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s4 not running boost....

Discussion in 'A4/S4 forum(B5 Chassis)' started by s4 osh, Mar 25, 2009.

  1. s4 osh
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    s4 osh Member

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    [Mar 25, 2009]
    hi guys, i have been on the s4 forum for a while and cannot seem to get any closer to solving my problem, so now i am looking for a new angle on things
    these are things that i have changed or inspected...
    02 sensor bank 1 no codes coming back now
    boost controll valve 'n75' code still there
    maf sensor, never been a code but the are known to be hidden faults
    checked all boost pipes including tbb and f hose

    there is no restriction on revving it drives so smooth, and every now andagain she will kick in full boost will be ok for a gear change, then its gone back........

    any input welcome, thank you in advance
    osian griffith
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  3. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator VCDS Map User

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    [Mar 25, 2009]
    If its making no boost the N75 valve code is the one to be looking at.

    What exactly is the code?

    We were getting an intermittant open circuit code on the N75 on our 1.8T and it turned out that the wiring was broken. I replaced the last 12" of wiring with that from a loom from a breaker and the fault vanished and my boost returned.

    It could be the valve itself though, have you tried changing it?

    If the valve or wiring is broken it will run at actuator pressure, which is about 4psi.

    An intermittant wiring fault like ours would likely cause the symptoms you describe, ie sometimes it will boost perfectly and othertimes theres nothing.
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  4. Lee Goodall
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    Lee Goodall ..got my head together!

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    [Mar 25, 2009]
    Yeah I'd def say either faulty N75 wiring (if the code is open circuit) or faulty N75.

    I had the same issue as Aragorn but the wire had snapped right near the N75 plug/connection so I just trimmed it down and rewired back into the plug.. job done!
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  5. Blue_Thunder
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    Blue_Thunder Active Member

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    [Mar 25, 2009]
    You haven't mentioned if you've run a pressure test. I'd look there first.

    Have you checked/swapped any of the common failure points, ie: DV's, TBB, F-hose, etc?

    You should also run some Vagcom logs if you are able to.

    Which S4 forum have you been looking at?
    #4
  6. Lee Goodall
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    Lee Goodall ..got my head together!

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    [Mar 25, 2009]
    If there is a fault showing with the N75 then that will be causing boost control issues.

    A boost system pressure test would obviously rule a boost leak out too though.
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  7. Blue_Thunder
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    Blue_Thunder Active Member

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    [Mar 25, 2009]
    Sorry, just re-read the original post and saw that your N75 code is still there.

    Get it sorted!
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  8. s4 osh
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    s4 osh Member

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    [Mar 25, 2009]
    no i havent done a pressure check, i really want to do it just for peace of mind, can any one put me through the steps of doing it, and an easy way of making a tester rather paying £50 for one haha.....
    the n75is a new one genuine audi part,,, didnt get the codes cleared but the car has been driven roughly 500 miles, same as always.
    i will check the wiring on the weekend.
    cheers for the input guys, my car is running 4 psi by the way haha
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  9. Broken Byzan
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    Broken Byzan Photographic Moderator Staff Member Moderator VCDS Map User

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    [Mar 25, 2009]
    4 PSI is Limp mode, you deffo need to get that N75 checked out
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  10. andybnwc
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    andybnwc Member

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    [Mar 25, 2009]
    physically check intercooler pipes throttle body and F hose ie take it off and check the underside .
    #9
  11. Blue_Thunder
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    Blue_Thunder Active Member

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    [Mar 25, 2009]
    You didn't get the codes cleared??
    :wacko:

    1) Get on Vagcom, go into Engine module, then 'measure blocks' then choose block 32 and post up the values.
    2) Clear the codes.
    3) Go for a decent drive.
    4) Check for fault codes again.
    5) Let us know how you get on.
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  12. s4 osh
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    s4 osh Member

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    [Mar 27, 2009]
    the engine management should re evaluate all reading after 300 miles so it should have picked the new one up by now, i have checked all pipes, removed and checked all over,,,,, twice.... ill check the wiring on saturday ill let you know how i get on
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  13. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator VCDS Map User

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    [Mar 27, 2009]
    The codes will remain, even if the fault has been gone for several thousand miles...

    The only way to know if they are current is to reset them, and then see which return.
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  14. Lee Goodall
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    Lee Goodall ..got my head together!

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    [Mar 27, 2009]
    I do seem to recall reading that the ECU will only keep codes stored for 300 or so miles.. So in theory if you've corrected the issue the codes will be erased.
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  15. Blue_Thunder
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    Blue_Thunder Active Member

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    [Mar 27, 2009]
    I'd not heard that.

    300 miles is a lot of laps round Anglesey! lol

    I picked up my S4 from Anglesey. Spent it's first 8 years there.
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  16. s4 osh
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    s4 osh Member

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    [Mar 27, 2009]
    wel hopefully i will have vag com tomorrow to see what the hell is going on lol, was it from holyhead you got the s4 from, negaro blue???? if so i believe thats a good buy
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  17. andybnwc
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    andybnwc Member

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    [Mar 28, 2009]
    nogaro
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  18. Blue_Thunder
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    Blue_Thunder Active Member

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    [Mar 28, 2009]
    No, mine's a Minger. Bought it from a guy called John in LLanfechell just over 2 years ago.
    #17
  19. s4 osh
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    s4 osh Member

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    [Mar 31, 2009]
    coolio i know someone had a nogaro in holyhead, but ihad no idea bout the one in llanfechell, my missus's step dad lives there... small world
    #18

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