Anatomy of a Failure: VNT actuator wear (images)

58% Fe, 24% Ni, 15% Cr, and you didn't find any molybdenum ... It's stainless steel, but I don't have a book in front of me that would narrow it down to a specific type.

Within the measurement error of SEM, this is similar to
Alloy A-286 except for some missing Molly.
Percent
Al - 0.18
Si - 0.5
Ti - 2.13
V - 0.3
Cr - 14.8
Ni - 25.5
Fe - bal
Mo - 1.3

There may be a better match - I just dug for a few minutes.

Geoff - did you see any molly?

i cranked the accel voltage up to 30 so I could maybe get the K line to stand out, but 17.481 wasn't a tick over background so I'd have to say no moly where I was looking.

I'm somewhat suspect about the V as well. The Ka peak for it overlaps the Ti Kb and the Kb peak overlaps the Cr Ka. So i'm kinda letting the computer factor the calculations. But it could be close.

I was expecting to see some Moly - and after the injectors have purposefully looked for it.

Maybe I'll try polishing down an area to get it smooth and run for a long time . . . see if I can't get something.

Wow... bad news, Geoff. But yet another entry into the Circus Sideshow of Mangled TDI Parts! I have a feeling that there will be much discussion about the possible causes once you materials dorks finish up with your analysis

I have to make a comment about the stop screw... if I understand your photos correctly, it looks like it DOES make a difference. remember, the stop screw adjustment is intended to OPEN the vanes at idle to reduce the rise time and the boost spikes. it looks like in your photo that there is definitely a difference (albeit a small one, but Boundy covered that already) between the stop-screw limited travel and the internally limited travel. sure, if your intention is to CLOSE the vanes more, you'll hit the internal limit pretty quickly, but setting it to open them more should work.

If the ring and lever arm really are stainless, then this could become a common failure in higher mileage cars.

Stainless rubbing on stainless is not good. Stainless is not a good bearing material. Actually, stainless is not a bearing material. I think the nickel nodules break out of the structure and become abrasive particulates that propagate more wear. These nickel particulates are far more abrasive than soot. They make soot look like baby powder. Maybe the soot was thought to be a dry lube for this 'bearing' area to reduce the stainless on stainless contact and reduce the liberation of nickel abrasives.

I had a really bad experience with stainless on stainless friction.

But, in this application, it was probably the best price/performance compromise material selection.

Geoff,

The Sulfur K alfa peak you are looking at is more likely Moly L lines. It may be that there is some sulfur present also, but not likely in the base alloy. Try re-running the analysis forcing the ZAF quant program to use the Moly L peaks instead of the Sulfur K. Watch the curve fit. If it's got moly, then the peak should have a widening towards higher Kev side. Sulfur is a sharper more symetric peak. Now if you've got all three, S, Mo, and Pb it can be very heard to get good quants, but with only two of them, one being S, it's not too bad...

dj

This ain't right. The L-shell X-ray cross-section is much lower than that for the K-shell. If Geoff hasn't seen the ~17.4 keV X-rays for Mo, he certainly won't see the Mo L-shell photons. Surface contamination is always an issue in these experiments.

How many other high mileage turbos have beeen looked at for this type of wear? There seems to be many TDI's with equal or more mileage that don't have "super overboosting turbos".

You guys are the best BS artists ever, all that metal analysis talk almost sounds authentic .

-Jack

To my knowledge, this is the first publicly documented failure of this type. By no means does this imply that the failure has not occurred before!

As a general rule, where two parts are to rub together, it's best for them to be dissimilar materials. (For example - the piston-to-cylinder interface has chromed steel rings running against cast-iron block, aluminum-silicon alloy pistons running against cast-iron block, and chromed steel rings running against the aluminum ring grooves.) Stainless-steel against stainless-steel is asking for severe galling in the long term.

we have one car lined up that more than likely has the same issue with overboost and an equally exhausted diagnosis of every other single component. And at some point i'm guessing we'll have that vehicle apart.

About the Mo and S lines.

The profile of the peak is definately closer to Mo than S. I didn't try the ZAF correction on the quant subtracting S and replacing it with Mo. BUT I did collect the data at 30kV from the previous injector work dj was directing. The 17.ish eV line (i'm not by the periodic table and I don't have it memorized) was not above the background. BUT if it is a low enough concentration that group of lines might not make it to the detector above background.

Load might be a big issue here in wear? exhaust gasses might have been coming out faster than design, putting more force on the acutaor? (just another guess - don't remember if I mentioned it before).

If anyone has an inside source at Garrett ask em about this - it woudl be interesting to get their perspective. As for this happening much or not? The dealer has been replacing turbos more often than usual lately, could be a wear issue. Problem is dealer service just takes the turbo off and sends them to Canada to be rebuilt (so any of yall nort of da border know someone at the rebuild place maybe we can score a ring )

Is this something that can be "felt". Meaning can we disconect the linkage to the diaphram and feel play in the rod. That would make for easy troubleshooting.

Hm. I'm wondering if high soot levels from chipping/injectors contributed to this. Isn't soot super-abrasive? Maybe combine that with higher bearing forces on the lever due to full vacuum bleed from the dawes device, and you get accelerated wear.

I thought GeWilli used that soot-free super fuel, Amoco Premier?

Oh, was this car hammer modded?

Well, upsolute + .205 makes lots of smoke regardless of the fuel you use. Actually, upsolute isn't bad in and of itself, but throw the .205s in and you get a freight train.

Soot buildup in the turbo can be an issue, too. I've got stock chip and PP520 nozzles... if I dont give 'er a full throttle run for a few days, even though I typically have VERY little to no visible smoke out the back I get a GIANT CLOUD of soot the first time I punch it from a stoplight. Usually after someone's been tailing me... So... all that soot just sits in the exhaust... and I'm sure some of it was on the VNT mech, until I blow it out.

Makes me really want a little more boost to lower the soot output a bit.

could have been lack of hammer mod.

my soot level in the oil was as low as anyones. For the whole duration i was using biodiesel (duration of ups+205s) in B20-B50 range. AND I was still making soot at WOT or even 60%. I was fully expecting to see the turbo looking like the one Thomas cleaned up. But no - the turbo was completely clean almost.

Which might pose this: Was my car too clean? Was there not enough soot? Hard to imagine since I could create a huge black cloud if I wanted. After doing the "hammer" mod (mechanical adj of the IQ so I could get to 6 instead of 3) the smoke was better. Maybe the soot was the hard complete stuff not soft fluffy crappy fuel soot? And yes It is Premier, i've got fuel analysis that lrpavlo and I did.

Yes it is 'possible' to feel the rod linkage doing this if you take the VNT actuator off (2 10mm bolts and a spring clip - not bad) but ya gotta move it back and forth quite a bit to feel it. the orientation on the engine block makes it a bit more difficult to feel it (relative to taking the whole manifold off the engine).

but yeah Cage, if someone says they are having over boost limp modes (and a fault code stored - it doesn't give a CEL but it does store a code), after checking the vacuum and the N75 and boost pipes and the intake and the EGR and making sure everything is okay, check the duty cycle on the vag-com at a steady state cruise, and then take the VNT off.

The duty cycle really is the clue here. there is a good target for a working one and the car does compensate in movement and in relative vacuum. So possibly it isn't even necessary to take the VNT off (thinking as I'm writing, a rather revealing and not so private process i guess). Just watching the data log and the duty cycle as reported by the VNT (block 011) might be enough.

Ya know, in looking at the opening pics again, it could be that the lever & the ring weren't properly aligned to each other.

Notice how the lever was worn and a similar wear pattern on the ring. The lever and ring did not line up such that all of the thickness of the bearing surfaces of both parts were used. That's why a portion of the thickness has worn away on both parts. It looks like 2/3 of the thickness of the lever was used and 3/4 of the thickness of the ring was used. This is not good. This causes much higher contact pressures, which results in much higher bearing stresses, which results in higher wear rates.

If those two parts are the same thickness, they should line up edge-to-edge. If one part is thinner than the other, it should line up between the two edges of the thicker part.

I think there was a misalignment that caused a reduction in the bearing area which caused excessive contact stress, and resulted in excessive wear rates.

From what I remember, the actuator pivot shaft had a lot of play in it's bore as well. This could have added to the misalignment problem.

I don't do SEMs, but I have a engineering background in engines. I might make a few observations. First, 165,000 miles in a turbo with moving vanes isn't too bad, especially if it has been hot-rodded. Second, the turbo will see more heat with the chip and injectors and if it has no EGR, then it is hot and highly oxidizing in there. Less boost and EGR is a lot better for the turbo. Get nice oxidized bits of metal in a sliding mechanism, (versus soot) and you will accelerate wear. The aforementioned extra heat and oxygen might have been the contributing factor to the formation of the abrasive?

10 years ago you would have been happy to get 165,000 on the turbo and you would have had a crappy internal wastegate, not VNT. I suspect that we are near the edge for longevity with this turbo size and power output, at least as far as living much past a warranty period at an elevated power level. perhaps this is part of why we get 90bhp and Europe can get more? I am assuming that the warranty period in Europe is less generous, but I am not sure on that point.

lemme restate/clarify something

Car until 1-2 weeks before replacing turbo had close to stock levels of EGR gass recirculated.

For 15-20 of the 25 or so thousand miles before replacing the turbo (the last few) boost was kept to a minimum (right foot) and peak cylinder pressure conditions were reduced (no significant fuel below 2,000 RPMs).

Looking back at the Data logs prior to big injectors, i'm 100% convinced it was a problem long before they went in there. ESP since the car has acted like it has had a +1 VNT for a year or so (soon after being chipped by Ups).

This car is not hot-rodded - spends 95-99% of the time driving on the highway at cruise control.

Should have had no problem. And mileage hasn't decreased - infact I'm above average.

Will this next turbo last as long as the old one now that the EGR is bypassed completely? Dunno. Am I terribly worried about it? Nah. Am I scratchin my head as to how it happened? well . . . in a way yeah but I know HOW it happened, and why it happened? well that is something i want to know. It shouldn't have happened IMHO, not based on my driving, and from what it sounds like more data points might be coming in proving that this is a typical failure of the VNT turbos in the 1.9 application (even stock).

I hope to have more info on the turbo this evening, and if nothing else I'll see if I can't run a few more spectrums to see if I can find any Moly in there...

Re: Anatomy of a Failure: VNT actuator wear (image

Geoff,

Anything come of your continued analysis>? or did you ever get aa chance to do it? have you seen any more oddly worn actuators?

17 year old thread revival! (I'll start a new thread as well, but wanted to add a couple of pics that I think demonstrate what this thread is about. The original photos are no longer visible.)