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How to defeat the dreaded AFM

Posted: 10/28/25 12:22pmMessage 1 of 8

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Thomas Koenig Sr

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Clackamas, OR - USA

Joined: 10/13/2025

Posts: 5

Vette(s): 2024 C8 E-Ray HTC Red Mist Metallic

Want to keep your C8 from changing from V8 to V4 and eventually destroying your engine !!

Just select Drive and then Manual and use your Paddles on the steering wheel to Shift up or Down … It’s that Simple 🤠

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Re: How to defeat the dreaded AFM

Posted: 10/28/25 2:45pmMessage 2 of 8

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Adam Wartell

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Eagleville, PA - USA

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💬Thomas Koenig Sr said:

Want to keep your C8 from changing from V8 to V4 and eventually destroying your engine !!

Just select Drive and then Manual and use your Paddles on the steering wheel to Shift up or Down … It’s that Simple 🤠

@Thomas Koenig Sr For those of use that don't know, can you explain a bit more about what you mean by "changing from V8 to V4"? And how/why will that destroy your engine?

Thanks!

-Adam Wartell
NCM Lifetime Member #1222
Founder: Vette Registry

My first Vette, now owned by JB79:

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Re: How to defeat the dreaded AFM

Posted: 10/28/25 3:36pmMessage 3 of 8

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Norsky

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Burke, VT - USA

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💬Thomas Koenig Sr said:

Want to keep your C8 from changing from V8 to V4 and eventually destroying your engine !!

Just select Drive and then Manual and use your Paddles on the steering wheel to Shift up or Down … It’s that Simple 🤠

I would have thought a setting somewhere would handle that. Lord knows engineers love settings...!!! 🤣

Jim Olson

"The Toys"...!!! Save the Wave!

Where I've been in a Corvette...!!!

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Re: How to defeat the dreaded AFM

Posted: 10/28/25 9:36pmMessage 4 of 8

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Adams' Apple

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Duncanville, TX - USA

Joined: 11/8/2003

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Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

GM's "Active Fuel Management/AFM" system allows the engine to run on either 8 cylinders, or 4, by dis-ableing 4 of the lifters in the engine, all PCM/software controlled. Most people hate the system, as I do. It makes the engine run rough at times, especially at slower speeds. In theory, it allows better fuel economy, but in my experience(My '09 Burb has AFM), it does NOT. At light throttle, cruzing speeds, it can feel like there is a mis-fire, or torque converter shudder, but it is the AFM system forcing the egnine into 4 cylinder mode. The system can be "overiden" by using the "manual" mode of the transmission, depending on what trans it has(6-8-10 speed). As far as I know, that will only work as long as you do not allow the trans to shift into high gear, being 6th,8th or 10th.

The AFM system lifters are prone to failure, mostly due to neglect...not changing the engine oil at the recommended mileage, or using the incorrect oil to begin with. Replacing the lifters requires removing the heads...not a fun job. The AFM system can also be programed out by using a HP tuner, or similar system.🙂

GM dint learn their lesson on crap like this from the fiasco of the "4-6-8" DFI Cadillac engines of the '80s....I hated those damn things too....

|UPDATED|10/28/2025 9:36:31 PM (AZT)|/UPDATED|

Joel Adams
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Re: How to defeat the dreaded AFM

Posted: 10/29/25 5:32pmMessage 5 of 8

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Thomas Koenig Sr

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Vette(s): 2024 C8 E-Ray HTC Red Mist Metallic

Okay, have had my C8 E-Ray since January of this year and started out only using Drive and Sport Mode. And of course. AFM and Auto Stop, so I immediately discovered that if I drive in Manual consistently it totally defeats AFM and Auto Stop. Now, I purchased a Range Device for my C7 with an A8 transmission and I didn’t have to worry about AFM or AS.

Unfortunately, they don’t make a Range Device for the C8 ☹️

And No, driving in Manual regardless of your speed and transmission gear 6-7-8 at least my vehicle, never activates AFM.

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Re: How to defeat the dreaded AFM

Posted: 10/29/25 5:44pmMessage 6 of 8

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Thomas Koenig Sr

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Clackamas, OR - USA

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Vette(s): 2024 C8 E-Ray HTC Red Mist Metallic

Actually Adam thought you explained the AFM characteristics rather well !!

Thank you

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Re: How to defeat the dreaded AFM

Posted: 11/2/25 11:04amMessage 7 of 8

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73shark

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Overland Park, KS - USA

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Vette(s): 1973 Orange Metallic Coupe (orig owner), L82, 4 spd (WR), PS, (A/C & PW (I installed from wrecked 73)), leather, AM/FM Stereo, ran with '65 FI unit earlier & will again some day. 2023 Accelerate Yellow HTC Stingray

💬Thomas Koenig Sr said:

Unfortunately, they don’t make a Range Device for the C8 ☹️

And No, driving in Manual regardless of your speed and transmission gear 6-7-8 at least my vehicle, never activates AFM.

Car Byte makes a AFM defeat plug-in module for a C8. I won one and tried it for awhile. It cost about 2 MPG on my 2023 C8. This is my third Chevrolet having owned two Tahoes previously that had AFM. About the only time the Tahoes would go into V4 mode was when you were on the highway at cruising speeds while the C8 goes into V4 a lot more. Personally I could never tell when it went in and out other than the dashboard display. One of my Chevrolet service managers said that they used to replace about two to three engines a month because of it but so far I haven't had any problems with it.