This is conjecture, if the diaphragm failed I would think under boost? My reasoning is if there is x pressure in the exhaust chamber that overcomes the tension on the diaphragm the wastegate opens. A ruptured diaphragm would be my opinion weak and result in under boost or no boost? So to get overboost seems like there's either extra pretension or the wastegate doesn't open at all?

As far as n249 if that were bad or the vacuum pressure lines leakey that would affect off throttle back pressure. That valve expels closed throttle pressure. Could that be the case and overboost spike at closed throttle???

Regarding n75 if the vacuum pressure lines were leaky then it wouldn't be monitoring the wastegate properly.

Assuming the two solenoids are functioning electronically as stated. The other possibility is a vacuum leak. These lines carry vacuum and manifold positive pressure.

In any event the vacuum test will be revealing.

 

Just FYI - I have this exact same issue and spoke with a VAG specialist yesterday and Bentley.

The N249 valve blowing is normal under overboost condition, it's what it's designed to do, so if that is happening, then the N249 is working correctly, so no need to talk about that anymore.

Max Vehicles - how did you do a test of the N75?

When I asked I was told that you can't with VCDS, you can only read the ECU input signal to the solenoid, if the ECU sends signal to the N75, it doesn't mean the solenoid is reacting correctly to that input signal.

The N75's are prone to failure and even with removing the valve and electrically testing the resistance doesn't guarantee good functionality. Old N75's tend to fail when warmed up / hot. So to test, you remove the N75 and put under hot water to heat, then check the resistance.

My car works "boosts" for a bit then has the same symptoms (when warmed up a bit), overboosts N249 blows the pressure and i'm running NA after that, sucking away any real power (usually over 2500-6000rpm, i.e. on boost). The N249 blowing the air gives me a MAF implausible signal DTC, cos well, all the air's been blow out a hole to the atmosphere, so that's fairly obvious. This tells me which bank is at fault, as only 1 MAF fault on my drivers side.

Vacuum lines split will cause overboost or a punctured diaphragm / sticking wastegate. So it's going to be 1 of those 3 (N75 faulty, wastegate or a line between the two).

I am planning on bypassing the N75 and running just on wastegate pressure, this will tell me which is at fault.

If the car now boosts fine(ish), then the wastegate is fine and the N75 is jammed. If the symptoms are same then the N75 is probably working but the wastegate is punctured / jammed. I will check all the lines when doing the bypass to remove that option from the table.

But still not fixed yet, so will keep you posted if I do fix it. My car comes off the road over winter, so i'll strip it down then and fix it. It's still driveable, so I'm not going to worry about it till then.

I don't drive the car hard tbh, so i'm just going to drive it leisurely till winter (off boost). And then fix it over winter (October-).

 

For the record the Volkswagen turbos experience the same n75 and n249 failures. However their wastegate is a different design. Looking through those threads one sees a pattern similar to the Bentley. N249 also fails in the closed diverter valve position allowing for excessive pressure in a closed throttle position. The result is damage to the turbo overtime. Performing the output test after the engine is quite hot will also check if the solenoid is clicking at least.

When stated earlier to check the vacuum on the wastegate. One can also hook a vacuum line to the rubber hose which goes back to the turbo solenoids. With a helper one could do the output test and apply vacuum and see what is happening if solenoid is modulating allowing vacuum pull and then turning off. Not positive but I think this rubber line goes back to the n75.

 

There is a lot of great info in these replies, thanks. I am going to try to make some time over the next few days and do some of these vacuum tests. Dav, you are right, I was not able to output test the N75 (I just data logged what boost level the ECU was commanding, see post #5 and 22) , it has been a while since I did it. For n249 I was able to do an output test in VCDS on it and it's counterpart on the other bank. All I did was confirm that n249 made the same noise as the one on the other bank. I will update after some more testing.

 

When I investigated vcds output test on the engine address 01, showed output test for n75 but not n249. Where is n249 output test?

THX

 

There is a "turbo pressure bypass valve" in both engine address 01 and 11. It does not say n249, but I thought that the one in 01 is n249 (also called C00667) and the one in 11 is the same thing for the other side (C00668). I could be wrong though. The picture from IETIS below illustrates what I am talking about.



Does anyone know how much vacuum pressure it takes to open the wastegate? It held at 7.5HG for a few seconds, so I assume the diaphram is ok. It did not open at 7.5HG so I wanted to ask before I put too much vacuum on it and blow the diaphram.

 

I just tried 10PSI of positive pressure and the wastegate still did not open. I think it must be seized. I am probably going to try and open it with mechanical force if I can figure out a way to get a hold of the rod. I am assuming it is seized internally somewhere where I can not get penetrating to, so I am pretty much out of ideas.

 

15 PSI positive pressure does not open it. I was able to apply some mechanical pressure to the shaft but not much, it did not budge. The view is a little obstructed by the old hose clamp, but the red arrow shows where I disconnected the air line and hooked up one that I am using for positive pressure and vacuum. Am I doing something wrong? The yellow arrow shows where I am thinking about removing the rubber boot and spraying in some penetrating oil, but I am not sure if that will help.

 

Looks like the wastegate is stuck, not sure what the vacuum threshold is. On my spare turbo I can move the wastegate a tiny bit by just sucking on it 🤤.You are connected to the correct space. Which turbo is this? Will you be checking the other one, since this would be a good comparative indicator regarding vacuum and movement.


Further testing the output test n75 which is connected to the hose. Blow into the rubber hose while it cycles and see if it turns on and off allowing air to push through and then stop.

 

If the wastegate is stuck and the diaphragm is holding vacuum that is a better scenario than a ruptured diaphragm. The next challenge will be getting lubrication to small shaft through the exhaust side of the turbo where the wastegate pivots. That pivot point may need lubrication. Should that be the case forget about WD-40 go right to penetrating oil. This maneuver will take some creativity not a lot of space to work with.

 

The pivot point is circled in red where the lubrication is required. Please don't mess with the diaphragm. Oil on the diaphragm will guarantee failure of the diaphragm. The circled pivot point is the shaft that operates the actual wastegate. Historically on many turbos this has been problematic.

 

Sorry to bump an old thread, but I wanted to update it with the correct resolution. I had done the wastegate test incorrectly. The wastegate did work when vacuum was applied to the bottom nipple. When the car is boosting the wastegate is actually controlled by the top nipple (I believe positive pressure). The air hose from that nipple goes to the boost control solenoid (in this case bank 1 N75 solenoid).

I bought the boost control solenoid (get it from VW part # 06a906283e for $100 instead of $400 plus from Bentley for the same VW solenoid), and I had a great local shop replace it. That did not work, but the tech found a crack in that air hose that runs from the wastegate to the boost solenoid. That fixed it, $1,500 total cost, but would have been about $500 if I did not have them unnecessarily replace that solenoid.

Thanks for the help, turned out to be an easy fix. One caveat is that this shop does a lot of things that are supposed to be engine out without pulling the engine. They did this very tricky hose replacement with long pliers and other tools to reach, it did not sound fun. A lot of shops would probably take a costlier approach.

TLDR: The bank 1 overboost was caused by a crack in the air hose between the N75 boost control solenoid and the wastegate. The crack allowed too much air too escape so the wastegate was not opening. Fixing the hose completely resolved the overboost issue.

 

Thank you for your update!👍