No boost... Not turbo actuator and not N75

I am trying to figure out a problem that has me completely stumped. One day, my ALH seemed to lack power, and I could no longer hear the turbo spool. First thing I did was check the turbo actuator, and found it to be slowly leaking vacuum. Replaced that, and no change (no, the vanes aren't stuck). I then discovered that there was no vacuum going the actuator at all, so I tried swapping in few spare N75s I have laying around - still no change. Turns out that there doesn't seem to be any measurable vacuum going to the N75 valve at all for it to even regulate. Shouldn't there be full vacuum at the inlet line? The weird part is that all the vacuum lines are new, and all other parts of the vacuum system seem to be functioning fine. EGR valve is getting vacuum, and anti-shudder valve is working. I cycled the N75 with VCDS in output tests, and still no vacuum going to the actuator at all. I can't find any leaks, and the hoses are routed correctly as well from what I can see. I even replaced my vacuum pump because it was leaking a bit around where the hose connects. Only thing I can think of is the possibility that my N18 valve is somehow failing and hogging all the vacuum? I see that the N75 is T'd into that. Not sure. Could the heat from the engine cause my crappy eBay silicone vacuum lines to collapsing when vacuum is pulled? Also, NO CODES, and I've been driving for about 2 weeks with no boost! Any help would be appreciated.

This might not be of much help, but here's a couple thoughts.
Just got back from gearhead garage
(shout out!) where I was adjusting/checking my actuator chasing dropping fuel economy. I changed out the actuator's vacuum hose, but did not have the proper size ear clamp for the actuator nipple. So I just stuck the hose on and went. Boost is even worse now. I suspect that clamp is essential to prevent pulling in atmosphere.

Only other thing comes to mind is check your check valve? The little black and white disk right up there by the N75.

Bench test the n75 to make sure it's opening. Is there vacuum at that hose?
Check with the engine running to see where the vacuum stops appearing. The old
leaking actuator may have allowed some rust to clog one of the connections.

Thanks guys, it turned out to be the vacuum check valve. It was completely clogged. It held vacuum when I checked it and I didn't think anything of it. I bypassed it completely and my boost returned. What a difference!

STDOUBT said:

This might not be of much help, but here's a couple thoughts.
Just got back from gearhead garage
(shout out!) where I was adjusting/checking my actuator chasing dropping fuel economy. I changed out the actuator's vacuum hose, but did not have the proper size ear clamp for the actuator nipple. So I just stuck the hose on and went. Boost is even worse now. I suspect that clamp is essential to prevent pulling in atmosphere.
Only other thing comes to mind is check your check valve? The little black and white disk right up there by the N75.

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@STDOUBT I think that as long as you are using the correct size vacuum hose you should be fine. None of my vacuum hoses are clamped on and they don't leak.

cwatson9510 said:

Thanks guys, it turned out to be the vacuum check valve. It was completely clogged. It held vacuum when I checked it and I didn't think anything of it. I bypassed it completely and my boost returned. What a difference!

Click to expand...

FWIW, I got rid of those about 5 years ago.