Do these VCDS traces explain my limp mode?

[VAG-Abound]

Turbo on our BKD has been dropping into limp mode pretty reliably on motorway trips for a few months. I've been underneath and dropped the turbo actuator from its mount so that I could wiggle the VNT lever around, and it seems to shift pretty easily. I've also put a hand vacuum pump on the actuator and watched the action from along the exhaust tunnel. It seems to run smoothly to the stop and back in the expected vacuum range. Also popped the vacuum manifold / distribution plate off and all the lines that I'd expect to do appear to hold a good vacuum.

I did a bit of VCDS logging and noticed that when the actual / specified on the boost control vary significantly from each other there is often a similar variance on the EGR control, such as in the areas I've marked with a red rectangle below. Is there likely to be a connection between these, such as vacuum system losing (negative) pressure? Could EGR action be dragging down turbo response and triggering limp mode?

BTW, the car went into limp mode towards the end of this trace. Looking at the traces I'm a bit surprised that the area in the first rectangle didn't do this as well. Is there a grace period of some kind before it will actually trip?

[Hmmm. I'm struggling to make image large enough to see easily. Top trace is boost pressure (blue=specified, red=actual) and lower is EGR (green / purple respectively).]

20190313a.jpg

[Crasher]

You could do with extending the time base as it helps see what is going on but you have clear overboost flat lining on the sensors max measuring pressure so it is pretty certain the turbo needs a rebuild.

[VAG-Abound]

Thanks Crasher, something like the new attachment?

I've filtered the data to show just the sections where the actual boost (purple line) hits that 2601 limit. They're all joined up (the timebase on the bottom helps pick them apart) with the peaks highlighted with the green ovals. It looks like in each case:

• Engine revs are climbing sharply, presumably having dropped down a gear? (red, left hand scale)
• Boost demand (green, left hand scale) and actuator duty-cycle (pale blue, right hand scale) are going from high to very low

So some sluggishness but not completely seized behaviour in the vanes is most likely? The correlating bounces in EGR behaviour from the original post not being related?

20190313c.jpg

[Crasher]

Is the turbo howling?

[VAG-Abound]

Sorry Crasher, missed that in some holiday madness.

I'd describe it as a strong whistle, like having a window down a fraction with air coming in at speed.

[Crasher]

Classic BKD sign of a failed turbo.

[VAG-Abound]

Classic BKD sign of a failed turbo.

Thx.

Failed as in sticky that a strip and clean would cure, or some other failure?

[DieselMonster]

Worn out. If it goes into limp mode without the howling and the vnt mechanism is stuck then it means the vanes are clogged usually so a strip and clean will do but in your case, it's either the bearing, turbo wheel that's been badly damaged which can be caused by low or bad oil, foreign objects or just wear and tear after many miles of use. You have a few options really, it depends on whether you are keeping the car and how much you are willing to spend on it, either get a brand new turbo, refurb, second hand or rebuild your own. Regardless of which option you choose, get a new turbo gasket set, oil feed line and give your existing oil return line a good clean. With the oil return line being a bigger diameter pipe, it can be cleaned unlike the oil feed line. The reason for a new oil feed line is if they clog they can cause low oil pressure going to the turbo.

[VAG-Abound]

Thx.

There does seem to be some oil leaking around that area. Is it worth trying to trace where it might be weeping from the turbo to see if a fuller bearing cheers things up? No obvious drop on the engine level though, so it must be pretty slow if it is there.

It's a 58 plate in generally good condition, but not great value. Thinking of changing it anyway, so a good service of what's there may be a reasonable investment, but new turbo probably not since it doesn't cause too much trouble on the typical about town / commute runs. Happy to tell future owner about that and let them replace it if they see fit.