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VW MKIV-A4 TDIs (VE and PD) This is a general discussion about A4/MkIV Jetta (99.5-~2005), Golf(99.5-2006), and New Beetle(98-2006). Both VE and PD engines are covered here.

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Old June 18th, 2011, 12:26   #1
edmcnierney
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Join Date: May 2011
Location: Groton, MA
Default Yet Another 2002 Jetta P0234 Overboost Problem

Hi, folks; I've already gotten a LOT out of these forums and I've been spending a few weeks diagnosing (and researching here) an overboost problem on a 2002 Jetta TDI. I've done a lot of homework and I'm developing a theory and would appreciate comments/feedback/suggestions. I apologize for a wordy introductory post, but I really have been trying a few things and need specific guidance rather than being advised to read "Diagnosing and Fixing Limp mode for A4 1.9TDI" by CanadianGrizzly (been there, done that, it's great - thanks).
Background
This is my daughter's car, purchased used with about 90,000 miles on it. She had an accident shortly afterward which required replacing the turbo intercooler and connections. The aftermarket turbo boost gauge was not reconnected after this repair. The car appeared to run OK and definitely did NOT set the MIL or throw any P0234 overboost codes. I used it and concluded the turbo was not working; some diagnosis showed that the actuator was kaput and not moving. My mechanic replaced the actuator (not the whole turbo) and I reconnected the aftermarket boost gauge.
Lack of Boost Pressure
The turbo was now being activated, but the aftermarket gauge showed little pressure. I guessed that the turbo vanes were gummed up after sitting in the exhaust stream for a while without being moved (due to the bad actuator). I removed the front exhaust pipe and did a good round of the Easy-Off Oven Cleaner turbo cleaning routing. This seems to have worked just fine. I could now hear the whine of the turbo where I couldn't before, and the boost pressure gauge clearly shows a turbo boost at the right time.
Excessive Boost Pressure
In the process of diagnosing the turbo problems I replaced all the vacuum lines. In the process of doing that I broke the N75 valve (cut that old tubing off the old plastic gang, don't pull it!) and replaced it. So the N75 and tubing are all new. My mechanic checked the tubing and found it to be good.
I then started getting excessive boost pressure and the MIL set with P0234 (Overboost). The aftermarket boost gauge showed the boost pressure jumping up quite high (30 PSI or so) very quickly. There was no doubt that the turbo was doing its thing, but was doing it too well. The result is that after a few minutes the P0234 code gets set, the MIL turns on, and the car goes into Limp Mode. This happens very reliably, every time you take the car out for more than 5 minutes. While the MIL stays on, the Limp Mode is reset after the car is switched off then restarted; after a few overboost episodes again the car goes back into Limp Mode.
The Sticky Vanes Theory
So I hypothesized that while I had cleaned the turbo vanes and the actuator was good, maybe the vanes were a little cruddy and the boost pressure would go UP okay on demand, but maybe came DOWN too slowly (sticky vanes) when I eased off the throttle, causing an overboost condition. I drove it for a while, not being too gentle, in the hopes this would clean/loosen things up. No change was observed so I needed more data.
Time for VAG-COM
At this point, just replacing the turbo whole seemed like an expensive option, so I jumped into the Modern Era and got a VAG-COM cable from Ross-Tech and started analysis. This would allow me to compare the actual boost pressure to the requested boost pressure, which would help confirm or exclude my sticky vanes theory.
First, I checked for faults and got this:
1 Fault Found:
16618 - Boost Pressure Regulation: Limit Exceeded (Overboost Condition)
P0234 - 35-10 - - - Intermittent
Here's a graph of some of the data:

That says to me that the actual boost pressure shoots up quickly far in excess of what the requested boost pressure actually is. This is NOT consistent with my sticky vanes theory. The Ross-Tech wiki suggests this problem is caused by
  • Hoses incorrectly connected, disconnected, blocked, or leaking
  • Boost Pressure Control Valve (N75) faulty
  • Boost Pressure Sensor (G13) faulty
  • Turbo Charger stuck/faulty
I'm reasonably confident that I've cleared three of those four possibilities, and I'm left wondering about the Boost Pressure Sensor.
What Causes Over-Responsive Turbo Boost?
I found a few references to the Boost Pressure Sensor but was a little puzzled by the Bentley manual, which seems to have the wrong drawing (page 21-15) for the 1.9L intercooler, showing no Boost Pressure Sensor at all! I found some online shops selling such a sensor for the 2002 TDI and got a Bosch part number for it. I can see that there is such a Bosch part where I expected it to be, on top of the intercooler just in front of the windshield washer reservoir. So I know I've got one of them, and I'm wondering if it could be the problem. But I can't quite figure out HOW it could be the problem. It must be the thing reporting the boost pressure the VAG-COM is reading, and it seems to be doing that correctly (it roughly correlates with the aftermarket boost gauge, so I'm inclined to trust both of them).
Summary
For those who feel asleep partway through:
  • Engine reliably throws P0234 and goes into limp mode
  • N75, vac lines, and turbo actuator are all new
  • Turbo interior cleaned with Easy-Off
  • Aftermarket gauge confirms very high boost pressure
  • VAG-COM shows P0234 and no other faults
  • Actual boost pressure is seriously higher than requested
Now, just to show I have been doing my homework, I have NOT tested the MAF Sensor by disconnecting it. I do not understand how that could be causing this problem (although it could be implicated in other Limp Mode situations). Ross-Tech doesn't mention it as a cause of P0234, and I couldn't really figure out what "runs better when MAF Sensor is disconnected" means in this case.

Thanks for hanging in there this far - I could really use specific suggestions that take the information I have so far into account, so I've tried to tell you everything I know. I appreciate any input/ideas/suggestions, especially those confirming whether replacing the Boost Pressure Sensor makes sense given the boost pressure chart data above.
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Old June 26th, 2011, 16:55   #2
edmcnierney
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Let's not all jump on this at once, gang!

I have diagnosed and solved the problem myself. An experienced reader would have seen the clue in my posting above (the silver lining of being thorough and posting a small novel):

Quote:
My mechanic replaced the actuator (not the whole turbo) and I reconnected the aftermarket boost gauge
The actuator/turbo unit is often replaced whole, and the actuator (in this case) was not calibrated from the factory. In fact, it was about as far out of calibration as one could get. The adjusting rod was screwed all the way down to the stop, making the rod as short as possible and the turbo way too sensitive to a boost request from the vacuum line.

Looking at various photos of actuators, I eyeballed the right length, got something that looked good when I pulled vacuum on it, then gave it a test. No limp mode, no CEL, and a boost graph that looked just right, with a 78% duty cycle at 4,000 RPM.

I'm VERY glad I didn't give in and have the whole turbo replaced, since that would have also solved the problem and left me thinking things were far more complicated/expensive than they really were.

So when you look at those lists of "possible causes for P0234 overboost", add "badly adjusted actuator adjusting rod" to them.
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Old June 26th, 2011, 17:25   #3
IndigoBlueWagon
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Looks to me like your N75 valve is tired or you have a leaky vacuum line. My vote is for the N75. You can take it off and try to clean it out with brake clean or compressed air. It may work better, although it may not fix the problem permanently.
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Old June 26th, 2011, 17:32   #4
PeterV
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YOU did miss a great GTG not far from you with the northeasts best GURUs.

keep an eye in the east coast forum section for local assistance.
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Old June 26th, 2011, 19:20   #5
Jerry Freeman
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I just found this thread.

If I'd seen it in time, I would have asked if you'd adjusted the actuator properly.

Best wishes,
Jerry
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Old April 3rd, 2013, 18:38   #6
mel johnson
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Fuel Economy: 96 Passet=>46 always. 2002 Jetta wagon 42.
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I have the same problem and have done the same things and more. I removed the turbo and cleaned it and replaced the actuator so I matched the length of the old rod but will give this much more attention now that I know this solved your problem.
thanks for the post as it at least gives me hope. Ps I also put an analog gauge in because I didn't trust the electronics.
Mel
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Old April 4th, 2013, 18:33   #7
mel johnson
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Join Date: Mar 2013
Location: Pa
Fuel Economy: 96 Passet=>46 always. 2002 Jetta wagon 42.
Smile

As you guys talked about, I gave the new actuator rod more length and so far the longest trip without any code or limp mode. The boost still goes 25+ for a second but comes down to a good pressure very quickly.
I am confident, because of everyone's help and postings I am the road to survival.
Thanks all for sharing.
It works,
Mel
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Old April 5th, 2013, 21:07   #8
Deadend
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TDI(s): 2001 Jetta
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Having pulled my turbo and replaced the actuator recently, I didn't have this problem. What I did was calibrate the actuator on the bench to that it reached both positions at the right amount of vacuum with my pump.

Also wanted to note: Did the same bloody thing with my N75 nipple. When replacing all the vacuum lines it was the first one I tried to pull off and - SNAP!.... I used a knife to cut the rest of the connections as you say.

I was however too cheap to replace the thing for want of a plastic nipple! I managed to super-glue the old one back together. Without gluing my fingers together or to the table! Then I gasket-makered the crap out of it. It'd not pretty, but it works. (I think the super glue connection even failed, but all that silicone seems to be keeping it together just fine.
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Old September 22nd, 2013, 17:42   #9
jjpsuperdad
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Default actuator lever is out of adjustment

Quote:
Originally Posted by edmcnierney View Post
Hi, folks; I've already gotten a LOT out of these forums and I've been spending a few weeks diagnosing (and researching here) an overboost problem on a 2002 Jetta TDI. I've done a lot of homework and I'm developing a theory and would appreciate comments/feedback/suggestions. I apologize for a wordy introductory post, but I really have been trying a few things and need specific guidance rather than being advised to read "Diagnosing and Fixing Limp mode for A4 1.9TDI" by CanadianGrizzly (been there, done that, it's great - thanks).
Background
This is my daughter's car, purchased used with about 90,000 miles on it. She had an accident shortly afterward which required replacing the turbo intercooler and connections. The aftermarket turbo boost gauge was not reconnected after this repair. The car appeared to run OK and definitely did NOT set the MIL or throw any P0234 overboost codes. I used it and concluded the turbo was not working; some diagnosis showed that the actuator was kaput and not moving. My mechanic replaced the actuator (not the whole turbo) and I reconnected the aftermarket boost gauge.
Lack of Boost Pressure
The turbo was now being activated, but the aftermarket gauge showed little pressure. I guessed that the turbo vanes were gummed up after sitting in the exhaust stream for a while without being moved (due to the bad actuator). I removed the front exhaust pipe and did a good round of the Easy-Off Oven Cleaner turbo cleaning routing. This seems to have worked just fine. I could now hear the whine of the turbo where I couldn't before, and the boost pressure gauge clearly shows a turbo boost at the right time.
Excessive Boost Pressure
In the process of diagnosing the turbo problems I replaced all the vacuum lines. In the process of doing that I broke the N75 valve (cut that old tubing off the old plastic gang, don't pull it!) and replaced it. So the N75 and tubing are all new. My mechanic checked the tubing and found it to be good.
I then started getting excessive boost pressure and the MIL set with P0234 (Overboost). The aftermarket boost gauge showed the boost pressure jumping up quite high (30 PSI or so) very quickly. There was no doubt that the turbo was doing its thing, but was doing it too well. The result is that after a few minutes the P0234 code gets set, the MIL turns on, and the car goes into Limp Mode. This happens very reliably, every time you take the car out for more than 5 minutes. While the MIL stays on, the Limp Mode is reset after the car is switched off then restarted; after a few overboost episodes again the car goes back into Limp Mode.
The Sticky Vanes Theory
So I hypothesized that while I had cleaned the turbo vanes and the actuator was good, maybe the vanes were a little cruddy and the boost pressure would go UP okay on demand, but maybe came DOWN too slowly (sticky vanes) when I eased off the throttle, causing an overboost condition. I drove it for a while, not being too gentle, in the hopes this would clean/loosen things up. No change was observed so I needed more data.
Time for VAG-COM
At this point, just replacing the turbo whole seemed like an expensive option, so I jumped into the Modern Era and got a VAG-COM cable from Ross-Tech and started analysis. This would allow me to compare the actual boost pressure to the requested boost pressure, which would help confirm or exclude my sticky vanes theory.
First, I checked for faults and got this:
1 Fault Found:
16618 - Boost Pressure Regulation: Limit Exceeded (Overboost Condition)
P0234 - 35-10 - - - Intermittent
Here's a graph of some of the data:

That says to me that the actual boost pressure shoots up quickly far in excess of what the requested boost pressure actually is. This is NOT consistent with my sticky vanes theory. The Ross-Tech wiki suggests this problem is caused by
  • Hoses incorrectly connected, disconnected, blocked, or leaking
  • Boost Pressure Control Valve (N75) faulty
  • Boost Pressure Sensor (G13) faulty
  • Turbo Charger stuck/faulty
I'm reasonably confident that I've cleared three of those four possibilities, and I'm left wondering about the Boost Pressure Sensor.
What Causes Over-Responsive Turbo Boost?
I found a few references to the Boost Pressure Sensor but was a little puzzled by the Bentley manual, which seems to have the wrong drawing (page 21-15) for the 1.9L intercooler, showing no Boost Pressure Sensor at all! I found some online shops selling such a sensor for the 2002 TDI and got a Bosch part number for it. I can see that there is such a Bosch part where I expected it to be, on top of the intercooler just in front of the windshield washer reservoir. So I know I've got one of them, and I'm wondering if it could be the problem. But I can't quite figure out HOW it could be the problem. It must be the thing reporting the boost pressure the VAG-COM is reading, and it seems to be doing that correctly (it roughly correlates with the aftermarket boost gauge, so I'm inclined to trust both of them).
Summary
For those who feel asleep partway through:
  • Engine reliably throws P0234 and goes into limp mode
  • N75, vac lines, and turbo actuator are all new
  • Turbo interior cleaned with Easy-Off
  • Aftermarket gauge confirms very high boost pressure
  • VAG-COM shows P0234 and no other faults
  • Actual boost pressure is seriously higher than requested
Now, just to show I have been doing my homework, I have NOT tested the MAF Sensor by disconnecting it. I do not understand how that could be causing this problem (although it could be implicated in other Limp Mode situations). Ross-Tech doesn't mention it as a cause of P0234, and I couldn't really figure out what "runs better when MAF Sensor is disconnected" means in this case.
Thanks for hanging in there this far - I could really use specific suggestions that take the information I have so far into account, so I've tried to tell you everything I know. I appreciate any input/ideas/suggestions, especially those confirming whether replacing the Boost Pressure Sensor makes sense given the boost pressure chart data above.
I found for the code P0234 always coming back it is the lever adjustment was to low on our turbo. thank you for you tests this help me find the problem after several weeks and 200 hundred in parts . For me the VNT lever was to low causing over boost code! Thank you for your help.
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Old July 19th, 2014, 15:21   #10
caspa
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hi guys iv got a vw jetta 4 1.9 tdi thats not boosting. I have had the turbo repaired the cat removed. I have cleaned the egr valve the intake and the entercooler. I have replaced the vacuum pipes all of them and one of the n75 units. I changed the maf sensor and i also blocked off the egr valve from the down pipe to the turbo. I can hear the turbo when it does engage but thats only when im in third gear and at 3500rpm. I just cant understand where the fault still is. Please guys could you help out. If you could give me feed back via email it would help alot because i get my email straight to my fone. Anyway here is my email address. bergm37@yahoo.com thank you. Michael
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Old July 19th, 2014, 16:10   #11
keaton85
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Location: Maine
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Bringing up a year old topic is not the best answer, and no one is going to email you a solution.

First guess: graph the boost pressure via VCDS. Nothing else to do before that is seen.
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Old July 20th, 2014, 01:06   #12
caspa
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thank you for the reply.

i will check the graph with vcds and then i will let you know what it reads.
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Old July 21st, 2014, 12:11   #13
caspa
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Location: ladysmith kzn sa
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hi guys

well just to let you know i sorted out my tdi. all that was stopping me was the vacuum pipes they were not routed properly, but once i checked the vacuum diagram that i got from this forum i sorted it out and know she go's great again.

thank you
michael
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