I can't fix "limp mode" how exactly does it work?

Discussion in 'Audi TDI forums' started by Daventry, Oct 25, 2016.

  1. Daventry

    Daventry New Member

    Oct 7, 2016
    Audi A4 Avant 1.9TDI 2002
    Hertfordshire UK
    2002 Audi 1.9DTI, 130BHP,150K miles
    Having read around and in looked at forums and watch videos I've looks at the usual suspects. My code reader tells me that it is an overboost rather than underboost.
    Took my turbo apart - not dirty at all. I can move the actuator just by sucking on the vacuum hose.
    Cleaned the EGR valve (since it was apart).
    Changed the MAP sensor
    Changed the N75
    Changed most of the hoses around the N75.
    Still going into limp mode.
    I tried disconnecting the vacuum hose from the turbo actuator and the turbo still makes boost (according to my live data)

    Now I have run out of ideas. and I'm sure I don't understand how its supposed to work. Or my understanding is wrong. The trouble is that most of the videos that I have watched include a phrase like "and that puts it into limp mode" Sure - but HOW???

    As I understand it:
    the MAS sensor signals a fault if it detects too much (in my case) pressure.
    The N75 works by closing when more boost is required, this prevents manifold pressure entering the vaccum, which causes a lowering of pressure in the vacuum line.
    When there is a low pressure in the vacuum line it causes the turbo actuator to work the vanes and create more boost. (low pressure = more boost)
    Limp mode instructs the N75 to open fully and defeats the turbo charger. ( no vacuum = no boost)

    But my understanding must wrong because I get boost if I disconnect the vacuum line! - or I rebuilt the turbo incorrectly.

    So please enlighten me. What am I misunderstanding? Does limp mode do something other than switch off the turbo?
  2. Seatman

    Seatman Well-Known Member

    Jul 9, 2010
    2000 Mk4 Golf Estate 1.9tdi 174 bhp
    PP764's, turbo and mapping
    If you're able to move the actuator just by sucking I'd suspect maybe the spring in the actuator is weak. It should be pretty stiff to move it.

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