My 2008 coupe has the NPP muffler option and has decided on it's own to "open" when just cruising around.
At first I thought it may be the Sunsetorange Exhaust Commander that I have used reliably for the last three years so I removed it and went for a drive. I ran the car though a couple of gears and everything worked as the General designed it. The exhaust valves opened at 3500 rpm's and closed when I let off of it. A few minutes later on the way home I'm thinking I'll have to contact SSO then again out of nowhere the exhaust valves open up again while just cruising along at 40 mph. The valves remain open until you shut off the car and it sits for awhile.

Any ideas would be appreciated. Thanks in advance.

 

Since open actuators means no vacuum you must have a vacuum leak someplace between the source and the actuators.

Start looking at the easiest spots, the hoses to the actuators and if that doesn't do it you'll have to follow the vacuum lines back to the source.

 

The easiest to fix problem would be if a vacuum tube simply came loose from one of the valve actuating solenoids - just take a look under the rear of the car to see if the tubing is loose or split back there.

I've had 2 vacuum leaks and they were both not that easy to find.

One was a split in the tubing under the hood - it was hidden under that piece of corrugated plastic cover that is sitting on my hand. The picture below shows the area at the back of the driver's side of the engine where the vacuum line comes off the manifold and attaches to the line that goes back to the exhaust valves:




The other leak I had was back near the trans/diff where the line goes from the left side of the car over to the right side and then ultimately back to the reservoir and the valves. The lines coming from the front to the rear are a combination of metal lines and hard plastic tubing. One of the plastic sections melted where it passes over the exhaust. In the picture below I had cut out the damaged section of tubing, and you can see the long split in the tubing caused by the heat of the exhaust:




Good luck finding the problem, but like haljensen, I'm quite sure it's a vacuum leak.

Bob

 

Why would they put plastic type tubing on something that is so close the exhaust. Kind of crazy. Did you replace it with a different type of material or zip tie it to something that doesnt get melting hot?

 

Here it is before I cut it out. I've already disconnected the fitting that attaches that plastic tubing to the metal hard line in that area - the green fitting is the one on the metal line. This is looking up and back - you can see the left exhaust pipe going up and over the axle. The driver's side is to the right in the picture, and the back of the car is at the bottom:




I used a heavy duty rubber tubing and wrapped Cool Tape (same stuff I wrap around my rubber ball joint covers to protect them from brake heat) around it to help protect it from the heat:







I've done several thousand miles of street driving and a couple track events with no further problems since repairing that split tubing problem.

Bob

 

Nice fix Bob! Good thinking!

 

Thanks for the replies. I will put my car on the lift and report back soon.

 

I finally had the time to look into all the suggestions. I put the car on my lift and checked everything, twice, no visible signs of any damage to the vac lines. Hooked up my Mity-Vac at the hard line in the engine compartment pulled 15* and it held until I released it.
I drove the car for 45 min. never going over 3000 rpm no issues nice and quiet. About a mile or two from home I floored it and the exhaust valves opened as designed I let off the gas but the valves did not close. This leads me to believe it's something else causing the issue. When I returned home with the valves still open I shut the car off and waited a minute then restarted it, valves stayed open. I went into the house and came back 30 minutes later and restarted the car the valves closed as designed.
There has to be a control/relay that the ECM tells the exhaust to open and close.
Is there a schematic showing the NPP system? Any additional help will be appreciated.
Thanks

 

I found this one any other input would be great.

http://forums.corvetteforum.com/1577171390-post14.html

 

The item# 5 in the schematic put up by Tech Contributor Bill Dearborn,

(5) Exhaust Tail Pipe Flow Vacuum Valve Solenoid

is, if I remember correctly, accessed by removing the right taillight. You can see it in there and access it to hopefully determine if there's a loose connector or loose ground.

However, when I had the problem with the plastic tubing above the exhaust, it acted sort of like your problem on your drive - it was initially intermittent.

I think that when I first started driving the tube was firm and didn't leak, but after driving a while and heating up, it would get softer and leak.

So....I still would suspect a vacuum leak. If you do a drive similar to what you did today and the valves remain open at low RPM, put your Mity-Vac back on it and see if you still can pull a steady vacuum. When you do pull a vacuum from that engine compartment connection, it should close the valves - so check that as well.

Good luck!

Bob

 

Not sure when I can look at it again but I will be sure to try your ideas when I do.
Thank you.

 

Update:

I have been busy the last few weeks and finally had time to look into this further. It is not a vacuum leak of any kind I tested the system front to back it held 20" until I released it. I disconnected the (5) Exhaust Tail Pipe Flow Vacuum Valve Solenoid in the schematic and tested the system both ways back to front and from the solenoid to the exhaust valves, no leaks. I tested the vacuum valve solenoid on the bench by applying vacuum to it, again no leaks. I then checked resistance 23 ohms. I put 12v to the terminals while it was holding a vacuum you could hear it click and see it release the vacuum.

I have narrowed it down to the Exhaust Tail pipe Flow Control module.
Where exactly is the Flow Valve Control Module and how do you check it?

My car has headers and it is extremely loud with the valves open you can't drive it around town like this so I left the Flow Control valve Solenoid out for now and added a small piece of vacuum tube so I can drive the car until I figure this out. Now when you shut off the car the valves stay closed so that is another reason I know there are no leaks in the system.

Thanks in advance.

 

I have a code reader and checked for any trouble codes, there are none.

 

Found this thread it looks like I need to buy one of these.

http://forums.corvetteforum.com/c6-c...is-module.html

 

I ordered one today from GMPH should be here in a week. I'll report back part #'s etc. maybe it will help someone else.

 

The module has shipped hopefully a couple of more days. Now all I have to do is figure out how to remove the right rear interior panel over the fender well to replace it.

Stay tuned.

 

 

Received and installed the new exhaust flow control module today. Long story short it is still not functioning properly. Valves stay closed under normal driving floor it and at 3500 rpm they open as designed when rpm's drop the valves stay open.
Looks like it's off to the dealer the ECM controls all of it so I'll have that looked into. Not happy.

The exhaust flow control solenoid is N.C. when power 12v is applied to it from the exhaust control module it opens and allows vacuum to close the exhaust valves. Vacuum closes the valves, no power to the solenoid, no vacuum to the valves I guess GM wanted them to fail in the open position.


Will report back soon.

 

Thank you for sharing your experiences with heat and its result on the vacuum lines; it allowed me to plan a different route for my install.

 

Sorry this took so long but things kept popping up around the house that came first. Long story short it was the #5 Exhaust Tailpipe Flow Control Valve Solenoid. Picked one up at my local Chevy dealer part #325794609 $94.48 list they sold it to me for $67.31 which I thought was fair. Have your VIN number handy because there are two numbers for this part.