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flosho · Posted: Sat Apr 22, 2006 2:07 pm Post subject: Turbo Bark

Tonite when I was driving, I would notice that the turbo was making a wierd noise. When I shifted the turbo would make the noise... it sounds like turbo bark from a turbo diesel.

Its kinda dark, but I'm wondering if the recirc valve is leaking and not functioning properly or a that turbo pipe connected to block came loose again.
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flosho · Posted: Mon Apr 24, 2006 10:24 am Post subject:

No one has had this happen before?

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Khal · Posted: Mon Apr 24, 2006 10:44 am Post subject:

I have absolutely no idea... don't even know what you're trying to describe?

But the thread looked so empty I thought I'd better drop a reply!

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john h · Posted: Mon Apr 24, 2006 11:24 am Post subject:

Could be the "o" at the throttle body has worked loose giving the bark. have a look and see if you can see it at the junction of the charge tube and throttle body.
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flosho · Posted: Mon Apr 24, 2006 11:31 am Post subject:

Well the best I can try to describe it is like the pop-off valve is not functioning properly, and when I let off the gas to shift, the air is rushing back into the turbo and makes a squeek. This "squeek" sounds similar to some BOVs not the PSHH sound but a wierd sound, sometimes the diesle guys describe it as "turbo bark".
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goturbo · Joined: 15 Jun 2004 Posts: 88 Location: Massillon Ohio, USA

What does the boost gauge do when this is happening? (assuming you have one...)

flosho · Posted: Mon Apr 24, 2006 12:52 pm Post subject:

Well the boost gauge is going back to vacuum, but this is happening when im shifting, so it normally does that...

It might have just been my imagination, but I thought it felt kind of sluggish at times.. I don't know, I'll have some time tomorrow evening to look at it some.
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goturbo · Joined: 15 Jun 2004 Posts: 88 Location: Massillon Ohio, USA

My turbo did the same thing just a short time after I got the car. As it turned out the squeek was the sound of the compressor impeller rubbing the housing! Seems as if the thrust bearing was on it's way out (later discovered when the turbo was removed due to the rear oil seal failing-surprise). Mine also was in between shifts, but the sound was more like a squeek.
But hopefully your problem is something different-just sharing my fun little turbo squeek story...

Tigger937 · Posted: Tue Apr 25, 2006 2:24 am Post subject:

flosho · Posted: Tue Apr 25, 2006 4:33 am Post subject:

Wow.. I did not want to hear that..

I certainly hope it has nothing to do with the turbo, as this turbo probably has 5000 miles on it, max...

I'll get under there, and see if all the lines are connected and in good shape, check and see if I can hear the pop-off valve functioning properly, maybe take it out for a drive too..
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Tigger937 · Posted: Tue Apr 25, 2006 7:45 am Post subject:

goturbo · Joined: 15 Jun 2004 Posts: 88 Location: Massillon Ohio, USA

flosho,
At this point I would bet my money on a leaky elbow joint that connects the Pressure relief valve on the turbo to the bottom of the intake manifold. Two things make me think this. First, the squeek could be the sound of air being sucked through a small hole or tear in the rubber elbow during high vac. between shifts. Secondly the fact that you said the car runs 'kind of sluggish'. One thing that needs to be clarified is the function of this valve. It DOES NOT have a direct control on the boost pressure. The waste gate is the primary control for the boost presure. This valve is used to increase the effectiveness of turbo operation during sudden changes in engine RPM. When the car is running at high RPM and high boost and the throttle plate is suddenly closed or partially closed(during shifting) the turbo does not want to slow down and it continues to build pressure in front of the throttle plate. At this point two things happen. The waste gate opens up on the exhaust side of the turbo to prevent overboosting and the pressure relief valve opens up (with vac. assist) to allow the compressor to 'free spin' and not feel a braking effect from trying to move air that can't go anywhere becaues of a closed throttle plate. In effect the compressor is blowing air into its self in a loop when the valve is open and feels very little resistance. What does all that do? Well it keeps the compressor/impeller spinning as fast as possible so that when the throttle plate is again opened and the system retunds to 'normal' operation, there is reduced or very little 'spool time' for the turbo to come back on to full boost.
The fact is you don't need the pressure relief valve. If your elbow is cracked or broken I would temporarily cap off line where it connects to the intake manifold. That tube is big enough that it can cause your idle to increase, and that air is not passing through the fuel distributor-which is never a good thing.
Your car will run fine without the pressure relief valve connected, but it will feel sluggish because the turbo system is not being optimized. It is far better to have a little prefomance loss than a vacuum leak.

Anyhow a bit wordy but I hope this helps. Most of us have struggled with issues in this area and know how frustrating it can be, so I hope you get it figured out.

Tigger937 · Posted: Tue Apr 25, 2006 11:33 pm Post subject:

goturbo · Joined: 15 Jun 2004 Posts: 88 Location: Massillon Ohio, USA

Tigger-how do you explain the thousands of cars that are driving around all over the world that didn't have a pressure relief valve from the factory???

I have 3 different turbo cars and the Porsche is the only one that came with one from the factory. There is no additional danger in driving the 931 with the pressure relief valve disconnected.

I've measured the system without the valve connected while doing some other development work and the velocity pressure of the surge charge when the throttle plate is suddenly closed from 7000 rpm does not exceed 13 psi while the static pressure is barely over 11 psi. That is negligible. The boost is too low to worry about any damage to the compressor etc. It's a small system with limited CFM output and the charge density is reflective of that. If someone was running ++ psi with a bigger compressor and an intercooler- well that is a different story. (perhaps a better story....

)

The real danger is if there is an air leak and #1 cylinder goes lean because of the additioal air being pulled in through the pilot line that connects the valve to the manifold. I just hope that the problem is that easy to trobleshoot and repair.

I have seen where those valves get worn and sloppy in the housing. That gets a bit more challenging to repair...

sequential · Joined: 18 Nov 2005 Posts: 500 Location: BANNED