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  1. #1
    Veteran Member Four Rings f4m0u5's Avatar
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    Aug 04 2014
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    Gloucester MA USA

    car bucks when letting off throttle at part throttle in boost

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    Started happening recently never happend before. The car is stage 2. Fuel trims are fine so dont think its a boost leak.

    Car doesnt buck/hesitate when leting off the throttle at wot it only does it in boost and part throttle.

    Car has forge 007s with yellow springs and 1 shim that i just re greased last month.

    Any idea of what it could be? Its as soon as i let off the throttle it will buck slightly.
    Last edited by f4m0u5; 01-12-2016 at 12:36 PM.

  2. #2
    Veteran Member Four Rings vavJETTAw36's Avatar
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    Jul 04 2012
    AZ Member #
    96276
    Location
    Woodbridge, VA

    I get this as well... +1 for finding an answer


    Sent from my iPhone using Tapatalk

  3. #3
    Veteran Member Four Rings xalents23's Avatar
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    Oct 26 2011
    AZ Member #
    83081
    Location
    CA

    Did you scan your car for any codes?

    Maybe your EGT's but you need to check to see if, your car is throwing any codes first.

  4. #4
    Veteran Member Four Rings f4m0u5's Avatar
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    Aug 04 2014
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    270797
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    Gloucester MA USA

    Quote Originally Posted by xalents23 View Post
    Did you scan your car for any codes?

    Maybe your EGT's but you need to check to see if, your car is throwing any codes first.
    No codes stored.and my egts have been coded out for over a year

  5. #5
    Veteran Member Three Rings cinimod25461's Avatar
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    Dec 04 2013
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    133598
    Location
    CA

    Does it happen in every gear?

  6. #6
    Veteran Member Four Rings christianb5s4's Avatar
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    Aug 24 2014
    AZ Member #
    277489
    Location
    Newport Beach, CA

    Engine/trans mounts?
    Imola 2001 Stage 3 S4: 324K - 157mph 1/2 mile - 543whp/530ftlbs Mustang Dyno - Built BEL Block - RS4 cams/intake - TTE600s - Ringer Racing Stage 5 - Etspec - SRM V3 Intercoolers - AA built trans - 4:1 Diff - Vast cooler - JHM Trio - 034 - H&R Coilovers - OZ Racing - SRM/SSAC exhaust
    2024 M3 Comp xDrive
    2016 A6 prestige w/ s-line, APR Stg 1, Melen TCU, PS4S, valcona S6 interior parts

  7. #7
    Veteran Member Four Rings Timtheguru's Avatar
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    Dec 30 2009
    AZ Member #
    52711
    Location
    Seattle

    Throttle body alignment can do this. Do you have your n249 intact? Perhaps chec the DVs again.

  8. #8
    Veteran Member Three Rings
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    May 20 2012
    AZ Member #
    93814
    Location
    San Jose

    Also the infamous green check valves.

  9. #9
    Veteran Member Four Rings f4m0u5's Avatar
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    Aug 04 2014
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    Gloucester MA USA

    Yes the n249 is being used. And yes evry gear. It doesnt happen all the time. But it does happen more offten in higher gears on highway.


    I have 034 mounts in there that are a year old. This feels like itsthe motor itself having a problem and kicking not like a bad mount


    Ill do a tba woth vag com tomoro before the drive to work

  10. #10
    Veteran Member Three Rings
    Join Date
    Mar 07 2013
    AZ Member #
    110864
    My Garage
    2015 Mercedes GLK350
    Location
    Rancho Cucamonga, CA

    Quote Originally Posted by f4m0u5 View Post
    Started happening recently never happend before. The car is stage 2. Fuel trims are fine so dont think its a boost leak.

    Car doesnt buck/hesitate when leting off the throttle at wot it only does it in boost and part throttle.

    Car has forge 007s with yellow springs and 1 shim that i just re greased last month.

    Any idea of what it could be? Its as soon as i let off the throttle it will buck slightly.
    not sure ive had this happen before, maybe something similar. check all the basics before getting to crazy with diag. it would appear related to the dv's since it does it when you let off the throttle. could be a slight crack in one or both dv diaphragm. that's where I would start. throttle body boot is a good place to start. cover all your bases on boost leaks. small cracks or tears can be really difficult to see, but they open up when boost is present. weird you don't have any codes.... also leads to believe its something related to dv's off throttle. just my .02
    2008 Ibis RS4....GETing tuned

  11. #11
    Veteran Member Four Rings christianb5s4's Avatar
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    Aug 24 2014
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    277489
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    Newport Beach, CA

    Quote Originally Posted by f4m0u5 View Post
    I have 034 mounts in there that are a year old. This feels like itsthe motor itself having a problem and kicking not like a bad mount
    Ah, gotcha. Thought to mention it since it sounded like possible mount issues since it happened in all gears getting off power after the engine was under load.
    Imola 2001 Stage 3 S4: 324K - 157mph 1/2 mile - 543whp/530ftlbs Mustang Dyno - Built BEL Block - RS4 cams/intake - TTE600s - Ringer Racing Stage 5 - Etspec - SRM V3 Intercoolers - AA built trans - 4:1 Diff - Vast cooler - JHM Trio - 034 - H&R Coilovers - OZ Racing - SRM/SSAC exhaust
    2024 M3 Comp xDrive
    2016 A6 prestige w/ s-line, APR Stg 1, Melen TCU, PS4S, valcona S6 interior parts

  12. #12
    Veteran Member Four Rings
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    Nov 04 2013
    AZ Member #
    128426
    My Garage
    stg 3 widebody, 1990 miata, '05 gsxr 1000
    Location
    Palo Alto, CA

    could also be a wastegate or tune issue no? I like the DV, throttle body, and N249 ideas though.

    This doesn't happen any other time besides partial throttle, and only when you lift?

  13. #13
    Veteran Member Four Rings MacDaddy's Avatar
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    Aug 06 2008
    AZ Member #
    31677
    Location
    Canadia

    Sounds like my car when the TBB started tearing. Leak test it.
    the B5 S4 is like the mafia... there is only one way out!

  14. #14
    Veteran Member Four Rings f4m0u5's Avatar
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    Aug 04 2014
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    270797
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    Gloucester MA USA

    Yup thanks tho!

    Correct only on lift of part throttle .

    Ill boost test it this weekend again, ive gone through a few tbbs, but now it has a t bolt clamp and has been ok il have to double check it again!


    Id say its not tune related. The car was fine before for 10 months
    Thanks guys

  15. #15
    Veteran Member Four Rings f4m0u5's Avatar
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    Aug 04 2014
    AZ Member #
    270797
    Location
    Gloucester MA USA

    Yesterday when i was at the garage flushing my heater core i did a boost leak test with out finding a single leak.

    This morning while bleeding my coolant system i heard a hiss coming from that area. Found out the back side of my brakebooster hose at the secondary fire wall was leaking vac because the rubber grommet wasnt secured in the firewall correctly.


    So far no more heaitation when off throttle!!

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