edmcnierney
New member
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
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:
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.
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
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
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.