I have a 2024 C8 and the AFM sometimes gets in a situation where I can hear the engine lugging. If you ever owned an old school manual and tried to accelerate in too high a gear, that noise.

It happens somewhere between 1000 and 1500 RPM at low load. I can get on the gas or get off the gas to stop it. Anyway...

Is there any known solution for this besides running in Manual or Track mode all the time? I don't mind AFM except for this issue. I know it's not healthy for the engine. Pretty sure it's low speed knock. When the warranty is out I will get the ECM unlocked and turn AFM off if there isn't any other options.

Update. I have determined it is not coming from the engine, it's some sort of rattle. Doing some research it seems C7's had a similar issue that was usually related to a weak serpentine belt tensioner.

 

My C7 would do this occasionally...typically b/c I got a tank of crap gas in the boonies. That's the first place I'd start.

I'm sure you're already running premium, but it may be time to switch stations/brands as not all pump gas is created equal.

 

That's an excellent thought. I can't be sure but I believe it spans multiple tanks of gas and different stations. But I'll pay more attention now.

 

Before my ECM was unlocked I found it easier to bypass the V4 than using the console dial was to use the Z button set on track on the steering wheel. Nothing else besides unlocking the ECM works.

Personally if I had a new 24 C8 in case you have any strange early issues since they usually occur early, I would keep the warranty for the first 9 months. Before the unlock take up GM on the free oil/filter and DCT filter and hydralic flush.

In the next 9 months add some mods to get ready for the tune. After 9 month I would go for the ECM unlock E85 Flex Fuel Tune. The ECM unlock will also delete the annoying cold start.

 

Accelerate harder to make the transmission downshift. At 1000 rpm in anything but 1st, or maybe 2nd, you ARE lugging it.

 

Try using Z Mode with powertrain set to Track.

 

AFM should switch back to V8 mode once you add some throttle or the car starts to climb out of an almost unseen dip in the road. You might not be hearing lugging sounds it could just be normal V4 operation since the engine doesn't sound very good when running in V4 mode.

The engine should be perfectly happy running at 1000 rpm. All Corvette engines have sufficient torque at 1000 rpm and WOT to accelerate quite well.

Bill

 

Agree with Bill. V4 sounds funny, but in the situations I hear it, it isn't lugging.

 

I agree with both of the above. The C8 will NOT lug - if it does there is something wrong and you should take it to the dealer. However, if you are not accustomed to the C8 and AFM mode you might be hearing the normal AFM sounds. Lugging is something you can feel as well as hear and the computer in the C8 will not allow it unless something is wrong.

 

Your description is interesting. Many say the V4 mode transition is seamless and they can tell no difference in V4 mode. I believe them, but some like to suggest if you can hear it, it is in your head. That is not the case for me. While I agree that the difference is subtle, and I don't worry about most of the time, I can hear and feel a difference in the powertrain in V4 mode. And for me, it is very similar to the lugging sensation - sort of a very very low subtle rumble - not as obvious as real lugging. My Suburban does the same thing. In my case its not at an RPM/gear that constitutes lugging, it just a change when in V4. (Edit: I notice you mentioned a knock. I hear nothing like that. If you have a consistent knock, that is another issue).

I believe that in automatic mode, and even in manual mode, the car will always downshift to avoid actual lugging. In any case, while it does not bother me most of the time, if I am cruising on back roads at a leisurely pace, I find that it will frequently enter and exit V4 (I hear and feel it - but I use the Track Display theme which does not display the icon regardless of Driver Mode) between going up and down little hills and then short straight sections for road. To avoid it, I now use M under those conditions.

And to avoid it, if that is what you want - I think you listed the ways already: M, or set engine/shift to Track.

 

The dealer said it's normal but I am not buying their explanation as the car did not exhibit this behaviour before.
What my C8 does at < 1500 RPM (any mode except Track of course) is that when it goes into V4 mode (mine can be felt sometimes as it makes a clunk noise) and there's enough incline to provide some load to the engine, there's a faint chirping noise reminiscent of pinging but less loud than pinging and also a womp womp womp like low frequency typical of lugging. I know everyone says to floor it and what not but that is plain indifference to the problem. Since the C8 is computer controlled (the DCT and the motor) it should not allow itself to lug. Especially when you are in traffic that can't just accelerate towards.

 

Doesn’t clicking a paddle engage manual mode and kick it out of AFM?

 

Yes. Temporary Manual Mode. Its temporary though.

 

Isn’t lugging temporary? To me if I don’t like the way a computer is programmed to automatically shift, then I’d take over manually to prevent it.

 

As long as the car stays under 1500 rpm (in 5th gear) and has a slight load like going uphill. What I am noticing though is that mine may have gone away now that we are in summer conditions again.
I bought the car in May 2023. I had no lugging. Lugging started sometime in late fall/winter when the temperatures dropped. It seems that now with warm temperatures being back, lugging has gone away again.
Maybe GM algorithm does not take temperature (or the effects of temperature) under account or it could be the fuel. In the northeast they switch to 'winter blend' at some point and then back to normal again. The octane rating does not change though, I use 93.

 

Edit: this is not a response to the post above. I was writing it when it was posted. I'm not disagreeing with anyone that is sensing actual lugging - I'm just presenting some stuff on how it is supposed to work.

I do not believe the car can lug (if it is working right) and I do not believe that it lugs in AFM -not if it is working right. It is supposed to force a downshift even in manual shift mode. OP mentioned hearing knock. I never hear any engine knock, not even briefly. As I described in post 10, in AFM , I can sense something in the drivetrain the is reminiscent of lugging, but it is not lugging. And most do not even notice it.

As for actual lugging, the manual give no definition but says don't lug and don't let the engine labor. It is never defined, but refers to driving car in a gear that is too high for the speed and especially trying to accelerate like that. The resulting sensation is a low rumble and shaking sensation. Difficult to achieve with the C8 even in manual mode. It will automatically downshift, even in manual mode. A clue can be had to what lugging might mean by looking in the manual for the minimum upshift speeds for the Z and non Z car. Upshifts are inhibited below those speeds. Presumably the car will also downshift below that speed if the gear is too high. Using those tables in the manual, and table of speed vs RPM for each gear, for the non Z car, I calculated about 1100 RPM for first gear, and just under 1200 RPM for all the others. For the Z car, I calculated 1000 for first gear, and a little over 1200 for the other gears. The differences might just be rounding errors - the speeds give in the manual are to the nearest 1 mph - and the manual says they are "approximately" the listed speed. But around 1200 RPM +/- for all gears except first. Some may consider that lugging already. I won't disagree - its just what does. And, it may be that there is some hysteresis between the upshift allowed and the downshift required - probably is - so the automatic downshifts may be a touch lower than these numbers. I have never actually watched it while driving.

As an example, the table in the manual says a minimum upshift for the Z car into third is "aproximately" 16 mph. Using tables for speed vs rpm, that corresponds to 1250 RPM. For upshift allowed. Automatic downshifting may be a touch lower.

Here is the Z table.

 

The car actually has knock sensors and the Service Manual claims it will adjust the timing to eliminate the knock. It says it always seeks to return the timing to the nominal advance. There are limits to the adjustment it can accomplish, which is, I suppose, the reason the minimum recommended octane is 91. As you probably know, the manual actually says "If heavy knocking is heard when using gasoline rated at 91 or greater, the engine needs service." This kind of implies that you might hear some occasional light (whatever that means) knocking with 91. I just don't know about that with 93 octane though. If you hear it much with 93 octane it might be an issue - or - try a different source for the 93 and see what happens.

 

 

Anyway, I just came back from a ride. 90 degree weather and 93 octane. The car was chirping a lot, it even did it on the highway in 8th gear very slight incline (while in V4 mode of course).

 

So, we have lugging, knocking, and chirping. What is chirping? Mine sometimes makes what I would call a chirp as it transitions between modes, but not always. I think it is the sound of exhaust flow control valves in the muffler opening or closing - they occasionally squeak. I can sometimes hear them squeak just sitting and change the engine sound setting. And some hear a "swoosh" perhaps like air, but only at transition. I don't know where that one comes from. Some say its from actual transition of the valve train. I think it might be just exhaust gas rushing past the valves as they close, but I do not know. Are you hearing something else - a different sound?

If I am on a very very slight incline in AFM mode, I can sense that feel that is reminiscent of lugging (low rumble) but it is very subtle, not pronounced at all and could easily go unnoticed. It is nowhere near the actual sensation of lugging from yesteryear. And no knock. Anything more pronounced should not be evident - its designed to be seamless - and many/most say it is. Any more than very very slight incline or even just slight increase in throttle angle and it will pop out of the V4 mode.