VNT Turbo Actuator Issue

I recently finished installing an ALH in a swap but I'm having a weird issue with the turbo. With the engine running at idle, I heard a sound similar to rushing air coming from the exhaust and noticed the turbo actuator was maxed (rod fully sucked into the actuator). I've triple checked the vacuum hose routing and that isn't my issue. When I unplug the N75 while running then plug it back in the issue disappears. I also don't get any codes thrown for my MAP sensor. Is it normal for the N75 to have vacuum in the hose going to the actuator at idle, or is my actuator diaphragm worn out? Thanks!

At idle, the actuator rod is pulled all the way "in" (vanes are aimed at the outer edge of the turbine wheel) position ready to create boost. The rod is not released until the boost has met the ECU's requested amount.

JETaah said:

At idle, the actuator rod is pulled all the way "in" (vanes are aimed at the outer edge of the turbine wheel) position ready to create boost. The rod is not released until the boost has met the ECU's requested amount.

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So I guess it's technically working as it's supposed to. Thanks for the info. But, I still don't understand what this horrible noise coming from the exhaust is. No matter where I have the rod adjusted, it makes this noise sounding like a rush of air coming out of the exhaust. It's straight piped, but I've heard other straight piped TDIs and they certainly don't sound like this.

At idle the vanes are closed:
80 = closed
25 = open

https://i.ytimg.com/vi/XFqjagNSk0U/maxresdefault.jpg

pdtdiarl said:

At idle the vanes are closed:
80 = closed
25 = open

https://i.ytimg.com/vi/XFqjagNSk0U/maxresdefault.jpg

Click to expand...

So, for someone unfamiliar with VNT turbos, does this mean the rod is "sucked in" all the way, or "pushed out" all the way?

pdtdiarl said:

https://www.youtube.com/watch?v=Ppif4qC560U

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Sorry, can't see youtube at work and trying to diagnose this issue before I get home. I'll throw that link to my phone. Cheers.

Tacoma_TDI said:

Sorry, can't see youtube at work and trying to diagnose this issue before I get home. I'll throw that link to my phone. Cheers.

Click to expand...

When you apply vacuum, the rod is pushed out and closes the vanes.

pdtdiarl said:

At idle the vanes are closed:
80 = closed
25 = open

https://i.ytimg.com/vi/XFqjagNSk0U/maxresdefault.jpg

Click to expand...

You are referring to N75 duty cycle?
That depends on the engine. On an ALH 25% duty cycle = vanes closed.
On a PD engine it means open.

That sound IS air (or exhaust) rushing through the closed down vanes which creates a higher velocity to push the turbine wheel at lower RPMs.

pdtdiarl said:

When you apply vacuum, the rod is pushed out and closes the vanes.

Click to expand...

He has an ALH with a Garrett turbo. When you apply vacuum, the rod retracts.

1854sailor said:

He has an ALH with a Garrett turbo. When you apply vacuum, the rod retracts.

Click to expand...

I'm sure it doesn't make much a difference, but I'm running an ALH with a 1749VA from an 05 Passat TDI. I presume both function identically.

JETaah said:

You are referring to N75 duty cycle?
That depends on the engine. On an ALH 25% duty cycle = vanes closed.
On a PD engine it means open.

That sound IS air (or exhaust) rushing through the closed down vanes which creates a higher velocity to push the turbine wheel at lower RPMs.

Click to expand...

Is this normal? I speculated that this is what it is, but it doesn't sound normal. Maybe it's just the 3" straight pipe that permits one to hear this.

What JETaaH and Pdtdiarl are saying is correct. I've visually observed the Actuator function on the ALH in my Vanagon while someone started the engine. Immediately, the Actuator rod is pulled down solidly against the set screw. (Of course, there are a few that still don't accept this.)

So, "releasing" and "applying" vacuum is how the Vanes are operated. The strong spring inside the Actuator moves the rod far faster than vacuum can, thus, boost comes quick when you need it. In the fully down position of the Actuator Rod, the angle of the Vanes, creates little or no boost. Even if you rev the engine, virtually no boost (observation of both my Scan Gauge and mechanical boost gauge). But, once you put the foot down on the accelerator and ease out the clutch, a combo of things commence by the ECU and all the components it monitors....... thus, more fueling, boost, etc.

To be correct, the Vanes are never "fully" closed as there would be no place for the exhaust gases to go (no by-pass in a VNT Turbo).

AndyBees said:

What JETaaH and Pdtdiarl are saying is correct. I've visually observed the Actuator function on the ALH in my Vanagon while someone started the engine. Immediately, the Actuator rod is pulled down solidly against the set screw. (Of course, there are a few that still don't accept this.)

So, "releasing" and "applying" vacuum is how the Vanes are operated. The strong spring inside the Actuator moves the rod far faster than vacuum can, thus, boost comes quick when you need it. In the fully down position of the Actuator Rod, the angle of the Vanes, creates little or no boost. Even if you rev the engine, virtually no boost (observation of both my Scan Gauge and mechanical boost gauge). But, once you put the foot down on the accelerator and ease out the clutch, a combo of things commence by the ECU and all the components it monitors....... thus, more fueling, boost, etc.

To be correct, the Vanes are never "fully" closed as there would be no place for the exhaust gases to go (no by-pass in a VNT Turbo).

Click to expand...

I too observed this, but with this being my first experience with VNT turbos I wasn't entirely sure. Thanks for clarifying. This would also explain why, when I remove the line from the actuator, the noise goes away. I am still concerned that this "rushing air" noise is abnormal as I've never heard it before on a straight piped TDI.

Yeah, that YouTube is not for a VNT 15 ......... totally opposite in operation! (pushes vs pulls)

Tacoma_TDI said:

I'm sure it doesn't make much a difference, but I'm running an ALH with a 1749VA from an 05 Passat TDI. I presume both function identically.

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Yes, the actuator on the 1749VA pulls the VNT rod in when vacuum is applied.

Push or pull on the actuator depends on the design of the turbo/actuator.
The BRM (PD) engine pushes the rod out because the vacuum is applied to the opposite side of the actuator diaphragm from the ALH turbo. The BEW (also PD) pulls the rod inward like the ALH does.
In any case, vacuum applied means boost position.

The difference in duty cycle is dependent upon how the software was written.

Depending on the exhaust set-up, muffler, etc., you should hear the Turbo whistle at idle..

AndyBees said:

..... Even if you rev the engine, virtually no boost (observation of both my Scan Gauge and mechanical boost gauge)......

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That is because the ECU increases the duty cycle (less vacuum on ALH) to prevent boost when it is not needed.

AndyBees said:

Depending on the exhaust set-up, muffler, etc., you should hear the Turbo whistle at idle..

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It doesn't sound even remotely like turbo whistle. Almost sounds like a blow gun on an air compressor slightly open. I'll toss a video up later.

JETaah said:

Push or pull on the actuator depends on the design of the turbo/actuator.
The BRM (PD) engine pushes the rod out because the vacuum is applied to the opposite side of the actuator diaphragm from the ALH turbo. The BEW (also PD) pulls the rod inward like the ALH does.
In any case, vacuum applied means boost position.

The difference in duty cycle is dependent upon how the software was written.

Click to expand...

You're right, didn't noticed it was an old VNT

JETaah said:

That is because the ECU increases the duty cycle (less vacuum on ALH) to prevent boost when it is not needed.

Click to expand...

Well, the one thing I have not done, is observe the Actuator while someone revs the engine! That's on my bucket list ... LOL

I'm not sure how much it would move (boost) when revved with no load on the engine.
Have you observed it at start-up?

BobnOH said:

I'm not sure how much it would move (boost) when revved with no load on the engine.
Have you observed it at start-up?

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Yes! I have observed the Actuator at engine start-up as I stated up there ^^^^ in one of my other posts! Immediately upon starting the engine, vacuum pulls the Actuator rod/lever down solid against the set screw!

And, yes, I know there is no boost when revving the engine (Basically zero, as a hard rev will show a bit of boost on Scan Gauge.).