Clean or replace VNT15?

[McGuillicuddy]

Edited this post to reflect new information. I'm pretty sure I've narrowed down my occasional limp mode issue to carboned up internals of my VNT15. During the pandemic the car does a lot of short trips without getting up to temperature which I suspect is gumming up the vanes/control ring in the turbo, leading to an overboost condition on the highway when I make small adjustments to the throttle while cruising.

My question is for a relatively low mileage car (125K miles, Malone stage 2 if that matters), is it worth trying to find somebody who will disassemble/clean the turbo? I don't have the time/space to do this work myself (even though I would like to). Or am I better off just biting the bullet and ordering a new one from IDParts?

Edit: update below.

 

[McGuillicuddy]

Note I have replaced N75 and vacuum lines for anybody about to recommend I do that first .

 

[TurboABA]

I would suggest you do more troubleshooting to verify that the VNT is the issue.....

 

[super1]

Before you go & spend money on cleaning or a new turbo you should make sure that's your problem, you can take the clip off the actuator & see if the the lever of the turbo has full range & moves freely
might be able to just unbolt the actuator & do the same test

 

[McGuillicuddy]

I would suggest you do more troubleshooting to verify that the VNT is the issue.....

Well aren't there only so many options? For overboost condition it's either the N75 (which I replaced with new Pierburg unit), vacuum leak (lines replaced), actuator (checked it and moves freely, not rusted), or the vanes/control ring sticking. Anything else I'm missing?

 

[TurboABA]

Did you vacuum test the actuator? Does it HOLD vacuum or does it have a leak?
If the actuator moves as per your tests, how can the VNT be stuck? Or is the actuator not connected to the VNT arm?
Spray all the pivot\bushing points with penetrating oil in case there's rust and they hang in certain points, but my best guess without any other information would be to suspect a leaky actuator\diaphragm.

If you were using VCDS you could look a data as well as conditions under which it happens and no one would have to guess or throw random parts at it.

 

[TurboABA]

Also check that your heatshield is still on the actuator and that the vent hole isn't clogged up with crud\rust etc.

 

[greengeeker]

Well aren't there only so many options? For overboost condition it's either the N75 (which I replaced with new Pierburg unit), vacuum leak (lines replaced), actuator (checked it and moves freely, not rusted), or the vanes/control ring sticking. Anything else I'm missing?

It's even simpler yet...a vacuum leak results in an underboost, not overboost. Not a candidate.

Best bet is to disconnect and remove the actuator. Swing the lever arm through its full range of motion with your finger. It should move freely with the lightest touch, no 'catchy' spots. Similarly for the actuator off the turbo, when releasing vacuum it should smoothly extend without any pauses.

Upon reinstallation of your actuator, ensure it is adjusted properly. Hits stop screw @ 18"Hg, lifts @ 3-5"Hg.

What you can't glean from either of these tests is control ring wear. I'm not aware of a way to physically hold the vanes inside the turbo whilst it is assembled (appreciate ideas). I tried bending up a coat hanger but no dice. If you can hold them you could wiggle the lever arm to get a feel for any wear in the control ring. THIS was the problem on my own ALH after 350k miles. New turbo cured my overboost issue.

HTH.

 

[TurboABA]

Could also be a flaky connection\wiring to N75 or something if all other tests pass..... just because the valve is new doesn't mean it's getting the proper voltage from the ecu, etc.

 

[McGuillicuddy]

Could also be a flaky connection\wiring to N75 or something if all other tests pass..... just because the valve is new doesn't mean it's getting the proper voltage from the ecu, etc.

I would expect that to throw an error at random, no? In my case I only ever get limp mode in 5th gear during highway cruising. It is very situational. If I'm thrashing the car around town (accelerating/decelerating) it never goes limp.

 

[TurboABA]

You haven't shared any codes\errors.
What are you getting? Can you see FF data?
Could be that you're dropping some other sensor reading like MAF or something, under a certain load\condition. Just saying you're getting overboost is very generic.

 

[McGuillicuddy]

You haven't shared any codes\errors.
What are you getting? Can you see FF data?
Could be that you're dropping some other sensor reading like MAF or something, under a certain load\condition. Just saying you're getting overboost is very generic.

Sure, fair enough. I don't know much, but here's what I do know:

I'm getting a P0234 Overboost error, quite reproducibly under specific conditions: it only appears on the highway when I'm cruising in 5th gear. Hard acceleration and general thrashing of the vehicle around town does not cause a problem, nor does lower speed cruising in 3rd/4th. I don't have any more advanced diagnostics than a bluetooth code reader. As mentioned I've replaced the N75 and the vacuum lines. My local TDI guy looked/played around with the VNT actuator on the car and found that it moved freely and was not rusted at all.

My understanding was that the MAF could not cause an overboost code so I have not looked in that direction. I don't know if there is a boost pressure sensor that could be mis-reporting the pressure, but I think it would be odd if it was only doing so under my specific error conditions.

 

[TurboABA]

The ecu looks at MAF\TPS\MAP\IAT\BARO\N75.... even more to control everything.... not being able to see what all these are doing, or not having any freeze frame data from when the codes get triggered will leave you guessing. If your dongle can display values, try to look at these to see if anything looks weird. (or get a proper tool)

 

[McGuillicuddy]

Sorry, I should mention that only once in a while do I get a CEL. 4 out of 5 times (ish) there is no CEL. My understanding based on https://forums.tdiclub.com/index.ph...r-a4-1-9tdi-low-power-troubleshooting.179589/ is that the MAF would always through a CEL. But I will try unplugging the MAF and see if it still goes limp.

 

[TurboABA]

16618/P0234/000564 - Boost Pressure Regulation: Limit Exceeded (Overboost Condition)
Possible Symptoms

• Malfunction Indicator Light (MIL) ON
• Boost Pressure too high
• Reduced Power Output

Possible Causes

• Hoses incorrectly connected, disconnected, blocked or leaking
• Boost Pressure Control Valve (N75) faulty
• Boost Pressure Sensor (G31) faulty
• Turbo Charger stuck/faulty

Possible Solutions

• Check Charge Pressure Control System
  • Perform Output Test (to check the Turbo Charger)
  • Perform Basic Setting (to check the Charge Pressure Control)
  • Perform Pressure Test to locate Leaks
• Check/Replace Boost Pressure Control Valve (N75)
• Check/Replace Boost Pressure Sensor (G31)
• Check/Replace Turbo Charger

 

[McGuillicuddy]

Guess I'm buying a MightyVac.

 

[burpod]

Take an 011 only log and post it up, that includes various percent throttle inputs at various rpms, and holding each throttle press firmly in steady position for at least 5-10 seconds, and if possible, for longer 10-20second for boost to fully stabilize. Lower fuel pedal positions don't need as much time for boost to stabilize.

 

[burpod]

Also, I didn't see it mentioned, but how long has this problem been around? Before or after tuning?

 

[McGuillicuddy]

I don't have any way to take logs right now. I'm guessing i need a VCDS/vagcom cable setup for this? I've had the tune since mid-2019, so it would surprise me if it was suddenly causing a problem.

 

[McGuillicuddy]

Update: so after all that talk about expensive repairs, I realized that my air filter was quite dirty (it was well overdue) so I replaced that and (probably more importantly) cleaned the MAF with some SensorKleen. Today I took 2 separate 1hr trips on the highway and I was unable to induce limp mode no matter what I did (hard pulls on hills, casual cruising, cruise control on/off, etc.). This is after being able to reproduce it fairly easily before. Is it possible that I've fixed this with the MAF cleaning + air filter change? The only other variable that I think has changed is the ambient temperature. It's a fair bit warmer today than it was the last time I induced limp mode on the highway. Not sure if that could be a contributor or not.

 

[TurboABA]

 

[McGuillicuddy]

Is that a "yes"?

To be fair to myself, I've had this thoroughly checked out by a VW mechanic and he was the one suggesting that this was a turbo issue. I'm glad I started poking around myself before jumping off the turbo cliff if indeed the MAF cleaning has solved the problem.