pointy shaped tip) on it that would extend and retract in a corresponding orifice that allowed air to bypass the throttle blade. When you stepped on the gas the IAC would lift off it's seat to "catch" the idle on the way back down as the throttle was released. Because the sizing on the orifice was about as big as your pinky finger it could respond pretty quickly. If the valve was seated all the way in not bypassing any air the engine would idle pretty low usually about 200-300 below what the computer would target, if it was fully open with the throttle blade closed it would idle about somewhere about 2000 or so. That gave the computer a pretty sensitive and far ranging idle control. On TAC vehicles the position of the throttle blade dictates minimum airflow and idle speed. If you think about how the area increases as the throttle blade is opened it changes pretty drastically close to where the blade sits at normal idle speeds. They slow down its closing close to idle to make sure the vehicle doesn't stall. It happens while driving also but not as noticeable as when it is being revved with no load.

Any way to reprogram this behavior? Something that can be done with tuning?

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SavageSierra · February 24, 2017

Well I was getting an idle drop at stop lights that would make truck shake but then it would recover. So I was messing around one day and noticed this behavior while lightly revving it. What I am curious is if it is normal and other see this?

Did you ever find a solution to the idle drop? My truck is doing the same thing at stop lights and it makes the truck shake then recover just like yours.

alkaspeltzar · April 18, 2017

Did you ever find a solution to the idle drop? My truck is doing the same thing at stop lights and it makes the truck shake then recover just like yours.

No I didn't. next Thursday I am dropping it off for the service manager to drive for a day. Feels like when the idle drops the truck is going to stall and die. I hate that I take care of this thing and I amgetting plagued with this dumb stuff.

joshc64311 · April 18, 2017

No I didn't. next Thursday I am dropping it off for the service manager to drive for a day. Feels like when the idle drops the truck is going to stall and die. I hate that I take care of this thing and I amgetting plagued with this dumb stuff.

Lol i know what you mean, my second vehicle was a civic. I beat the living shit out of that car, it lived at red line for 200k . Only thing it needed was gas and an alternator.

Spencer Bailey · July 25, 2023

On 4/18/2017 at 4:03 PM, alkaspeltzar said:

No I didn't. next Thursday I am dropping it off for the service manager to drive for a day. Feels like when the idle drops the truck is going to stall and die. I hate that I take care of this thing and I amgetting plagued with this dumb stuff.

Did you finally get it figured out? Im having that issue now.