can someone tell me the role this plays in the motor? i have mine vented to atmosphere.. seems to not affect anything...

 

Bump for you. I have this same question and also am wondering if it can be plumbed into the boost pipes instead of the intake. I have a hks manifold and t04 and there isnt much room for a metal pipe to connect a vacuum line too. i was hoping to just be able to use an elbow.

 

That makes sure that the emissions crap goes into the TID and gets sucked into the motor.
If you look at the design, it is next to impossible to get positive boost into that section if the TID and back into the hose, but Mazda didn't want to take chances and inserted the check valve there.
If you happened to get positive boost into the hose, it would pressurize the "crankcase" and charcoal canister.
Pressurizing the crankcase will cause the motor burn oil and smoke through the exhaust.
Pressurizing the charcoal canister can cause problems with the fuel system.


-Ted

 

I don't think that's the main reason Ted. It took me ages to figure out why it's there, but if you follow the system right back to the purge control valve and manifold, you'll see that without the check valve the purge valve would allow air to bypass the throttle at low load, i.e. sucked in through hose in the TID and into the manifold post-throttle. Given the size of the hoses, I expect this would raise idle speed considerably.

Leaving the hose and check valve open-vented will have no effect other than allowing crankcase emissions into the atmosphere instead of being sucked into the engine and burnt.