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BMW labeled the boost solenoid nipples VAC and OUT for a reason. The boost solenoid is a sophisticated PWM-controlled turbo component that continuously meters vacuum to the wastegate. To operate, VAC must be connected to the vacuum source and OUT must be connected to the wastegate. By swapping the two hoses, the boost solenoid won't function properly due to being stuck in a continuous backward and forward loop, which can result in the flutter sound you heard.
When I saw the bent and deformed vacuum hose, my first thought was a kinked garden hose that can't spray water.
My recommendation would be to rule out these^ items first before replacing the boost solenoid.
Are you absolutely sure that the two vacuum lines connected to the boost solenoid (pressure converter) near the vacuum canister are not swapped and are not pinched?
With the engine cold, reach down and wiggle the wastegate actuator arm. Does it rattle or have a lot of play? It should be tight.
In this post you shared, it shows the following routing, which matches how I have it currently setup:
Which one of these is the correct way? Also, what a terrible diagram am I right? Why can't they keep the lines separate a little bit so we can clearly see where they are meant to go. If you look close enough though it does look like it should go through the Vacuum Canister first.
Edit: The reason I question this is because you mentioned that the VAC on the Boost Solenoid must be connected to the vacuum source... Directly to the vacuum source? or through the vacuum canister? (Maybe this is just a misunderstanding of how vacuum works, but I feel like it could matter if the vacuum canister was before or after the solenoid?)
Okay, I will absolutely 100% confirm tonight this is how it is routed... I was thinking, since the boost solenoid is electrically triggered via the ECU, wouldn't it be a good idea to pump 15in Hg of vacuum through from the "vacuum pump" (Handheld) while manually triggering the solenoid electrically, to see if its properly opening the wastegate? I know the best way to do this would be to visually watch the wastegate close with a bore scope, but physically feeling that lever go stiff would probably be good enough...
My only question would be how I can safely trigger this solenoid, since in order to verify the signal it normally uses, would be to somehow measure this during load, via scan tool or multimeter while you're driving? lol...
I do have ISTA+ which I believe has the capability to send an artificial signal from the ECU to close the wastegate.
Actuating the entire system and seeing the wastegate close only tells me that the boost solenoid can fully close the turbo wastegate, and I could measure the vacuum after the solenoid, but doesn't really prove the boost solenoid isn't intermittently failing under load, which brings me back to square one.
Wow okay, well assuming this is accurate, this should be very nice. I want to say the only thing related to vacuum in ISTA+ the last time I looked was WasteGate activation. Thank you for your help and will update later with my findings!
So… Just ran a test… unhooked vacuum hose from vac pump, hooked up handheld pump, pumped to 15Hg and then manually actuated the turbo wastegate. This dropped the vacuum down to 13Hg, then when the turbo wastegate de-actuated, it stays at 13Hg and makes a little spew sound like it shot air out somewhere below the intake manifold. Probably the boost solenoid? Was expecting it to jump back to 15Hg. Do we think this is normal behavior?
How does this prove the boost solenoid is bad? This was while the car was off and not creating any new vacuum from the vacuum pump, I can’t tell exactly where it is coming from.. the vacuum is not meant to escape after the wastegate re-opens?
As you stated, during your test, the hose running from the boost solenoid to the wastegate is not under vacuum, whereas the rest of the turbo vacuum system is.
The hose that runs from the boost solenoid to the wastegate is instead under normal atmospheric pressure. When you manually actuated the wastegate, it compressed the air in this hose. The air pressure wave traveled all the way back to the OUT port of your boost solenoid and caused it to abnormally expel vacuum from its atmospheric vent. A good boost solenoid is regularly and more severely subjected to turbo wastegate back pressure, but it withstands this abuse without venting any of its vacuum. This explains the flutter sound while driving. I believe that you can now confidently replace your bad boost solenoid.
So the best way to prove this immediately without the new boost solenoid (aftermarket replacement will be arriving tomorrow), is to do this exact same test on another, properly running R56, if the same thing occurs, then that means it is intended, otherwise, I have finally determined the issue!
Also, I do understand what you’re saying, sort of… but what are you trying to say the expected outcome of my test should be if what you’re saying is true, and the boost solenoid is bad?
Yes, that's an excellent verification test. If you install the known good boost solenoid in your car, it is predicted to remain silent (no vacuum loss sound) without a vacuum drop when subjected to the same test. If so, you could also drive the car to see if the flutter is gone.
Last edited by Maybe, maybe not; May 19, 2026 at 05:42 PM.
Well, I’d need to get the new boost solenoid in it before driving it to see if the flutter is gone haha. We have lots of MINIs, but only 1 known good boost solenoid in a running, driving car
Well, it didn’t do as either of us suspected, when loaded with 15Hg of vacuum and wastegate actuated, it drops by the 2-3 Hg, but then still slowly leaks vacuum until reaching approximately 10Hg. Then comes to a stop. I also did not hear the instant leakage of vacuum after stopping wastegate actuation like I do on the problem car. I will replace the boost solenoid and update my findings after
I am happy because I am like 99.9999% sure this is still the problem component, because the part I bought is dead on arrival (Chinese clone garbage)
i am sad because i need to wait a few days to get a good part, the one i bought is clearly junk. I can pump vacuum straight through the solenoid. How do people get away with this **** man :(
