Overboost condition 2.0 TDi 000564 / P0234

[CDF]

B6 2.0 TDi 140 PD, DSG, 160,000 miles.

The car went into limp mode the other day on the motorway whilst accelerating and the glow plug light illuminated. After stopping and restarting, the car behaved normally.

A VCDS scan showed up the above code(s) and I've read the Ross Tech wiki for it. I've also seen the many results in Google and on this forum: I just wish to clarify some things before proceeding.


I've been out in the car today and monitored some live data in VCDS (with a passenger): MAP pressure (absolute), RPM, calculated load and ECT. I really just wanted to see the MAP reading under various loads. I see a peak of 2.55 BAR under full throttle, which subsides to a sustanined 2.4 BAR after about 1 second (these figures are absolute, remember). Not once was limp mode triggered.
Does anyone see anything of note in those figures?
As I understand it, the MAP sensor very rarely fails. Can it be reasonably concluded that my MAP sensor is accurately reporting the boost pressure to the ECU?

I have removed the middle vacuum line of the boost pressure control valve (N75) and sucked on it.
The actuator (for the VNT vanes, right?) then moves easily and clearly stops with an audible stopping sound (not mushy resistance).

Does this definitely mean that I don't have to remove the turbo to clean the vanes and that I can rule out the vanes are the cause of my overboost condition?

Are there any definitive tests I can run on the N75 to either condemn it or rule it out? Is it simply a case of take a punt on a new one and see if it works?

There is a vacuum diaphragm to the left of the inlet manifold. I can get the rod to retract when I suck on the vacuum supply line. Could this component (is it a swirl flap?) in any way force an overboost condition?

I haven't yet checked thoroughly for boost leaks in any of the hoses (or the intercooler itself). Would a leak cause an overboost though? Logically, it seems like it wouldn't because there'll be less boost pressure at the manifold. Also, my MAP readings above should be lower if there's a leak?

Of course I've replaced nothing yet: I'm trying to narrow down the fault before throwing money at it.

Thanks for any help!

[Crasher]

More than likely it needs a turbo rebuild. Do you have a registered copy of VCDS?

[DMitch16]

The vacuum solenoid near the inlet controls the manifold (swirl) flaps as you state but the usual fault for this mechanism is the "Inlet Manifold Runner Control Deviation" where the flaps are usually sticky and not in the expected position. I get this often as the plastic mechanism allows the vacuum to pull the actuating lever completely vertical causing the flap rail to stick, then suddenly snap back under the spring pressure when you lift your foot right off the gas catching the ECU unaware and Ping fault light on. There's nothing wrong with my flaps or rail just the wrong amount of actuation. I am fitting a stop so that the rail mechanism cannot rotate to a fully vertical postion which should solve the problem.

You can therefore probably rule out this part.

[CDF]

Thanks for the response guys.

Yes my copy of VCDS is registered. Why do you say it's likely to need a turbo rebuild? Simply the mileage? I would be horrified if the turbo on my other cars needed a rebuild at this mileage!
Would a clean of the vanes not be a good solution?

[Crasher]

Cleaning and rebuilding are the same job and it has managed a good 60K miles more than most.

[DMitch16]

Replaced mine at 103K with a new one as I managed to get the next "upgraded" Garrett version for £500 new. Took the old unit which still had good bearing seals, split it, cleaned it to within an inch of its life, greased up the rotator mechanism with anti seize, reassembled it and sold for £350. So the new turbo cost me £150 !! The car the old turbo is now on has just reached a further 85000 since the clean / rebuild and the turbo is still fine. If I had not had the offer of a cheaper new better replacement I would have put the original one back on. Just shows that a few hours work can save hundreds if you know how to remove and re-install.

[CDF]

Ok, fair enough.

The price of a boost solenoid is ~£40.
The price for a garage to remove, clean/supply a new turbo and then reinstall will be a good few hours of labour plus hundreds of pounds in parts.

The price differential between these two numbers is vast. Enough infact for me to buy a new solenoid just to see if the problem goes away. It has only happened once so far and could not be provoked into re-occurance yesterday.

If the solenoid does not cure the issue, then so be it: I'd rather 'loose' £40 overall than spend many hundreds on a new turbo to find it was a £40 solenoid all along.

I'll try to remember to keep this thread updated periodically with any news.

[CDF]

Mitch, I hadn't seen your reply when I was composing post #7. The turbo seems like a total ball-ache to remove on this car. Is that true?
If I could do it without dismantling half of the engine bay I might be tempted! It'd save some money in labour...

[DMitch16]

What is your engine code? (On the white sticker in spare wheel well area under boot carpet). You could try working the actuator leaver up and down at the pivot point to see if there is any roughness in the rotator ring. Any stiffness or roughness felt may indicate a deposit build up preventing the ring from moving smoothly which may cause the variable "nozzles" (vanes) from returning to their correct position after you take your foot off the gas. This may be a sign that the turbo would need cleaning.

Mine started with a single over boost fault then a month later a couple more, then more and after that nearly a permanent limp mode as the VNT unit had stuck.

[Crasher]

Removing a BKP engine turbo takes about two hours from below if you have the car in the air.