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Weird ECU/Turbo Lag.

Discussion in 'A3/S3 Forum (8L Chassis)' started by aragorn, Mar 30, 2012.

  1. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    I've posted about this before, and i've also posted it over in the B5 section, but since the AJL is similar to the AGU i figured someone here might have seen this before.

    So, i've noticed this oddness when driving the car hard, in that theres a lag after a gearchange.

    So you go full throttle thru 2nd gear to say 6500rpm, change to third and go full throttle again, and the car starts to accellerate, then after a moment, perhaps 0.5-1seconds, it starts accellerating harder.

    Today i noticed that if you watch the boost guage when this occurs, the needle quickly flicks round to 5psi, and sits rock solid on 5psi for this 0.5-1sec period, then it sweeps round to 10-12psi and the car stars accellerating more. So clearly its not mechanical lag, as the fact the turbo hits 5psi instantly suggests the turbo has spooled instantly but the wastegate has simply opened at actuator pressure.

    This in turn suggests its as though the ECU itself isnt requesting boost quick enough...

    I first noticed this ages ago on the Avant, and put it down to the old turbo being a bit shonky, however i've started noticing shed does it as well, and avant still does it with its new turbo.

    First off does anyone else have this phenomenon, and if not, any ideas of the cause?
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  2. Jason.s
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    Jason.s Active Member

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    Mine does it and has done for months. Thought the mapping would clear it up but it does it still. Last few days it hasnt done it when driving fast. Sods law it`ll do it now i`ve said that on the way home..

    No idea on the cause, most things have been replaced other than the N75 but Rtech said it seemed pretty healthy so didnt bother looking to replace it.
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  3. Prawn
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    Prawn My other car is a MINI!!!!

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    Maybe it's because Jesus hates you Kev?
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  4. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    i can imagine it will only get worse with the mapping, as it will be an even larger step when it jumps from 5psi to 20 or whatever...
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  5. Jason.s
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    Jason.s Active Member

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    Seems about the same tbh. Spool is so fast it's not a huge about from 8 to 14psi. Be good to know what it could be though as its quite annoying. Especially when doing a rwyb day. Last year changing from second to 3rd there was a huge hesitation/lag, biggest ever I've experienced but it was still my fastest run lol.
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  6. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    yeh it does seem to vary, sometimes i dont notice it at all, others i go for an overtake and shift part way thru and it feels like i've stuck the brakes on.

    I've also noticed that if you drive less hard, perhaps shift at 5k, its less noticable.
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  7. Paradox1
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    Paradox1 Active Member

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    Funny this came up, I get a similar one too, when driving calmly, in low gears after a gear change it revs just over 2k, hesitates the back on boost again. It never happens when I floor it though. Thought it was a boost leak but checked everything but it still does it
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  8. s3dave
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    s3dave TFSI Hybrid

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    Checked the n75 logs when this is happening i take it?
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  9. knoxie
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    knoxie K4OXE

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    i also got/get this, mainly when its cold weather, cant say i have felt it since the map went on about two weeks ago tho
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  10. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    s3dave: no, i've not looked at n75 duty, will need to see if i can log that.

    I did log airflow on the Avants old turbo, and plotted it as a power/torque graph, and you can really see the effect (bear in mind the sample rate is fairly low here):

    [​IMG]
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  11. badger5
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    badger5 RIP S3dave Site Sponsor

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    tried another n75 valve?
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  12. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    Bill, no i havent, maybe i should get a new one and try it.

    Will try and log the N75 duty first though, no point changing the valve if the ECU isnt requesting any boost!
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  13. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    Just to round this out incase anyones searching for it in future, it was the N75 valve.

    I was somewhat bemused when i removed the old valve and discovered it was date stamped 11-96, which means not only is it 14 years old, it came off my old FWD A4 having done 195k, and has since done another 10k fitted to this engine. I think when i built the engine i picked the best looking TIP out the few i had, and this one happened to still have its N75 installed so i left it there.

    Not entirely surprising that it was acting oddly!

    Replaced it yesterday for a new one, and now it works great.
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  14. Prawn
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    Prawn My other car is a MINI!!!!

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    hahahaha, so this mystical problem that you've been chasing for months and months, turned out to be something as simple as a pre-historic N75?

    I'm happy for you :) but that is quite funny!
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  15. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    aye, the new N75 has been sat on my desk for ages, but only got round to fitting it now.

    TBH i wasnt really "chasing" the problem, it was there and it was annoying me, but i hadnt actaully tried anything to cure it.

    I also fixed my missfire by swapping a couple of similarly prehistoric coilpacks from that same 195k AEB with some less ancient ones i had in the garage. :p
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  16. boost-addict
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    boost-addict Member

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    i have a ajl motor, mine does this its like it hold back sometimes after a gear change, its like it pauses to think what to do then its off

    i put it down to either the **** recirc valve i have, the fact that i have a decat and big tip, or that the remap i have is ****. or even a sticking actualtor
    as i run 18psi but it dosnt feel 18psi fast!


    but if you found it to be the n75 i might have to get a different one and try it out, but it dosnt happen often
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