What sound should a check valve make when air is blown through it? I took off some valves today to see if they were working, most of them were pretty hard to blow through but quiet, one made a low whistle when I blew through it. Is that normal?

I'm trying to troubleshoot a whistle under secondary boost.

 

It's not really the sound that is important. What's important that the check valve allows air to flow through one direction, but blocks air the other direction.

 

But if it whistles when i blow through it, I imagine at full boost it would whistle loud enough to hear over the engine. Could a whistle indicate a bad check valve or is that normal?

 

With vacuum lines connected on the ends, it's not going to make noise like that.

A check valve is bad if air flows BOTH directions.

 

Secondary turbo control trouble must me be the problem of the week. ttb and I just had these same sounds coming from the extension manifold area. Typically, there is a secondary boost problem under WOT after the 4500 switchover rpm. The secondary turbo doesn't come online unless you generate a vacuum by coming off of the gas (to create 20 inches Hg vacuum) and then reapplying the gas.

The whistling sound is most prominent between 3000 and 4500 rpm range when the secondary turbo is spooling up. During this time, the charge relief valve (CRV) is venting secondary turbo boost through the air intake, until 4500 rpm when the CRV closes and adds secondary boost to the primary turbo's boost.

After exchanging emails with Dave Barninger of KD Rotary and reading the twin turbo trouble shooting guide (rynberg clued me in to this site), http://www.autosportracetech.com/RX-7/rx7stuff.htm, you'll find the culprits are either one or ALL of the following:

1. Vacuum chamber 1-way check valve
2. Cracked vacuum chamber
3. Turbo control solenoids (under the extension manifold)
4. Disconnected vacuum line between turbo control actuator and rat's nest
5. Turbo control actuator
6. Charge relief valve (commonly known as the secondary turbo "blow-off valve")
7. Charge control actuator

FWIW, I've replaced my 1-way vacuum chamber check valve, and still have the secondary turbo control issue, and whistling. ttb's problem were bad solenoids, so I'm headed there next...

Happy reading and best of luck!

Let us know what your turbo control problem/whistling ends up being...