Raven65

Hi all,

Shiny, new (55-yr-old) member here. Just bought a gorgeous 38k-mile '12 GS 6-speed manual coupe a couple of weeks ago - which is my first Corvette - and the attainment of a lifelong dream. I'm LOVING it so far. It has the dual-mode exhaust option (NPP), but it is currently stuck on LOUD. Don't get me wrong, it sounds GREAT, but I'd like to at least have the OPTION of going quiet if I want. I'd actually like to have a switch choose the mode (and I see one is for sale in the vendor section right now), but first I need to know why it's stuck open. I assumed the fuse had been yanked, but it is there - and it's a good fuse. The original owner had replaced the entire audio system with aftermarket stuff - a Kenwood double-DIN head unit, a JL Audio amp behind the passenger seat, all JL Audio speakers and subwoofer in a custom enclosure in the right rear corner of the hatch... so there's non-factory wiring going to the fuse box. I hope they didn't cut the wires to the NPP instead of just pulling the fuse. What-all/where-all should I check???

Thanks!

cdm747

I was going to say check the fuse but you already did. You could check to see if there is vacuum where the lines connect to the actuators.

twinarrow99

My bet is bad NPP solenoid. Same thing happened to me. Behind right taillight if I recall.

Raven65

Hmmm... how do you tell if it's "bad" or not? Is there a way to test it?

Landru

Start at rear of manifold where vacuum originates, track back to cans.
Vacuum fitting located rear of manifold's been reported to crack.
Cracked it loses its ability to maintain vacuum.

Then, there's the solenoid. Not unusual one craps-out.

Also OP do you know for certain it didn't have a M2W, installed?
Should've received 2 xmitters, the 'switch' you're looking for, however often enough the fobs get lost in the shuffle at sale.

Landru

A vacuum generator hooked-up to correct side of solenoid, per manual's troubleshooting steps, w/could check solenoid viability.
Not complex yet kind of a PITA, IIRC. Working in the blind, by feel.
Solenoid's located behind rear facia between tail lights and one must be removed for access to the little gem.
Bet there's uTube video(s) detailing a DIY, as I said it isn't complex aside from working blind.

Most shops probably follow The Book, or something similar.
Dealer might cut to the chase & replace solenoid, owner pays either it works or not.

Raven65

I only got one transmitter/fob with the car. Already have another one that I need to program. I believe the M2W plugs into the fuse slot where the NPP fuse goes, right? If so, it doesn't have one... there's just a stock fuse (that is not blown) in that lower/right slot.

I just found a real good thread discussing the issue... should have searched more before posting. Appreciate the help though! I'll post up the resolution once I get it figured out.

https://www.corvetteforum.com/forums...d-fried-2.html

AdrenalinRides

Have the exact same issue. Great write up on diagnosing and replacing here: https://www.corvetteforum.com/forums...placement.html

C5forGreg

And here is yet another thread - https://www.corvetteforum.com/forums...t-working.html - as you can see, this comes up quite a bit. Most likely the solenoid and easy to swap out. Mine was stuck open, with a good fuse and swapping the solenoid fixed it. Now I have a M2W switch on it that I keep open most of the time anyway.

Raven65

Well, I almost ordered a new solenoid assuming that had to be the problem as it seems to be very common, but it seems in most cases, when the solenoid burns up & shorts out, it causes the fuse to blow - and my fuse is intact, so I suspected a vacuum line problem instead. I finally got a chance to look around a bit today... starting under the hood... and I found my vacuum line severely deteriorated. Literally like... soft... rotten... falling apart. I'm guessing the heat got to it?

I found this:


In the area indicated here:


It's hard to tell from those photos, but the deteriorated section is the same line that leads up to the fitting with the green locking tabs.

I have questions:

1. Where in the world does the other end of that line end?
2. How do you get to the other end? (from under the car?)
3. Should I buy the GM part (anybody know the part#) or just get a length of vacuum line from any auto parts store to replace it?

Thanks!

Landru

Yup, believe you found the issue.

Vacuum's generated by engine, vac's plumbed to solenoid, then from solenoid to actuators.
Precise routing used IDK, however IIRC there've been diagrams posted showing location.
Just have to find those threads.

Trying to source proper part(s) via dealer might be a PITA, depending on dealer parts dept and being gouged a certainty.
OTOH there's nothing specialized about required parts, need properly sized tubing & an appropriate connector.
Pneumatics uses such components where vacuum/air pressure is required.
If you're the handy type, a fairly easy DIY.
Your call.

Repair line, NPP will function.
There's still time for solenoid to fail, later.

Raven65

So, I take it this vacuum line goes to the back of the intake manifold, correct? Can it be reached without disassembling anything? Is it just a friction fit on the manifold or does it have some kind of clip-connector like the other end at the firewall?



Thanks

Landru

Know I've personally not done this job, I do recall others who did.
IIRC access is a genuine PITA simply due to close quarters, can be done though.
Better be the patient type.

While removing manifold may make work easier, it's also more work overall as well more chances to screw something up.
Pick your poison.

Did a search using 'C6 NPP routing' and turned this, and a lot more info:
https://www.bing.com/search?q=c6%20n...D5287F9C047540

See if there's anything helpful perusing through the links.
Will learn everything A to Z re C6's bimodal exhaust, how it works.


Appears tube(s) are press, slide over flared ends of plastic joint.

Raven65

Thanks for the reply! Yeah, I've done tons of searching already... on here, YouTube, and all over the 'net... but haven't found anything specifically about attaching that vacuum line to the back of the manifold - only stating that's where it goes (and it took quite a while to find THAT much). I guess it's obvious to most people who've been working on LS engines for a while, but I'm brand new to them, so I have a lot to learn. My last V8 was the DOHC 4.6L in my '96 Mustang Cobra... which, of course, was a whole different world from the LS... and that was a long time ago anyway. Anyway... Thanks again.

449er

Raven, its just easier to leave the flaps open & let her sing

Vet Interested

Nice job Landru

Raven65

Oh, no doubt. Don't get me wrong... it sounds GLORIOUS and I love it - and it will be that way the majority of the time... but I would like to have the option to be quiet if/when I choose.

oldcanuck

Not trying to sidetrack this post but I need to tell you what happened to me.
I ordered a mild to wild switch even though my exhaust always sounds pretty good.
Got the switch in mail yesterday. Went to install it today.
Pulled fuse. No real change in sound.
Put in switch. Pushed buttons on both remotes.
No difference in sound. Oh darn. Must have gotten a bad switch.
Put fuse back in.
Crawled under car (hard for this old guy) to see if actuator may be stuck.
Guess what?
I have an after market exhaust so no more oem NPP. Never would have guessed.
Are you sure you have an NPP.
Just wondering.

Raven65

Yeah, I'm sure mine is stock. I can see the actuators above the mufflers from the rear and I can see the butterfly flaps (open) just inside the inner exhaust outlets. They're there... just not functioning - I assume due to the severely deteriorated vacuum line under the hood. There's certainly NO vacuum getting to the rear.

dr_gallup

Have to think you have a pretty big vacuum leak too?