My 2016 C7 (Z51, M7, Mag Susp and NPP exhaust) has 23,000 miles, mostly short trips to the grocery store, gym, golf course. Recently a load "clanging" noise started appearing intermittently. After some investigation I visually confirmed it is the two NPP valves on the outboard exhausts repeatedly slamming shut, opening, and slamming shut.

Issue occurred first time starting the car in stealth mode after an hour long stop at coffee shop. It now occurs on about 25% of cold starts in stealth but has also occurred while driving with exhaust in track mode. When it starts clanging away while driving, changing exhaust modes (e.g. from track to stealth or touring and back) or simply hitting the gas makes it stop.

My dealer checked for codes and there are no saved codes. I didn't have time to leave car for a tech to start extended testing, but that is the only option I know of to try next.

Any ideas on what could cause these symptoms? Thoughts on next step(s)?

 

Sounds like the NPP valves just need to be replaced.

 

I caught the issue on video. This seems pretty unique, haven’t seen any other discussions of similar issues.

Any thoughts on root cause (bad sensor(s), corrupted software,...???). Having trouble understanding how this could be failing actuator.

 

Do an "Advanced Search" in the entire C7 section of the Forum for NPP valves. There are MANY relevant threads. All the best.

 

Thanks to all the previous authors of NPP explanations, very helpful. Does anyone have a wiring diagram and/or logic table that provides details of the sensor inputs that command the NPP valves to close?

I am hoping to at least narrow down the potential causes of my NPP problem before I hand it over to the dealer and say "please fix it". After a fair amount of testing I can confirm that 1) upon cold start-up in stealth the passenger side NPP valve often (but not always) bangs open/shut repeatedly, but if mode is switched to track the valves stay open, and close normally when switched back to stealth; 2) at idle the valves close and open as expected when switching modes between stealth and track, 3) when driving one or both valves sometime (but not always) start banging open/shut/open etc when in track mode but off the throttle - blipping the throttle to accelerate or downshift stops the problem.

What I would like to determine is if the issue is some sort of fault in the actuator itself, or are they receiving a corrupt/faulty signal. If it is a corrupt signal, is that due to a bad sensor or a fault in the ECM?

 

Looks like the passenger side has broken free of the solenoid and is just fluttering in the exhaust.

 

OK so the symptoms are changing a bit, perhaps the issue is progressing some (or maybe I am just starting to understand what I am hearing and seeing a bit better). The valve on the driver's side seems to work normally. As seen in the video the passenger side valve tries to close on command but re-opens, and this sequence repeats itself. Or was.

At idle in track mode, if I switch the exhaust to stealth there is now one loud click but the passenger side valve flap remains open. I can reach in the exhaust and push it closed with a long screwdriver, and it stays closed (at least at idle). If I then change modes to track it will open, but upon changing modes back to stealth I get the one loud click again and the valve flap stays open.

So my non-expert diagnosis is that a fault is mechanical and within the valve actuator or part of the valve stem. What I don't know is if there is also a control system fault that keeps trying to cycle the valve, and if that repeated cycling and banging snapped something in the actuator (e.g. valve stem). Or if that cycling was caused by some sort of issue within the actuator itself.

Has anyone taken one of these apart? I can of course take it to the dealer and pay for a complete replacement, but that wouldn't be much fun!

 

Sounds like a bad actuator or electrical connection. I found the part number for the actuator 84560761 a bit costly around 135.00 Hope this helps..

 

I removed the actuator and there must be some broken/stripped teeth on gears in the actuator. Unfortunately it is a sealed unit so I can’t open it up.

So next step is buy new one. Priced at dealer from $205, so online for $135 works for me. But there are so many online GM OEM parts. I tried calling 4 or 5 to confirm stock and delivery time, couldn’t get through to a single one.

Any recommendations on which online parts supplier to use?

 

For a general part like this, I’d try Rock Auto.

 

Unfortunately Rock Auto doesn’t carry it

 

Cheapestgmparts.com has it listed You can ck. with them

 

So I purchased a new actuator from GMOutletparts.com, which is Ed Rinke Chevy in Michigan. Actuator arrived and I was able to install and connect fairly easily, only tool required was 8mm wrench.

PROBLEM: New part does not work. I reconnected old actuator and it dutifully turns when commanded (stripped plastic gears and all) but new actuator does not respond at all. Thinking it was defective I ordered another and prepared to return the first new one. One of my golf buddies mused that perhaps the ECM needs to be programmed to recognize the new actuator. BINGO! Called Chevy and a Tech confirmed they need programming.

Does anyone know what is involved with programming of ECM to recognize a new NPP actuator? Having gotten this far in DIY mode I hate to give up and take it in to dealer.

Pictures of new actuator attached along with picture of the valve stem interface for anyone who is interested.

Outboard side of NPP actuator

Inboard side of actuator showing valve stem drive

NPP valve stem interface

 

I think it's one of these:

 

Final step was getting the ECM to “relearn” the exhaust valve actuator. Fairly simple process as described in post from emperors61 above (Tail Pipe Exhaust Flow Control Valve).

I am fortunate to have an excellent small shop just down the road, they plugged their machine into the data link connector and followed the valve learn process. Cost me $40 (dealer wanted $165).

So, being the curious type and also being in possession of a Dremel cutting tool, I opened up to original actuator. In the pictures below you can see the drive motor and gear. Might be difficult to see in photo but two teeth on the smaller gear are missing.

In the photo of the inside of cover you can see a “rocker” gear (half of a full gear) that is driven by the smaller gear. One tooth is missing from this gear.

So my explanation of the original symptom (repeated banging sound) is that when commanded to close by the ECM the actuator would engage and drive the valve almost closed until the damaged parts of either gear came in to play. At that point the actuator loses its grip and the spring on the valve stem rotates freely open (and bangs open!). The motor continues to rotate, the gears re-engage, and the valve starts to open again, until a damaged tooth comes in to play. Rinse and repeat.

Again many thanks for the comments and suggestions, we got it done!

Two broken teeth on drive gear

Broken tooth on “rocker” gear

 

I’m also having a similar issue and planning to fix it myself.
Do you know what kind of scan tool was used for the valve relearn process?
I’m trying to find a scan tool that is capable to do that but it is difficult to identify.

 

I was dealing with this as well for the last few months. I just unplugged mine. Good to know how to fix it now. I like the stealth mode feature sometimes.

 

Sorry but I did not investigate scan tools. I have a very simple code scanner which was not sufficient, so local mechanic's offer to scan for $40 seemed the most cost-effective route for me.

 

Broken tooth on “rocker” gear

being part of the exhaust, why did GM use plastic parts for these gears. Something so easy to fix and they did nothing.

 

Its all about the money