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  1. #1
    Senior Member Two Rings Allaidback's Avatar
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    May 23 2008
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    29129
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    Columbia, SC

    S4 doesn't boost past 17psi???

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    Hey guys...I'm having an issue right now with my vast mbc. My car (awe stage 3) spikes at 20psi and then holds at 17psi consistently no matter how much I turn the boost up with the mbc w/ N75. Now it will cut boost back, but can't get it to boost past 17psi. Already pressure tested and everything is good, no cel and the car is running good. Before this issue I found a small leak on the t-fittings going to the mbc and n-75, but after I fixed the leak this is whats going on? The car was running great before I fixed the leak, hitting 22psi and holding at 21psi. The mbc is maxed out right now and still no increase in psi. I'm going to reset the ecu and see if that helps. Any other suggestions?

  2. #2
    Veteran Member Four Rings gointoscott's Avatar
    Join Date
    Jan 26 2006
    AZ Member #
    9931
    My Garage
    2001 Audi S4
    Location
    Milwaukee, WI

    Re: S4 doesn't boost past 17psi???

    do you have vag-com? log block 115 and see what requested boost vs actualboost is.

    what does the car boost to if you just use the n75?
    Original K03's, Ecodes, H&R Coilovers, Inmotion Chip/MBC, Piggies

  3. #3
    Senior Member Two Rings Allaidback's Avatar
    Join Date
    May 23 2008
    AZ Member #
    29129
    Location
    Columbia, SC

    Re: S4 doesn't boost past 17psi???

    yeah I got vagcom...the requested is 22psi. I'll take the mbc off tomorrow morning and post back with just the N-75.

  4. #4
    Veteran Member Three Rings
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    Sep 20 2007
    AZ Member #
    21230
    Location
    Harrisburg, PA

    Re: S4 doesn't boost past 17psi???

    Maybe it's your boost gauge.

  5. #5
    Established Member Two Rings jaredsogn's Avatar
    Join Date
    Oct 02 2006
    AZ Member #
    12215
    My Garage
    2001 Silver S4
    Location
    Watertown SD

    Re: S4 doesn't boost past 17psi???

    Please don't tell me you have it in parallel with the N75? If you do, you can turn that thing up as high as you want and its not going to increase your boost...if you are trying to increase the boost with the MBC, you need to run it in MBC only mode...

    Quote Originally Posted by Allaidback View Post
    Hey guys...I'm having an issue right now with my vast mbc. My car (awe stage 3) spikes at 20psi and then holds at 17psi consistently no matter how much I turn the boost up with the mbc w/ N75. Now it will cut boost back, but can't get it to boost past 17psi. Already pressure tested and everything is good, no cel and the car is running good. Before this issue I found a small leak on the t-fittings going to the mbc and n-75, but after I fixed the leak this is whats going on? The car was running great before I fixed the leak, hitting 22psi and holding at 21psi. The mbc is maxed out right now and still no increase in psi. I'm going to reset the ecu and see if that helps. Any other suggestions?
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  6. #6
    Veteran Member Three Rings Paulyworld1432's Avatar
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    Dec 07 2007
    AZ Member #
    23171
    Location
    Houston, TX

    Re: S4 doesn't boost past 17psi???

    I had a similar problem... FWIW I had an MBConly set to 22-23 psi and then one day it would only make 19 and hold... I immediately took out the MBC and place the N75only in. My boost fluctuated from 15 to 20 to 14 to 22. Pressure tested and I had a boost leak. Fixed it and problem solved. Although you said you pressure tested... are you sure you caught any leak that might have been there?

  7. #7
    Veteran Member Four Rings AudiSportB5S4's Avatar
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    Jun 14 2007
    AZ Member #
    18801
    My Garage
    2000 S4
    Location
    Connecticut

    Re: S4 doesn't boost past 17psi???

    Quote Originally Posted by jaredsogn View Post
    Please don't tell me you have it in parallel with the N75? If you do, you can turn that thing up as high as you want and its not going to increase your boost...if you are trying to increase the boost with the MBC, you need to run it in MBC only mode...
    x2... are you running the MBC in parallel or standalone? If you are running it in parallel it is only used to clip spikes. If it is in standalone that's where you should be able to take manual control over the boost.
    Current: B9.5 SQ5 | Past: C7.5 S6, B5 S4, C7.5 A6, 8V A3, B8 A4, B5 A4 | Videos

  8. #8
    Senior Member Two Rings Allaidback's Avatar
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    May 23 2008
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    Columbia, SC

    Re: S4 doesn't boost past 17psi???

    i'm running it in parallel with the n-75...i just can't understand why i had full control of it before. I could set it at 10 psi or 25psi, i guess it was because the leak was in the wastgate line. Would my dv's be causing this? they are baileys.

  9. #9
    Veteran Member Four Rings AudiSportB5S4's Avatar
    Join Date
    Jun 14 2007
    AZ Member #
    18801
    My Garage
    2000 S4
    Location
    Connecticut

    Re: S4 doesn't boost past 17psi???

    Quote Originally Posted by Allaidback View Post
    i'm running it in parallel with the n-75...i just can't understand why i had full control of it before. I could set it at 10 psi or 25psi, i guess it was because the leak was in the wastgate line. Would my dv's be causing this? they are baileys.
    no, but there has to be a problem somewhere along the way because unless you have the MBC set too low, then it would be hitting below requested... You said your requested is into the low 20's with PSI and if you're only getting 17 then there is a problem somewhere, especially if the MBC is turned all the way up. Turned up all the way or not you shouldn't get PAST requested unless you're spiking which is why you have a MBC in there to protect, but since it's all the way loose the set screw then it means something is preventing you from boosting. Try running the valve in standalone. All you need there is for the bottom line that is connected to the N75 to be into the MBC, and the line coming from the driver side of the N75 connected to the N75, the other stays in and so does the electrical connection. See what happens, but turn down that MBC unless you want to hit some really fun pounds of boost!
    Current: B9.5 SQ5 | Past: C7.5 S6, B5 S4, C7.5 A6, 8V A3, B8 A4, B5 A4 | Videos

  10. #10
    Senior Member Two Rings Allaidback's Avatar
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    May 23 2008
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    Columbia, SC

    Re: S4 doesn't boost past 17psi???

    i will have to bleed the hole if I run mbc only correct? I have the vast one to run in parallel and there is no hole to bleed?

  11. #11
    Veteran Member Four Rings gointoscott's Avatar
    Join Date
    Jan 26 2006
    AZ Member #
    9931
    My Garage
    2001 Audi S4
    Location
    Milwaukee, WI

    Re: S4 doesn't boost past 17psi???

    Yes you need the bleed hole if running in standalone.

    This is my experience with running mbc with n75:

    My chip was set to boost to 12-13 psi, pretty conservitive. I was able to get the boost to rise to 14-15 psi with the mbc with bleedhole running parallel with n75. So I was able to increase boost with mbc running parallel with n75. Then I learned that you shouldnt have the bleedhole running in parallel because it basically would be like a boost leak. So I plugged it, after that I couldnt get it to boost any higher then the 12-13 psi that the chip was set to.

    If I went standalone it would boost higher but then I would get codes for overboosting and it would cut out so I just went back to n75.

    What I finally ended up doing is having my tuner reflash my ecu and now it boosts to 17 and holds, runs great! I am very happy with that. I'm on stock turbo's so dont want to go higher then that.

    So what I'm thinking could be happening for you is when you had the boost leak it was esentially working like the bleed hole on the mbc allowing you to boost higher but once you fixed that leak, then you were back to square 1 where the mbc only was capping boost spikes since you are running in parallel??? This all depends how big of a boost leak you had.

    Going by my experience this would be a viable explanation why you are having the problem you are.


    Good thing you got the boost leak fixed thouh because that causes the turbo to work harder to achieve the desired boost which shortens turbo life.
    Original K03's, Ecodes, H&R Coilovers, Inmotion Chip/MBC, Piggies

  12. #12
    Senior Member Two Rings Allaidback's Avatar
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    May 23 2008
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    Columbia, SC

    Re: S4 doesn't boost past 17psi???

    damn guys whats going on...I ran the car with just the n-75 and car only boost to 10psi and hold steady. Logged block 115 and 118 and requested boost is around 2200 and actual boost is 1600. There is no fluctuation in boost at all...something is causing the boost to stop at a certain point, what could it be? Still no CELS! and boost check came out clean. What would happen if I ran the mbc without a bleed hole?

    *update* installed the mbc in standalone and worked great...boost to 22psi no prob.
    Last edited by Allaidback; 12-13-2008 at 05:31 PM.

  13. #13
    Veteran Member Four Rings gointoscott's Avatar
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    Jan 26 2006
    AZ Member #
    9931
    My Garage
    2001 Audi S4
    Location
    Milwaukee, WI

    Re: S4 doesn't boost past 17psi???

    It is not good to run the mbc in standalone without the bleedhole. The bleed hole is used to bleed off boost so it doesnt harm the turbo's. It basically acts like the n75.

    If you want to run mbc in standalone spend the 30 or 40 dollars and buy the mbc from vast that has the bleedhole.

    But something isnt right if you put n75 in and its requesting 2200 and its only boosting to 1600, to me that screams boost leak somewhere. I can see how that is puzzling since you checked for leaks. Maybe you missed something or hooked up the n75 wrong.

    One thing for certain dont keep running the mbc in standalone without the bleedhole.
    Original K03's, Ecodes, H&R Coilovers, Inmotion Chip/MBC, Piggies

  14. #14
    Senior Member Two Rings Allaidback's Avatar
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    May 23 2008
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    Columbia, SC

    Re: S4 doesn't boost past 17psi???

    thanks for the advice...yeah i am puzzled just because you think i would be able to find a boost leak of 12psi? I pressured the intake system to 21psi and it held for a good 20-30 seconds with no audible sounds. Couldn't it be something electrical or another valve acting up? This is a brand new n-75 valve. Also, couldn't I just drill a bleed hole? anybody know the diameter of the hole?

  15. #15
    Senior Member Two Rings Allaidback's Avatar
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    May 23 2008
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    Columbia, SC

    Re: S4 doesn't boost past 17psi???

    I figured maybe I hooked up the n-75 valve backwards when I was only boosting to 10psi so I decided to switch the hoses on the n-75 valve to see if it made any difference...exactly the same...boosted to only 10psi??? I know this can't be normal. It's like it doesn't matter what side of the n-75 you hook up the hoses to? So what's my problem?

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