Re-reading this, I wonder if you are missing a check valve. The 928 was designed for full-speed bahn-storming, and while it doesn't have the positive manifold pressure your turbo cars have, it still needs to maintain vacuum for the HVAC (and cruise-control, plus a resonant-flap actuator on the S4+ cars) under that condition. Porsche added a venturi eductor, but more importantly it has a vacuum accumulator (which name seems just a little backwards...), plus a small check valve between the engine and that accumulator. The check valve allows air to flow only from the accumulator towards the engine, so under hard throttle or boost conditions, the check valve will keep manifold air from entering the accumulator. The accumulator will keep the whole system available, at least until leaks, seepage, and cycling actuators eventually bleed enough air into the accumulator that there isn't enough 'vacuum' to hold things in position.Tom wrote: ↑Fri Sep 20, 2024 11:52 am Since rabbit holes are kind of my thing,
<...>
Just riff'n here, but if the solenoid were internally leaking such that A, B, and C all flowed freely between each other, one could imagine a scenario where there is still enough vacuum to pull the heater valve shut any time there is full engine vacuum (where the unlimited supply of vacuum overcomes the 'leak' at C), yet can't sustain vacuum for long under boost when relying on the oxymoronically named vacuum reservoir alone (given the leak at C), causing the valve to always be closed under vacuum and always be open under boost. No idea if the solenoids ever fail that way, however. Just thinking out loud. I'll stop now.
heater-solenoid.jpg
In the V8 car, there's also a restriction in the hose between the accumulator and the HVAC controller. It functions to slow down the actuators, but also protects the other clients (cruise, resonant flap actuator) from leaks in the HVAC system.
I don't know all the details of the 944 systems, but I do know that Behr very likely participated in the design and probably a lot of the parts and pieces as they did for the 928. Assuming they all shared a lunch table at some point, there's a good chance that they use a similar check valve. Those are not expensive at Porsche, tend to be even cheaper at Mercedes stores, and are actually pretty generic in shape and function. A decent POLAPS (plain old local auto parts store) is likely to have a generic one.