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Sudden loss of power on a TDI engine

Discussion in 'A3/S3 Forum (8L Chassis)' started by VAGMAN1, Feb 20, 2006.

  1. VAGMAN1
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    VAGMAN1 Member

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    [Feb 20, 2006]
    This has happened twice to me today - going along at around 70mph and suddenly the turbo cuts out and the car goes in to limp mode. No warning lights appear on the dash and and all turbo hoses are connected ok. The engine is a TDI 110bhp (not PD).

    Any ideas please?!
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  3. chrisb
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    chrisb Member

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    [Feb 21, 2006]
    Sticky VNT mechanism (or dodgy vacuum hose) or a dodgy N75 valve are most likely.

    The VNT mecanism can only really be seen from underneath the car. It's a vacuum acutator under the turbo. You can work it back and forth by hand from under the car.

    The n75 valve is on the bulkhead. Go to http://www.tdiclub.com and search the forums for N75.

    Does cycling the ignition on and off fix it? (careful you don't put the steering lock on, and watch out for loss of power steering and brake servo........in fact, best if you don't try that while driving!)
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  4. VAGMAN1
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    VAGMAN1 Member

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    [Feb 21, 2006]
    thanks for that

    the problem does go if you switch off then re-start. all 3 times that it's happened over the past 2 days has been when the car is at around 3000 rpm - however it isn't happening all the time.

    an Audi dealer suggested that it could be:

    MAF - or

    turbo over boosting - or

    water temp sensor on the engine - i.e. it's over reading and assumes temp is way too high hence shuts down the turbo - but I havn't noticed temp going over 90deg C on the dash dial

    I tried un-plugging the MAF lead and the engine felt strangled and you couldn't feel the turbo - plugged it back in and all was fine again. So I doubt very much if it's the MAF.

    I'll pursue what you said anyway. I have twice had problems of a pipe coming off on one of the the valves at bulkhead and this produced the same effect, however re-connecting solved the problem.
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  5. GarryA3
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    GarryA3 Member

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    [Feb 21, 2006]
    [ QUOTE ]
    thanks for that

    the problem does go if you switch off then re-start. all 3 times that it's happened over the past 2 days has been when the car is at around 3000 rpm - however it isn't happening all the time.

    an Audi dealer suggested that it could be:

    MAF - or

    turbo over boosting - or

    water temp sensor on the engine - i.e. it's over reading and assumes temp is way too high hence shuts down the turbo - but I havn't noticed temp going over 90deg C on the dash dial

    I tried un-plugging the MAF lead and the engine felt strangled and you couldn't feel the turbo - plugged it back in and all was fine again. So I doubt very much if it's the MAF.

    I'll pursue what you said anyway. I have twice had problems of a pipe coming off on one of the the valves at bulkhead and this produced the same effect, however re-connecting solved the problem.

    [/ QUOTE ]

    Ive had that the past couple of days aswell mate.I also have a TDi.
    I was having the same porblem though through last summer and it turned out to be the MAF.
    Or so i thought,everything had been great but through this winter my car just hasnt been responding the same as it did.I have a new MAF so i dont have a clue what it is now.
    Same with me though,get to about 70mph then you feel something go and it goes into limp mode.Really annoying me my car just now.So many things have went wrong on mine in such a short time. /ubbthreads/images/graemlins/swear.gif
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  6. VAGMAN1
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    VAGMAN1 Member

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    [Feb 21, 2006]
    I've been looking at tdiclub.com and alot of evidence on there points to it being a valve called an N75 which is part of the turbo system. However I will be getting a diagnostic done this week to ascertain exactly what the problem is so I'm not changing parts blind. I'll keep you posted.
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  7. GarryA3
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    [Feb 21, 2006]
    Cool,let me know what you come up with mate.When i got mine checked the MAF code came up (appartently).Defo sounds like the same problem we have though.Im going to be getting mine checked soon me thinks.
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  8. VAGMAN1
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    [Feb 21, 2006]
    let me know when you find out more as well then!
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  9. VAGMAN1
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    [Feb 22, 2006]
    Had a diagnostic done today and it is almost ceratinly a faulty boost pressure solenoid valve (also known as an N75 valve?). The valve is becoming stuck open hence allowing a too much gas through which can over pressure the turbo hence the ECU shuts it down. I'm going to buy another although I have read that they can, perhaps, be cleaned......
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  10. chrisb
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    [Feb 23, 2006]
    Temp sensor sounds unlikely, MAF is possible, but my money is still on the N75, a vacuum hose, or the VNT mechanism.
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  11. VAGMAN1
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    [Feb 23, 2006]
    following a diagnostic, it's almost certainly a sticky N75 valve, although I'm also going to check out the VNT mechanism to see how stiff that is
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  12. VAGMAN1
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    [Feb 26, 2006]
    Replaced the boost pressure solenoid valve (N75) and all seems ok now. Also checked the VNT mechanism and you can work that easiley by hand so no problems there either. Fingers crossed it's sorted. Also dismantled the old N75 - they go sticky not due to crud build-up but if mine is anything to go by, a shaft corrodes which is part of the valve.
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