Turbo Overboost but can't diagnose why

[AlteaXLDriver]

Hi all,

I am new to this forum so I hope I've added this thread in the right place. My wifes 2007 Altea XL 2.0 TDI recently went into limp mode whilst motorway driving. I can not diagnose what the cause is but I have checked several things and have ran out of ideas.

The car has now done 105,000 miles and probably done 3000 since the problem reared its ugly head. It still has good power and torque upto limp.

I have an ELM adaptor and using the Torque app I can see it enters limp mode with P0234 Turbo overboost. It will reset with the reset command or on power down. At high revs the turbo can be heard whining but I'm quite sure the turbo is still good. If you hold the high revs/power for 5 seconds or so limp is triggered.

So far I have tested.

With a vacuum pump/gauge I can operate the actuator. The turbo VGT actuator moves with the correct pressure, its a garret and starts to move at 5 inHg and stops at 18inHg. It holds pressure (including the hose) for atleast 30 mins.

I removed the exhaust pipe and did the Mr Muscle trick in the turbines, I believe the vanes are free.

Using Torque App I can see the boost pressure reaching 1.6 bar before entering limp so I deduce the boost sensor is good.

The MAF sensor is working too. I can see this change with speed etc up to 100g/sec.

The EGR valve just before the inlet manifold opens and closes with my vacuum pump. ( I do not know the correct pressures but it opened at about 5 inHG and stopped at 15 inHg.

The butterfly valve below this in the same housing (throttle valve perhaps?) operates by hand atleast and the valve is clean. I did not test the electrics on this valve.

The N75 manifold/control gadget appears to be working. I say this as I fixed my vacuum gauge to the VGT actuator line whilst driving and I can see it goes to 18-20 inHg at idle and varies with driving conditions. It flicks to zero bar when lifting foot off the gas and it does reduce in vacuum as the engine gains rpm.

So now I've ran out of ideas. It is possible that the N75 can still appear to work, i.e. its's not totally dead, but does not work correctly? I do not know what other inputs are part of the VGT actuator control system.

I am reluctant to take it to a garage as I've read so many stories of garages just saying you need a new turbo and I want to avoid spending more than necessary getting it fixed.


Any ideas or advice would be greatly appreciated.

Cheers

 

[Dan FR]

Using VCDS you can check the operation of the VNT control. It can raise the RPM and cycle the N75 actuation so you can check that the vanes are moving as they should be. After that, data logging of the request, actual and N75DC% is needed. Torque doesn't show N75DC% and you need this to understand what is going on with the boost control

Have you removed the actuator arm to check that the VNT Lever moves smoothly and easily across its full range of travel?

If you are seeing 100g/s i would suggest you are mapped?

 

[AlteaXLDriver]

Firstl,y thank you for your reply.

I bought the car with just 2500 miles on the clock and the previous owner was Seat UK. From that I would guess it's original. I certainly haven't had it mapped. From your suggestion this sounds like a high value? I guess this backs up the overboost/limp mode and the turbo whine it can generate now.

I haven't actually removed the actuator arm on the VGT to move it with my finger tips, however when I apply a vacuum I can feel it move at 5inHg and it seems smooth and gradual all the way to 18 inHg when it hits the stop.

Perhaps I should unbolt the actuator from its bracket (it looks easier than removing the circlip that secures the end to the VGT pin) and feel just how free it really is.

VCDS is dealer monitoring equipment I guess, not something I can buy for a few £?

Maybe it's looking like something I can't fix?

Do you know what all the inputs from sensors are that ultimately effect the control of the VGT?

Thanks again.

 

[Dan FR]

What engine HP and code/model? I misread the opening post and assumed it was a PD105 rather than a 2.0L model, 140 or 170hp?

1.6 bar of boost may well be the limit of the sensor, so who knows how much boost you're actually hitting. If the turbo is whining then you could well have bearining damage and an imminent failure

I always unbolt the actuator having lost many a clip (not available separately!) but before any of that I would try and find someone with VCDS. Its not dealer equipment but as good as, and allows you to monitor more than just generic OBD2 data that Torque does.

Genuine is many hundreds, clones can be had for as little as £20-£30 from china, or £50ish on ebay/amazon if impatient. There's also a dedicated group on Facebook (VCDS Users UK) with a map showing locations of many users for you to contact someone local and arrange some data logging if needed

The Turbo is controlled by the N75, which is controlled by the ECU. The requested boost depends on many inputs such as pedal request, RPM, Load etc. and the ECU adjusts the N75 opening (Duty Cycle - shown as a %) to meet the requested boost. ECU uses PID control to manage deviation either side of specified, and if the deviation cannot be managed i.e. overboost throughout acceleration, it enters limp mode to protect engine components

Logging with VCDS will show what the ECU is requesting, what the turbo is delivering and what the ECU is doing to control it.

In all honesty, 9 times out of 10 it is down to the turbo in this situation

 

[AlteaXLDriver]

Hi Dan FR,

The car is the 140 hp, I don't know the code. After reading your reply and looking into the VCDS I have just purchased one. I figure if you use it only twice its paid for itself.

I'll unbolt the actuator tomorrow. Hope it's easier than it looks. Access is't great when it's still on the car. I will keep you posted.

My limited understanding of the turbo control system was leading me to think that perhaps the turbo is good but there was a sensor somewhere that was giving either false info or none at all.

Your comment about it being the turbo in 9 times out of 10, I'm not suggesting you're wrong but in mind the turbo is working (or at least doing what it's being told to do). Can you please explain a bit more?

I'm really starting to think that the days of diesels being the most reliable engines are behind us and that petrol is the way to go now. All the exhaust after treatment methods seem to me to be the weak point of the engines these days. Is it just me or is the tide moving away from diesels to petrols?

 

[Tezza]

Hi all,

I am new to this forum so I hope I've added this thread in the right place. My wifes 2007 Altea XL 2.0 TDI recently went into limp mode whilst motorway driving. I can not diagnose what the cause is but I have checked several things and have ran out of ideas.

The car has now done 105,000 miles and probably done 3000 since the problem reared its ugly head. It still has good power and torque upto limp.

I have an ELM adaptor and using the Torque app I can see it enters limp mode with P0234 Turbo overboost. It will reset with the reset command or on power down. At high revs the turbo can be heard whining but I'm quite sure the turbo is still good. If you hold the high revs/power for 5 seconds or so limp is triggered.

So far I have tested.

With a vacuum pump/gauge I can operate the actuator. The turbo VGT actuator moves with the correct pressure, its a garret and starts to move at 5 inHg and stops at 18inHg. It holds pressure (including the hose) for atleast 30 mins.

I removed the exhaust pipe and did the Mr Muscle trick in the turbines, I believe the vanes are free.

Using Torque App I can see the boost pressure reaching 1.6 bar before entering limp so I deduce the boost sensor is good.

The MAF sensor is working too. I can see this change with speed etc up to 100g/sec.

The EGR valve just before the inlet manifold opens and closes with my vacuum pump. ( I do not know the correct pressures but it opened at about 5 inHG and stopped at 15 inHg.

The butterfly valve below this in the same housing (throttle valve perhaps?) operates by hand atleast and the valve is clean. I did not test the electrics on this valve.

The N75 manifold/control gadget appears to be working. I say this as I fixed my vacuum gauge to the VGT actuator line whilst driving and I can see it goes to 18-20 inHg at idle and varies with driving conditions. It flicks to zero bar when lifting foot off the gas and it does reduce in vacuum as the engine gains rpm.

So now I've ran out of ideas. It is possible that the N75 can still appear to work, i.e. its's not totally dead, but does not work correctly? I do not know what other inputs are part of the VGT actuator control system.

I am reluctant to take it to a garage as I've read so many stories of garages just saying you need a new turbo and I want to avoid spending more than necessary getting it fixed.


Any ideas or advice would be greatly appreciated.

Cheers

 

[Tezza]

Turbo overboost this is caused by the vanes sticking in the turbo, p0234 code comes up I tried everything but unfortunately had to take the turbo off at 35000 miles.Took 3 days ,taking care not to break off exhaust manifold bolts ,can be done on ramps ,most of the unbolting is done from the top.

 

[Chris99]

Hi I’ve had a p0234 on my 2010 Leon 2.0 tdi sport and going into limp. Vacuum checked all pipework and corresponding components and all hold vacuum. Actuator arm moves turbo vanes. Only thing I noticed is when vdcs testing boost solenoid vacuum is 25” idle the same as the vacuum pump is creating but when not being tested I’m only getting 16.5” idle. Might be normal but I assume I should have 25” vacuum when the Solenoid valve is on. Could this be the ecu being sent wrong information by MAP or MAF sensor?