69 L46 4 Speed

With the car off, the valves should be in the open position. When you start the car your exhaust should be louder for a second or two while the valves are closing. They should remain closed while the car is idling. You have to get the RPM's above 3,000 to open them. If they stay open after starting the car, the solenoid is fried. The previous statement is true if you have not installed an override system or pulled the NPP fuse.

427#100

They butterfly valves should be open with the engine off. My situation was pretty easy to diagnose thanks to the advice I got on here. You can easily see if the solenoid is the problem by taking out both right side tail lights and disconnecting the vacuum hose from the solenoid to the tail pipe butterfly valves. If they valves open up when you do that the solenoid is the problem. I found out after I bought my new solenoid I could have saved about $30 by buying it online right from GM parts online.

NTHALP

Looked at the NPP gate valves this morning. Haven't started the engine for the past couple of weeks due to weather and residual road salt from the last snow/ice event. Gate valves are open as they should be. Am I correct in understanding that there could still be a problem with the solenoid or other component?

427#100

Yes, could still be a solenoid problem. It is easy to see how even if the solenoid isn't working correctly after two weeks the residual vacuum would bleed off. When you start the engine the valves should close and when you turn it off they should return to the open position, immediately.

NTHALP

Thanks 427. Next time I fire it up, I'll check. IIRC, the last time I had it out, I checked after parking it in the garage and noticed that the gate valves remained closed after killing the engine. If so, then I have "the problem."

427#100

Chances are it's the solenoid. It's actually pretty easy to change, just take your time, maybe take a couple pics with your cell phone camera before you start. The part # of my solenoid is 25794610, most likely the same for yours.
http://www.gmpartsdirect.co/oem-part...A0MTgwNSZyPTEx

MiamiCowboy

Solenoid went out on my 09 Z06. Found this thread on repair. Very helpful in my repair. Thank you.