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Overboosting?

k17rpo Jan 22, 2007

  1. k17rpo

    k17rpo Member

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    Hi, i was wondering if anyone has ever had this problem on their car? Basically my car has always had a problem where the power dies down around 3k rpm. I have as i read i many threads changed the Mass air flow sensor but it has not helped. i have then since put my car (1.8T quattro, AJQ engine) onto a diagnostics machine and it has come back with an Overboosting prooblem, anyone help??
     
  2. Are you sure you changed the MAF for a good one?
     
  3. k17rpo

    k17rpo Member

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    Its a Bsoch one, same as the one i removed but from GSF? I heard GSF can sell useless ones but i thought with it being the same make it shud be alryt?
     
  4. LaSarthe&Back

    LaSarthe&Back Member

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    N75 valve. It's a solenoid that controls boost pressure, when it starts to fail it'll go wierd, giving funny boost problems.

    Get a new one from stealers for about 50quid.
     
  5. k17rpo

    k17rpo Member

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    Where is this valve on vehicle pleaseee?
     
  6. DavidR

    DavidR Active Member

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    It's usually accesible from the top of the engine. 3 pipes into and out of it, near the main inlet pipe.

    Worthwhile thinking about the possibility of a boost leak too. In a closed system as the 1.8T is a leak can cause apparent overboost as the engine compensates.

    Have you changed the DV to a venting one? If so put the OEM bosch one back on and try it.
     
  7. k17rpo

    k17rpo Member

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    i have the standard dump valve on, but maybe its past it?
    just outta curiosity tho when i accelerate and lift of (eg to change gears) is the "chatter-like" noise normal for the 1.8T engine? cause i know on my old punto this was bad?
     
  8. DavidR

    DavidR Active Member

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    They all have a quiet chatter that's easy to hear inside the car.

    If your DV diapragm is torn the overboost symptoms can occur too - something else to check.

    You can take the DV off and blow through the larger pipes - if you can blow air down the pipes it's broken - a replacement is only £25 or so.
     
  9. The chatter you hear is compressor surge/stall, that's not normal. Most likely due to a faulty DV. Get a new one from your dealer - 06A 145 710N, it should be <£20.
     
  10. DavidR

    DavidR Active Member

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    Compressor surge / stall sounds nothing like that on the 1.8T. The chatter is the DV diaphagm flapping as it vents air back into the intake system and is characteristic of the 1.8T.

    A broken DV will not hold pressure (as the diphagm tears) rather than block the flow of boost leading to stall.

    The DV may well be suspect in this problem, you can be assured the turbo is not stalling / surging etc.
     
  11. IME the DV leaks between the boost entry pipe and the vacuum signal pipe. This causes the DV to leak under boost and the vacuum line cannot actuate the diaphram correctly, thus causing compressor stall. The vacuum line cannot flow enough air to prevent this.
    A healthy DV will not make a chattering sound.

    @ k17rpo. When you remove the old DV blow through each of the tubes, then report what you find. You may want to clean it a little before you do this.
     
  12. DavidR

    DavidR Active Member

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    Not so.

    The diaphagm tears - this means the DV is broken in the open position allowing overboost as the ECU tries to compensate for an apparent loss of charge pressure.

    All 1.8Ts make the chatter on throttle lift.
     
  13. WTF? BS!
     
  14. DavidR

    DavidR Active Member

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    OK.

    Please explain how a diaphagm DV can stick in the closed position and lead to the potential for compressor stall?

    The Bosch DVs are fail safe for this reason.

    Go do your homework before coming back and declaring BS.
     
  15. And you're proud of statements like that?
     
  16. DavidR

    DavidR Active Member

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    I'm not sure that a statement of fact is something to be proud of. But if you think it's worth of such a high laurel then thanks, I will be. Cheers! :)
     
  17. DavidR

    DavidR Active Member

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    And for the hard of understanding - Pictures!

    [​IMG]

    If the OEM DV fails then boost is ALWAYS vented - failsafe. The pressure behind the DV will invariably push it to open. Hence lost charge pressure and overboost recorded by the ECU.
     
  18. k17rpo

    k17rpo Member

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    Cheers to both you guys for the advice, no need to argue, wil try takin the DV off when possible to see what i find!
     
  19. Feel

    Feel New Member

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    How did you get on?
     
  20. k17rpo

    k17rpo Member

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    Replaced the dump valve with a new (standard) sump valve was awesome for a week but then went back to same problem and have not bothered with it since!
     
  21. WKDLAD

    WKDLAD Member

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    Dont suppose you have a REVO remap have you?
     
  22. simch

    simch Active Member

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    Is that a gravity fed system or combination...? lol
     
  23. k17rpo

    k17rpo Member

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    as far as i know, no remap!
     
  24. simch

    simch Active Member

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    Check your pipe from the n75 valve down to the actuator on the wastegate (turbo). Any leak here, or if there is a loose clip will result in an overboost problem too, as the vacuum used to operate the wastegate can be lost.
     
  25. k17rpo

    k17rpo Member

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    alryt, will have a look cheers fella!
     

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