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Thread: A6 (C6) Power Delivery Problem - diagnosis assistance pls?

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  1. A6 (C6) Power Delivery Problem - diagnosis assistance pls? 
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    Hello to all

    I have a 2007 A6/C6 Avant with a 2.0TDI BRE Engine.

    It has had a replacement EGR valve and throttle body (both VDO units) about 24 months ago. I've swapped out the MAF, and it has just been serviced. I was aware that the clutch was pretty worn, so was hoping that this being replaced would cure the issues, and a gen 3 piece clutch kit and LUK DMF have just been fitted. It made no difference.... In the past, I've had a "sticky turbo" where the veins have become clogged up with crud, but the local specialists have cleaned it out with Mr Muscle, and it's been a perfect treatment. I'm very aware of how the car is affected when the turbo is completely coked up, and when it's happened in the past, she couldn't even claw her way up a short (less than 1km), steep hill! (Ended up stranded and having to reverse back down and tow her home). It's worth adding, that the specialist cleaned out the turbo a month or two ago, but it made no difference this time sadly.... I'm confident that the veins are not presently clogged up, and I'm aware that she has a VNT turbo.

    The symptoms I am getting are that it seems to have the majority of it's grunt and pull from 1750RPM to about 3000RPM. I still think it's a little short on the grunt, but she seems to pulls ok'ish.

    However, she simply stops pulling after 3000RPM. She will go higher than 3000RPM, but will gather speed only very, very slowly indeed. To achieve actual acceleration, you need to change up a gear and get back down lower in the rev-range and she will start to pull again, back upto approx. 3000RPM, and then stop pulling.

    I'm booked into a local specialist, but I'd like to do as much diagnosis as possible in advance. I had a bit of a chat with the guys that are going to look at it, and he's mostly thinking that there's most likely to be a problem with either the turbo itself, or possible the turbo pressure sensor. It's worth noting that there are no error codes whatsoever being shown on VCDS to help point me in the right direction! Strangely (to me) he indicated that it was probably more likely to be the turbo as this didn't come online until about 2500RPM and beyond. Is this right? Whilst I'm no expert, I can hear (what I always thought) was the turbo whistling right down from 1500RPM on occasion, particularly if there is a load demand - for example a hill start on a steep hill, where a little more grunt is required than on the flat. I can still hear now, what I think/thought, is the turbo whistling away during acceleration.

    Therefore, I'm unsure where to look. I would be extremely grateful if someone could help me, by showing me what sort of testing, or data logging/extraction I can gather with VCDS to help demonstrate what part of the system might be causing the issue?

    Many thanks in advance for any replies.
     
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  2. Re: A6 (C6) Power Delivery Problem - diagnosis assistance pls? 
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    I’ve had a similar problem with an older V6 diesel, then the problem was found in replacing the MAF


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  3. Re: A6 (C6) Power Delivery Problem - diagnosis assistance pls? 
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    Quote Originally Posted by gerardh View Post
    I’ve had a similar problem with an older V6 diesel, then the problem was found in replacing the MAF
    Thank you for the reply.

    As I mentioned in my OP, that was the first thing I looked at and changed out. Changing the MAF made no difference whatsoever. I would very much appreciate any other suggestions of where I should be looking next!
     
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  4. Re: A6 (C6) Power Delivery Problem - diagnosis assistance pls? 
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    Sorry, looked over that part


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  5. Re: A6 (C6) Power Delivery Problem - diagnosis assistance pls? 
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    Quote Originally Posted by gerardh View Post
    Sorry, looked over that part
    No worries - it helps keep the post bumped!
     
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  6. Re: A6 (C6) Power Delivery Problem - diagnosis assistance pls? 
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    no codes so maybe not electrical - therefore we could be looking at fuel or air supply?
    I should note mine does the same when it shuts down the turbo actuator - but this comes with a code.
    Stewart
    2012 Q5 2.0D quattro S line Plus
    1986 coupe quattro (kv)
    1972 mini 1400 100BHP A series
    2016 F-Pace Portfolio X761 2.0D 180HP
     
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  7. Re: A6 (C6) Power Delivery Problem - diagnosis assistance pls? 
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    Quote Originally Posted by swwalker View Post
    no codes so maybe not electrical - therefore we could be looking at fuel or air supply?
    I should note mine does the same when it shuts down the turbo actuator - but this comes with a code.
    Hello Stewart

    Thanks for the reply. The logic works for me!

    I've been testing and logging data this afternoon with VCDS, and it seems to me (as a complete layman!) that the problem lies with the turbo data. I’ve attached one spreadsheets to this post to help pin down what’s not quite right. The garage that it will be going into suggested that the most likely candidates were either the turbo itself, or possibly the turbo pressure sensor, so I’ve particularly been looking at the pressure data, and I think I can see a sizeable problem?

    If you look at the Excel spreadsheet named: LOG-01-011-11E-13F-442 – I’ve focussed on logging engine speed / boost pressure (specified) and boost pressure (actual). The log started from a side road to a dual carriageway, and then was logging hard acceleration to get upto 70mph as quickly as possible. (EDIT: As I tried to upload the csv file to this post after writing all of this, the forum doesn't seem to want to let me upload or attach a spreadsheet to a post?! Does anyone know if I can share a spreadsheet file?)

    Row 45 is where the acceleration from joining the dual carriageway begins, and I can see that boost actual is significantly different from specified. I am assuming that this data means that specified is what the car states that it wants, and actual, is what it is actually getting, and that actual should be pretty close, or even identical to specified? In row 45 it’s under actual, and remains under specified through to row 49, and then appears to be overboosting in rows 50-52? In row 54 it’s significantly underboosting by about 35-40%, and then overboosting in row 59? As a layman, it seems that there is a very significant difference between what appears to be what the car is asking for, and what it’s getting – as then in other rows, like 68, it’s an exact match between specified and actual…

    I was wondering if this wayward data set points to whether the problem is the turbo itself, or the pressure sensor that it is at fault (or something else)? It looks like a problem to me, but I'm no expert and don't really know what the car should be doing!
     
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  8. Re: A6 (C6) Power Delivery Problem - diagnosis assistance pls? 
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    sounds like it could be a sensor - or a leak? try putting pressure in the system and see - blank off the air box blank off the manifold and pump up?
    Stewart
    2012 Q5 2.0D quattro S line Plus
    1986 coupe quattro (kv)
    1972 mini 1400 100BHP A series
    2016 F-Pace Portfolio X761 2.0D 180HP
     
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  9. Re: A6 (C6) Power Delivery Problem - diagnosis assistance pls? 
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    Quote Originally Posted by swwalker View Post
    sounds like it could be a sensor - or a leak? try putting pressure in the system and see - blank off the air box blank off the manifold and pump up?
    We may have made some progress....

    The car was in the garage a couple of days ago, and the mechanics were scratching their heads a little, as they couldn't understand why the car was "vibrating" (quite significantly) whilst you were trying to put the boot down and push through 3,000 revs. They don't think the turbo is at fault.

    What they have found is that the inlet manifold is in poor shape, and the swirl flaps are seized solid. One would imagine that the flaps are caked in filthy black crud deposits! They're recommendation was to remove it, soak it and then reinstall it, for which they wanted three to four hours of labour (which seemed a little high for this job tbh). There was no guarantee that soaking the IM would free up the flaps.

    It struck me that it was far cheaper to simply buy another IM from a breaker (circa £30 or so), with nice clean flaps, and simply swap them over. I reckon about an hours labour for that swap job, and I'll have needed a new gasket either way anyway. Their position was that they can't diagnose the power issue further until the IM issue was resolved (I agree with this logic), and there appears to be a good chance that it's actually the root cause - would be very interested in anyone else's view on what effect seized swirl flaps would have on the engine and power delivery? We're not sure if they are presently stuck shut, partially open, or fully open at the moment.
     
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  10. Re: A6 (C6) Power Delivery Problem - diagnosis assistance pls? 
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    Hey . Did you resolve the problem with a new im. Would love to know the outcome.

    PF.
     
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