dckotwicki

I own a 2016 Z06 with 3500 miles on it. Recently went into northern Michigan for a weekend get away. The roads are smoother and quieter with less traffic noise. Some roads have rumble strips imbedded into the side and center of the road to get your attention when you wander left or right. These get your attention big time. During the drive, I began to notice a similar but quieter noise. I began to study the road surface but could find nothing to connect to this noise. I started to look elsewhere for the source and discovered a direct connection to the AFM (active fuel management).

With the cruise control on and on a relatively flat road I began to notice this subtle rumble noise (similar to hitting rumble strips but much quieter) occurring when the engine was in 4 cylinder mode. When it went back to 8 cylinder mode, the noise went away. The noise always happened before returning to 8 cylinder mode. The duration of the noise ranged from 1-2 seconds to several seconds (5-10 seconds) and always disappears once the engine is back to 8 cylinders.

I am not a mechanic and do not know the mechanism that causes the engine AFM to activate, but I wonder if there is something acting up in the system that activates the AFM.

Anyone experience anything like this or have any possible explanation for what is happening?

The engine had the 500 mile oil change at the dealer and again at 1000 miles. Both with Mobil One. Oil level is where it should be and shows 65% oil life.

sTz

Edit: I heard something similar, but found that it was actually the road...

BOBSZ06

Yes! I have an A8 and experience the same noise when seeing the engine move from 8>4.

I believe it to be the belt tensioner vibrating when the 8>4 switch is made for which there is a TSB.
I am waiting to see if it starts leaking oil before taking it in for replacement.


Here is a link to the PIP 5185A...rattle or flutter, which describes it just fine.

http://www.stingrayforums.com/forum/...-pip5185a.html

subfloor@centurytrans

I also experienced this for the first time last Friday and when I first heard it I looked around me on the freeway expecting to see a diesel truck that was accelerating.

subfloor@centurytrans

I don't think the TSB accurately describes the problem, because the noise occurs right up until the car goes from V4 mode to V8, not when the transition occurs.

RichieRichZ06

I think the "rattle" from the belt tensioner is a separate noise and usually only effected earlier Stingrays. I have not witnessed that issue on 2015 or 2016 models yet.

I'm also curious as to what this noise is. Subfloor and another C7Z customer have been experiencing this and I can't find any TSB or other info from GM.

BOBSZ06

Mine only occurs when I see the V8 > V4 transition occur and lasts a few seconds. If I hit the throttle a little to go from V4 > V8, it stops immediately.

All occurrences during the transition stages. Yes, it can last longer but not always while in V4.

BOBSZ06

For those that wish to replace the belt tensioner:


Parts:
http://www.trunkmonkeyparts.com/auto...d-pulleys-scat

Install video:
https://www.corvetteforum.com/forums...uteginger.html

sTz

I only experienced this anomaly once, but will keep an eye on it. Thanks for the information

sTz

I just read the thread and viewed the vids posted by Bounty Hunter 6 (provided by BOBSZ06 above). He described the noise as a "whining sound, very much like what a car with a bad power steering pump would make, but increasing with throttle whether under load or not."

While the tensioner may still be the culprit, this is nothing like what I heard or experienced, or what the OP described. We might be talking about two different things...

I have an M7. I was in Touring mode and not in Ecco mode, so my car shouldn't have been sequencing from 8-4 and vice versa, right?

BOBSZ857

I have noticed this as well on long drives while cruising alone very smooth roads. I have an A8 and the noise seems to be coming from under the dash on the drivers side or the front drivers side in engine bay.

BOBSZ06

Correct, the noise in that video is not what this discussion is about.
The videos are for the replacement of the belt tensioner only.

dckotwicki

I have no mods at all.
Note: My noise always happens while in V4 mode and always before returning back to V8 mode.

Also: People seem to be connecting AFM (Active Fuel Management) to the performance exhaust. AFM is the system that toggles the engine between V4 and V8 mode for improved fuel economy.

What I am observing seems to be related to the Active Fuel Management system and not the exhaust system.

C7/Z06 Man

The light rumble strip sound happen to me today, coming home from having the airbag reprogram done. I had not really noticed it before. I have the A8. For me cruising around 69 mph the sound would happen on level road at very little throttle, if I increased the throttle by the smallest amount or let off the gas the sound stopped.

I was concerned because the dealer was "just in" the ecm for the airbag recall but after reading this I think the OP is right on the money! I will be checking this out again when I go for a drive later.

sTz

Unable to duplicate... Maybe it was the road

Edit: Confirmed, it was the road.

OP, good luck finding a resolution

7.0ltrpower

... could it be the small torque tube vibration in Eco-mode... Tadge discussed long ago... Saw in video..

gve

Mine did that also it's not good having your engine only run on 4 cylinders, that's why it vibrates under light load before going to V8. I bought the Range AFM eliminator which keeps it in V8 all the time. A 650 HP engine will not run smooth when exposed to light acceleration under load and only firing on 4 cylinders. The AFM is the worse thing for this 650 HP engine, plus they tend to use oil after you get more miles on the engine. I was told by the Corvette museum when I picked up my car to keep the A8 in Manual for the first 500 miles during the break in so it fires on all 8. Google Chevrolet AFM oil consumption and you will find more info.

C7/Z06 Man

Well I went for another drive including down the same road.

There is little doubt in my mind it is the AFM in my case.

As mention here there are products on the market that remove AFM.

wagoetzmann

I drive my A8 to work on a very smooth, hilly, 2 lane with very little traffic. With cruise control set at 62 mph every downhill coast the engine will go into V4 mode then at the bottom of the hill back to V8 mode. When going from V4 back to V8 the engine makes a farting noise, the car shutters and I see the vacuum gauge jump. I believe it to be the AFM valve lifters pumping back up. It is very annoying to say the least.

subfloor@centurytrans

You have to listen fairly carefully with the sound turned up, but in all three videos the growling sound it makes right before switching back to V8 mode is definitely noticeable. Please let me know if this is the same thing that other people have been experiencing.