Overboost Problem 130 tdi

[scotty33]

Hi all,

I bought myself a Cat D repairable passat. Unfortunately when we collected it, it was in 'limp mode'. Bodywork is now fixed and car MOT'd but the fault is becoming a pain.
Had fault codes read, I forgot the actual code, but the description was 'charge pressure - positive deviation'.

I have checked out the small bore pipes to the N75 and EGR system. I can't find any leaks. I tried substituting the N75 valve with the EGR control valve, this does not seem to have helped - not sure it would, they look identical, but one has suffix 'A' to it's part number?
The VNT actuator seems to work when vacuum is applied and returns to it's starting position. On the face of it, this does not seem to be sticking, and I'd prefer not to be taking the turbo off for no reason.
There is a possible boost leak, from joint between the intercooler hose and the hard plastic inlet pipe - joint is next to expansion tank. The wire clip and the lugs on the hose end are a bit worn. I have not heard air rushing, but there has been some oil coming from this area. Could a boost leak cause overboost conditions on a tdi?
Another thing that may help me is: does the VNT actuator give boost in it's 'natural' state or does it need vacuum applied to make boost?
Any help appreciated...

[Alan02]

Hi,have you removed and inspected all the boost hoses for splits?as they can be hard to see.Also look for signs of oil on the Intercooler core,if there are,poss.boost loss,not always though as if its near the rubber seal that houses the inlet /outlet hoses,generally normal.inspect the wiring to the boost sensor too.
I guess you mean VVT as opposed to VNT(variable vain turbo),sometimes the mechanism can stick when operating but in my experience it causes an Overboost situation,also causing the `Limpmode`fault.As you mention,its vacuum operated by the solenoid valve.
Hope this helps.

[kenney]

Hi You will not solve the problem by substituting the N75 valve with the EGR control valve,they are not the same.

[scotty33]

Thanks guys, clears up a few things, looks like time to bite the bullet and whip the turbo off

[scotty33]

I started to strip the turbo off today, first task was to remove the VNT actuator, at this point it became clear, the VNT is not sticking. It moves through its range freely, and the actuator also moves when vacuum is applied.
I swapped the egr and n75 valves back where they should be and tried a vacuum bypass, with a feed from the servo hose to the n75 only. (instead of the common vac line which services the EGR system and a coolant 'valve?')
When the engine is idling, the vacuum causes the VNT actuator to retract - is this normal?
When the vacuum line is seperated, the VNT actuator extends, but quite slowly.
My guess is the N75 valve is faulty, as I seem to suffer low torque at 2000 rpm where there should be plenty? And this overboosting at approx 3600 rpm
Any opinions welcome...

Cheers

Andrew

[scotty33]

Well I found the answer to my own question, applying vacuum causes the turbo to make boost.
Also a useful link for anybody that is interested in the VNT turbo:

http://www.technologie-entwicklung.d...t15-turbo.html

[scotty33]

Still having this problem, Vag com is picking up 17965 boost control - positive deviation.
I drove the car a little tonight. There is a definite lack of boost/throttle response below approx 3000 rpm. The power then comes in quite hard, and sometimes will rev cleanly past 4000, sometimes dies suddenly at approx 3600 - I presume this is when the overboost occurs?

I can see that a turbo problem would give me the lack of boost low down, but surely not the overboost as well?
I have checked everywhere for vacuum leaks, none found.
The VNT mechanism is very free to move when disconnected.
The VNT actuator/linkage can be seen working when revving the engine in neutral.
The N75 valve has been opened up and cleaned out.

I'm beginning to suspect the turbo and it's controls are not faulty, maybe the manifold pressure sensor is faulty, but how can I check this?

Or could a sooted up intake cause these problems?

I could now get this thing on the road if it did not have a 600rpm power band!! Any suggestions welcome

Cheers

Andrew

[kenney]

Hi If you have dignostic equipment you can check the turbo charge pressure (specified values) and charge pressure (actual value) in measured block values.You have cleaned the N75 valve.but you must remember it has an electronic function too,are you sure it is working as it should?I beleive you can check the pressure sensor too it should read atmospheric pressure with the engine not running.

[scotty33]

Thanks Kenney,

I only have freeware Vagcom, not sure if this does the monitoring you mention. I am not confident I would not mess things up either...
The N75 valve, it may have varying voltage applied to it by the ECU, but mine is just a basic solenoid valve. It has only two terminals, I presumed these are for the solenoid itself so probably no electronics in this case? I could not be certain that it is working properly, but it is hard to see what could be wrong? The core moves freely, the solenoid is ok, the diaphragm is ok and the seating is clean.
Will look into using measuring blocks, if I feel brave I'll give it a go.

[Col]

You can't do any harm by going into the measuring blocks.

If you can, use vagscope with vagcom and open (as Kenny has said) actual and specified boost. You will get a nice graph which will help you analyse what is going on. You will need the full version of vagcom though.

Is it in limp straight away? or does the boost drop out?

My guess and as you are suspecting yourself is that it is the MAP sensor.

However the N75 could be the cause, a knackered return spring or something inside it holding the vacuum to long and thus causing the overboost.