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Erratic idle and stalling

Discussion in 'A4/S4 forum(B5 Chassis)' started by AR-rkon, May 9, 2011.

  1. AR-rkon

    AR-rkon Mod back in a "B"
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    As per title, erratic idle at start then calms down after a few seconds, bus will also sporadically stall the engine when parking/stopping.

    No dtcs, measuring blocks seem ok, any suggestions?
     
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  3. aragorn

    aragorn "Stick a V8 in it!"
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    Engine?

    Our old 1.8T would do that. Start the engine and the RPM's would bounce between 1500 and 500rpm before slowly settling to a normal idle, and the revs would drop right down to 4-500rpm if you clutched in coming up to a junction. Its suggestive of a vacuum leak but i couldnt find anything.

    Ended up swapping out the tank vent valve after Mark suggested it and that fixed it.
     
  4. AR-rkon

    AR-rkon Mod back in a "B"
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    Yes, 1.8T.

    Will have a look for vac leaks, but didnt see anything obvious, do you mean the fuel tank vent?
     
  5. aragorn

    aragorn "Stick a V8 in it!"
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    yep the "n80" valve, it sits on top of the airbox.

    From memory i tested the theory by opening the fuel tank cap and starting the engine. The crazy bounce went away, and it popped a fault code about lean mixture or something (presumably because the open tank cap had introduced a huge air leak).
     
  6. AR-rkon

    AR-rkon Mod back in a "B"
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    Ok, tried the filler cap trick, no change. Could it be a boost leak? I know my Dv is on it's way out, hence I've just bought a 710N to fit tonight. Does seem a little "flat" on the drive out of work, almost as if it's holding back...
     
  7. AR-rkon

    AR-rkon Mod back in a "B"
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    Yep, it's definitely down on power past 3.5k, spent tthe last couple of hours searching out vac leaks (using various aerosols, carbcleaner and eZ start), nothing. same goes for blocks 2,11,31, and 15/16. All look ok.

    Anything i could have possibly overlooked or anyone got pointers?

    Cheers
     
  8. aragorn

    aragorn "Stick a V8 in it!"
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    Try a manual Throttle Body Alignment using Vagcom?
     
  9. AR-rkon

    AR-rkon Mod back in a "B"
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    Yep, done Basic Settings Block 60. says adp ok, still there (erratic idle at startup, then smooths out, but still tries to stall when coming to a stop.)
     
  10. AR-rkon

    AR-rkon Mod back in a "B"
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    Ok, well i was thinking about anything i'd touched recently, and the only thing was the TB and fitting a 710N, so packed out the gasket a little more with amalgamating tape to create a good seal and surprise surprise it seems to have cured it. Going to buy a new gasket today and see what happens. on a side note, i now have a 17965 DTC, no limp but is down on power, could the 710N cause this or is more likely to be a vac leak / faulty N75?
     
  11. aragorn

    aragorn "Stick a V8 in it!"
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    packed out what gasket?

    charge pressure Positive deviation can point to a vacuum/boost leak, or a faulty N75.

    I'd suggest pressure testing the inlet.
     
  12. AR-rkon

    AR-rkon Mod back in a "B"
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    TB to Inlet Manifold, Got a new gasket on order from TPS along with 2 new ones for the turbo so i can do my rebuild on saturday.

    Yeah, couldnt find any vac leaks. would it be worth testing the entire inlet (from post-AMM onward) or just the TB/Inlet manifold and also the pressurising the Vac system?
     
  13. aragorn

    aragorn "Stick a V8 in it!"
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    Just replace the AMM with the pressure tester and blow the whole lot up to 1 bar.
     
  14. AR-rkon

    AR-rkon Mod back in a "B"
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    Now have a full log done on [002] AMM(g/s), [115] Boost and [032] fuel trims. if anyone can tell me how i can get it up on here?
     
  15. aragorn

    aragorn "Stick a V8 in it!"
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    stick the spreadsheet on google docs?
     
  16. AR-rkon

    AR-rkon Mod back in a "B"
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  17. aragorn

    aragorn "Stick a V8 in it!"
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    cant really tell all that much tbh, the requested and actual boost seems a bit apart, and theres quite a lot of muliplicative fuel trim (-5)

    Try logging a 3rd gear pull from say 2000rpm to the red line.
     
  18. AR-rkon

    AR-rkon Mod back in a "B"
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    if you look on the boost list from T/Stamp 131.32 > 146.31. It starts at 1200rpm up to redline
     
  19. aragorn

    aragorn "Stick a V8 in it!"
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    ah missed that somehow the first time i looked at it

    What seems weird to me, is that your holding 0.4-0.5bar of boost, which is low indicating a standard tune, and is more or less what the ecu is requesting, yet you've got a peak MAF figure of 144g/sec which suggests ~180hp worth of air.

    The two dont really add up in my eyes. Either theres a boost leak, which may also explain the -5.5 multiplicative fuel trim, or the MAF is away.
     
  20. AR-rkon

    AR-rkon Mod back in a "B"
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    I didnt think they looked right. Maf is brand new unit, fitted about 3 weeks ago. Boost leak is possible, will be pressure testing tomorrow. Only thing thats changed is its had a 710N fitted. Cant see that being the cause though.
     
  21. AR-rkon

    AR-rkon Mod back in a "B"
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    UPDATE: fitted the new hot side housing with a functioning wastegate (not rattling). Still getting a Charge Pressure (Positive Deviation), However, no limp mode... but at least i know its not the wastegate causing it....

    Can't find any vac leaks (replaced most of the hoses).

    Would a duff N75 cause it? (im thinking it could be sticking occasionally)

    Any other ideas... its behaving itself at the moment (apart from the 17965 code)

    Though my turbo exhaust turbine is white... running lean or too hot???
     
    #20 AR-rkon, May 14, 2011
    Last edited: May 14, 2011
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  23. AR-rkon

    AR-rkon Mod back in a "B"
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    Done another run out today using my Autodiagnose MTpro.

    Got a peak MAF reading of 149 G/S :think: how? that would make 186 BHP(ish)?

    My maximum boost readings were (mbar)
    Requested: 1380(20.015psi)
    Actual: 1500 (21psi)

    Would an over-reading AMM/MAF cause the Overboost code?

    Still concerned by the white turbo innards? is that normal???
     
  24. aragorn

    aragorn "Stick a V8 in it!"
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    Those boost pressures are absolute, so you need to subtract atmospheric pressure.

    Meaning your requesting 5.5psi and getting 7.35psi.

    S4's typically give that code when they're leaking boost. Typically the car WILL make its normal boost pressure (as it just works the turbo a bit harder to compensate for the leak). The usual identifier is an over-reading MAF, as all the air, including the stuff thats leaking will have passed thru the MAF. You are flowing 186hp's worth of air, but theres no way your making 186hp on 7psi.

    You really need to pressure test the inlet.

    I think the white is indicative of silicone contamination or the fuel or similar, typically the turbine will be a dusty grey/brown color, more or less the same as a healthy spark plug.
     
  25. AR-rkon

    AR-rkon Mod back in a "B"
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    Cheers Aragorn, but forgive me if I sound dumb, what needs to be blanked off (other than the obvious at maf and tb) is it just the PCV where it meets the pancake filter?
     
  26. aragorn

    aragorn "Stick a V8 in it!"
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    yep, disconnect the pancake valve and block off the port for it on the tip, then remove the MAF and fit the pressure tester where the MAF used to be.

    Dont bother blocking off the TB, just pressurise the lot. It will leak down slowly past the rings etc but still gives you plenty of time to detect any leaks.
     
  27. AR-rkon

    AR-rkon Mod back in a "B"
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  28. aragorn

    aragorn "Stick a V8 in it!"
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    Looks like a good start anyway.

    Might want to check all the 1 way valves in the system to ensure they're still 1 way valves!
     
  29. AR-rkon

    AR-rkon Mod back in a "B"
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    which way should they be? white>black or visa versa (> being airflow)
     
  30. aragorn

    aragorn "Stick a V8 in it!"
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    I've never seen a late engine up close, so not sure what you mean by white and black, but the valves usually have an arrow on them indicating direction of flow.
     
  31. AR-rkon

    AR-rkon Mod back in a "B"
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    ah right, cheers. Never noticed that...

    As for the white and black. half of the valve is white and the other black (i would assume, to designate airflow).... :icon_thumright:
     
  32. AR-rkon

    AR-rkon Mod back in a "B"
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    Ok so the erratic idle and hesitation is gone (seems fitting a bosch OEM MAF solved that one). However car is still down on power, found the N80 had cracked on the White side, but as I understand it, the boost is not connected in any way... Or am I wrong and this may be the cause???
     

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