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A friend (not a forum member) asked me to post this tech note:
NPP EXHAUST FAILURE
I was driving along when my NPP EXHAUST SYSTEM WENT INTO DEFAULT MODE. This is caused by the loss of vacuum to the system. Nothing you can do about it but hit the local dealer, unless you like the loudness on steady. It cost me $500.00 to diagnose the problem.
The system starts up front at the engine and works it's way back to the mufflers. Part of the line is steel with rubber couplers along the way. So they had to work their way from front to back. This is what took the time and expense. So I'm sending this out to save somebody some money. If it happens to you have them go direct to the part they found on mine first. That way you save some time and money to. They DO NOT stock the part, it has to come from the USA. The funny thing is that the part is made in Germany for GM. Just a helpful hint.
Top tip: searching Google and this forum is free. 95% of the time you'll find a post by someone who has had the exact same problem and already knows the fix. This issue and fix has been posted on this forum a few times already. Latest post:
Well, still, it is a helpful hint, complete with a pic of at least one of the known culprits. By the way, Must Have, how much is the part that's pictured?
Man OP, that sucks that they ripped you off like that! $500?! WTF?! Anyone that's familiar with these cars should've been able to troubleshoot that right away. I had the same problem a few years again and I did a search on here and figured out where the solenoid was and ran it for a while with the solenoid bypassed and then later on I ordered a new one and changed it myself.
Here's the thread that I did back then on it: https://www.corvetteforum.com/forums...oid-fried.html
OP, you can also make the NPP system manually operational, by bypassing the electronic control.
Just remove and bypass the solenoid and remove the one way valve under the hood. It will work like that but instead of the exhaust opening at precisely 3000 or 3500, it will simply open when you get on the gas pedal past a certain point and the vacuum drops. I ran mine like that for months. The details are on thread that I posted the link to above.
Before replacing the exhaust vacuum solenoid, pull the vacuum hose from the solenoid input and put a vacuum gauge on it. This is actually the easiest vacuum hose to disconnect compared to the others. Start the engine and see if you actually have vacuum. I replaced my solenoid before doing this test and wasted a lot of time and money. I was not getting vacuum to the solenoid due to a cracked vacuum hose in the engine compartment.
Npp exhaust failure
