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S4 Vagcom output

aragorn Nov 18, 2008

  1. aragorn

    aragorn "Stick a V8 in it!" Staff Member Moderator VCDS Map User

    Ok i've had this posted up on RS246 but hopefully posting here will get a bit more coverage.

    Been doing some vagcom logging runs on craigs S4 and noticed it seems to be cutting throttle once it gets up the revs.


    Thats the last log we did (full throttle 3rd gear), which shows throttle angle as well as boost pressure, well at least its supposed to show boost pressure...

    As you can see the throttle closes down once the RPM's reach about 4800, the feeling from googling seems to be that throttle cut like that is caused by overboosting, so we want to look into that next, however with the logs not showing boost properly it leaves us a bit blind.

    I'm going to try it again, theres another few channels i can log, which are supposed to show boost pressure (117 and 118) so i'm going to try those first to see if it gives me any more sensible readings...

    Any ideas on what would be causing the overboost on a stock car? i'm thinking along the lines of a faulty N75 valve or a split vacuum pipe somewhere...

    Original post is here: http://www.rs246.com/index.php?name=PNphpBB2&file=viewtopic&t=73850
  2. Blue_Thunder

    Blue_Thunder Well-Known Member

    Faulty N75 would be one potential cause. Could also be a boost leak.
    A knackered wastegate could also cause overboosting but is fairly uncommon AFAIK.

    First things to check:
    Any DTC's stored?
    Pressure test
    Block 032 on Vagcom to check fuel trims

    Another thing is the DV's - if they are sticking and not opening fully, this would also cause overboosting, but you wouln't expect to see this at WOT, only on lift off.
  3. aragorn

    aragorn "Stick a V8 in it!" Staff Member Moderator VCDS Map User

    only DTC he's getting is "Power Supply Terminal 15 Voltage Low" and it only appears on startup i think.

    If you clear it with the motor running it stays cleared and doesn't return...

    The fuel trim i posted up over on rs246, -0.8% on one value and 0% on the other.

    I'm thinking along the lines of either N75 itself, or the pipe work to/from it...

    DV's are on the list of parts that he wants to replace anyway as a matter of course, and he wants to remove the F-Hose and do an N249 delete too.

    Not pressure tested it yet, as he hasn't bought the kit to do it, used to be someone selling them on ebay but they've vanished. Might have to knock something together myself to do it.

    Re the fhose/249 delete, we need to buy some pipe to do it, do you know what size it needs to be?

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