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Had my first track day with my 17 GS this past Saturday. Five 30 minute run sessions at PBIR in Jupiter, FL on a relatively cool day (ambient max's at around 75). Sessions were spread out from 9.30am to 5.30pm with plenty of cool down time. Everything was fine at the track, pressures and temperatures. Car ran great! On the way home from the track, the check engine light came on. Stopped at a gas station, fluids all fine, temperatures and pressures OK, no noises. Scheduled a maint appt for Thursday am. My wife took the car out for a quick spin today, she didn't get more than a minute down the street and all hell broke loose. Very loud banging sound from the rear of the car. She immediately crawled back to the house and parked it. Anyone out there have problems after a track day? Sounds like the rear diff? Thanks!
 

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Do you have an automatic transmission? I wonder if its an AFM exhaust valve malfunction. Sometimes that happens after heavy track use.
 

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Do you have an automatic transmission? I wonder if its an AFM exhaust valve malfunction. Sometimes that happens after heavy track use.
I have not heard of this. What is the relationship between whether he has an auto transmission and the AFM system? Wouldn't an issue with the AFM hold regardless of transmission?
 

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New for 2017 ... Manual transmission cars don't have AFM valves.
 

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New for 2017 ... Manual transmission cars don't have AFM valves.
I certainly could be mistaken, but I believe all C7s have the AFM system as standard on their engines (it is clearly listed as part of the 2017 engine specs on the GM build site). However, the system is programmed differently as to when it activates on automatic transmissions vs manual transmissions.
 

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Rick17GS, I've never tracked our C7 masterpiece, but if I were to ever experienced that type of problem I would be heading to the nearest Chevrolet service department ASAP!!

SF
Rick
 

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Rick - please follow-up after your dealer visit and let us know the diagnosis.
 

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I certainly could be mistaken, but I believe all C7s have the AFM system as standard on their engines (it is clearly listed as part of the 2017 engine specs on the GM build site). However, the system is programmed differently as to when it activates on automatic transmissions vs manual transmissions.
Hi Jeff, yes all C7's have AFM, but what I mentioned are the AFM exhaust valves just upstream from the muffler. These valves close down to increase the exhaust backpressure when AFM is active to reduce excessive NVH in that mode.

All Stingrays had these valves prior to 2017, but this year the Z51 and also Grand Sport M7's had those valves removed for reliability reasons. (N.B. Only very early production Z06's had the valves also.. they were removed very early on.)
 
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Thanks!
 
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You aren't very specific about where the banging is from. Is it driveline, suspension, or body? Does the noise change with speed, or turning? It could be a lot of things, (usually none of them good...lol)

But, even race cars occasionally break on the first lap... Sorry.
 

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Hi Jeff, yes all C7's have AFM, but what I mentioned are the AFM exhaust valves just upstream from the muffler. These valves close down to increase the exhaust backpressure when AFM is active to reduce excessive NVH in that mode.

All Stingrays had these valves prior to 2017, but this year the Z51 and also Grand Sport M7's had those valves removed for reliability reasons. (N.B. Only very early production Z06's had the valves also.. they were removed very early on.)
Thanks for clarifying, so how does the NPP operate on the Z51 & GS 2017s?
 

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From the great world-wide web:

Originally Posted by Tadge Juechter
Corvettes have long pushed the technology envelope to make a car that is both supremely fast and remarkably efficient. That often means we are doing things no other manufacturer would try. The combination of true cylinder de-activation (meaning keeping cylinder valves closed, not just turning off fuel injectors as many others do) in a track-ready car is an example. We did do our normal 24 track testing with the AFM valves in place without issue. Perhaps the way we run our test can help folks running on the track avoid problems.

A few important points: The valves are very sensitive to air flow under the car. Anything that restricts that airflow could increase the chance of an issue arising. These would include any aftermarket aerodynamic or cooling aides. Also, lowering the standing height (which many folks are tempted to do to lower the center of gravity) of the vehicle below spec has a measurable negative effect. We always run at least one complete cool-down lap before pitting. This is super-important because the relatively cool exhaust gases running at part throttle cool the AFM valve quickly. Finally, some tracks have noise regulations that cause people to run in stealth mode. That is a relatively recent trend, so we did not validate track durability in stealth mode. It was always intended to be a street feature.

The question cites the “plastic” actuator with the implication that we used non-robust materials in such a hot area. Although the accompanying photos show thermal distress in the glass-filled nylon case, that is an effect of the internal thermal failure , not the cause of it. Actually the internal magnets and electronics are the thermal weak link and there is no material substitution in the world that can be used to fix it.

So what can we do about it? For the 2015 model year we added insulating content into the muffler head to reduce the thermal load on the valve. That was a continuous improvement, but not a “clean kill” for every situation. For 2017, we have eliminated the valve on Z51 and Grand Sport (manual transmissions only). Manuals still have AFM, but the torque levels are low relative to the automatic so the N&V issues are manageable without it. Also, the AFM only works in Eco mode, so it is easy to avoid if you want to. The 2017 manual transmission hardware set is not an easy “bolt in” for prior year cars, so we are studying how to make that solution available. The automatic is likely to need the AFM valve for the foreseeable future.
 

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thanks Mobius. I had not seen that.
 

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Thanks for all the feedback, guys! I have an automatic. Unfortunately I'm traveling and away from home so relying on my wife's description of the noise. She's sure it's coming from the rear of the car and it doesn't seem to vary with engine or road speed, but she only drove it for a few minutes so not sure she really picked up on how the noise relates to engine or car speed. I had read about the AFM problem and thought that might be causing the engine check light. I did run a cool down lap before I finished each session. Car was flat-bedded to Schumacher in North Palm Beach this morning (any south Florida folks out there who have experience with them?), they still haven't had a chance to look at it. I called the Vette specialist, Kenny G, (great guy BTW and I highly recommend buying from them) at MacMulkin in Nashua, NH where I bought it. He thinks it might be a software or fluid issue with the rear diff, but I think he's hoping for a simple fix. I'll let everyone know what the diagnosis and fix is. None of this takes anything away from my track day experience or how much I love this car. If you haven't tracked your Vette yet-- DO IT!
 

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There was a long discussion thread on another forum from someone who had the differential blow up during an HPDE event. The dealer told him it wouldn't be covered under warranty because it happened while "tracking" the car. GM eventually agreed to inspect the rear-end to determine if it failed due to a defect or due to "abuse." He paid for the repair up front, and if they determine is wasn't "abuse" they'll reimburse him. Last I saw he was still waiting. (If you read the fine print in the warranty, it says something to the effect that "track use 'may' void warranty coverage."

You might not want to tell the dealer you tracked the car before it happened.
 

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Discussion Starter #17
The only fault found so far is that one or both of the NPP valves have failed. Still waiting to talk to my service advisor. Has anyone out there had an NPP valve failure? Can i make a loud metallic banging sound if the valve is trying to close?
 

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Thanks for all the feedback, guys! I have an automatic. Unfortunately I'm traveling and away from home so relying on my wife's description of the noise. She's sure it's coming from the rear of the car and it doesn't seem to vary with engine or road speed, but she only drove it for a few minutes so not sure she really picked up on how the noise relates to engine or car speed. I had read about the AFM problem and thought that might be causing the engine check light. I did run a cool down lap before I finished each session. Car was flat-bedded to Schumacher in North Palm Beach this morning (any south Florida folks out there who have experience with them?), they still haven't had a chance to look at it. I called the Vette specialist, Kenny G, (great guy BTW and I highly recommend buying from them) at MacMulkin in Nashua, NH where I bought it. He thinks it might be a software or fluid issue with the rear diff, but I think he's hoping for a simple fix. I'll let everyone know what the diagnosis and fix is. None of this takes anything away from my track day experience or how much I love this car. If you haven't tracked your Vette yet-- DO IT!

It's good to see that this issue isn't deterring you from using the car the way it was intended.


As far as the dealer not covering any issues because of the track, find a new dealer that honors your warranty which does cover non-competitive track driving.

I recently had a transmission leak (2015 M7 Z51) and took it to the dealer, filthy and covered in race rubber, large vinyl numbers & tow hooks. Not a problem covered without question.

To those that don't track the car, I highly encourage you do at least one track day to get an idea of what the car can do. You won't get anywhere close to the limits of the car on your first event and it is exceedingly safe especially for a new driver with an instructor sitting right seat.
 

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Has anyone out there had an NPP valve failure? Can i make a loud metallic banging sound if the valve is trying to close?
Good question, and I did quick look on the web and came across this on another site. It's from an individual who installed a Borla Atak on his C7 with NPP:

"...2 days before I went to reinstall the factory set, I get a check engine light. My son, who is a dealer mechanic, and did the installs brought over the code reader and sure enough, exhaust valve failure. He reset it and we hoped a reinstall of the factory exhaust would fix the problem. Instead, I immediately started hearing a loud clank clank when front valve should be adjusting. $400 later for a new valve, I'm now hearing the same sound, only much less volume and very infrequent, from the right side. I can't figure out if the Borla caused the valve problem or there's something up with the NPP system?"

He also posted a pic from his code reader (below). Now the thing is, that code is for the AFM valve(s) just ahead of the exhaust. AFM valves are a known occasional failure item for tracked cars. In that same thread it was mentioned that the NPP valves don't provide feedback to the ECU and can't generate codes.
img_6133_0e8d8cdd1f6ec4f3cd0e8bf98d197116f3c36dfd.jpg
 

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My NPP valves have also failed after a track day but I just get a check engine light (P12F0 code) no other noise or effect when driving the car.

It was a pain to convict the dealer that it was a NPP valve failure as they claimed that they never seen that... It's a problem over here as the number of C7 we have doesn't help to show reliable failures statistics and I had to insist and print many topics of diverse forums to convict them.
As this failure will certainly come back, my problem now is what GM will do for me after the last warranty year I still have.
 
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