Ever since getting my car put back together after a head gasket job, I've been trying to track down a pretty gnarly vacuum leak. Today I rigged up an adapter to pressurize the intake system to about 5psi & proceeded to spray soapy water EVERYWHERE in the engine bay. To my surprise, the BPV diaphragm casing started making bubbles like crazy!!

I'll probably just use the opportunity to swap it out with a Detroit Tuned unit. I am wondering, though- If myst BPV failures are due to a worn out spring, causing the butterfly to always be partially open, what would the symptoms be for the opposite? If the diaphragm is leaking & completely unable to open the butterfly, what other running conditions would one expect? I've been experiencing difficult/hesitant start, rough-ish idle, idle that waivers between 5-800 rpm and only pulling about 10 inHG vacuum at idle, rather than 15 which seems more typical.

I'll be ordering the DT BPV tomorrow, just curious if this could be the one fix to solve all my car's issues!

 

The BPV does not create a 100% seal, even our DT BPV leaks, the tolerances of the butterfly to the housing aren't cut to each other so you will always have a small amount of blowby.

Our valve is designed to seal quicker, tighter, and more exact over the stock valve, but again not a perfect seal.

The BPV can fail in multiple ways:
Broken spring
Torn diaphragm
Crack in the housing where the butterfly valve passes through the housing (which is more on the rare side)

A quick way to check if your valve is working is to manually operate the diaphragm by hand to feel for spring pressure throughout the diaphragms travel, and even apply vacuum to the diaphragm head to see if it'll pull/hold vacuum.

 

It's not the butterfly that's leaking, it's the diaphragm housing: