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  1. #1
    Registered Member One Ring Pete6641's Avatar
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    B5 S4 eating boost pressure sensors like candy

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    Hi I'm a new member I've been trying to use the search feature to find info on this but I haven't found anything I have put three boost pressure sensors in my stage 2 Giac B5 S4 in the past 2 months. I have been getting over boost codes for about a year but since I started driving the S4 more often I wanted to address that, i figured it out it's the boost pressure sensor changed it car ran great for a few weeks and then started doing the same thing again, on the highway in high gear if I get on it at all it seems to go into some sort of limp mode limits me to about 6 pounds of boost (normally 19) a boost pressure sensor seems to last about a week or so before going bad and doing the same thing all over again. I looked and it seems like ECS sells a high performance one but says it's specifically for an APR tune, will it still work?kind of stuck any ideas?20180912_190229.jpeg

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  2. #2
    Veteran Member Four Rings
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    are you sure you are simply not just pinning the MAP sensor and throwing a legitimate code? how do you know the sensor is defective? What PSI are you supposed to be running, and how much ARE you running? Have you logged your map sensor?

  3. #3
    Registered Member One Ring Pete6641's Avatar
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    Quote Originally Posted by S4James View Post
    are you sure you are simply not just pinning the MAP sensor and throwing a legitimate code? how do you know the sensor is defective? What PSI are you supposed to be running, and how much ARE you running? Have you logged your map sensor?
    I'm supposed to be at 19psi at WOT until higher rpms when it's supposed to taper off my boost gauge seems pretty accurate and confirms this... after the boost sensor seems to fail it can stray up to 25 psi and start to wipe out the plugs and go into a limp mode, I have not logged the sensor and I don't know what the safe max pressure is for the sensor I was hoping someone would have some info for me on here ... At my job we do have a rosstech and I could not find a specified or current boost pressure PID would you happen to know the channels I should be looking at?

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  4. #4
    Veteran Member Four Rings
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    06 9-3 Aero 2.8T, 96 Miata
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    I would be curious to try a different n75 valve.

    I'm running 30+ psi on the stock MAP sensor (pegging it, but that's OK for my setup. I'm not using the n75 to control boost).

    Stock map pegs at 2.5 bar (including atmospheric, so roughly 21.5psi boost.)

  5. #5
    Registered Member One Ring Pete6641's Avatar
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    Quote Originally Posted by DieselElectric View Post
    I would be curious to try a different n75 valve.

    I'm running 30+ psi on the stock MAP sensor (pegging it, but that's OK for my setup. I'm not using the n75 to control boost).

    Stock map pegs at 2.5 bar (including atmospheric, so roughly 21.5psi boost.)
    So going over the max the sensor can read doesn't seem to damage it then? I go no where near that much... I think I might try a manual boost controller to see how that does... I have one in my GTI to keep the boost down.... It's just so weird that after I change the sensor it seems to run so much better and doesn't go into limp mode at all for at least a week or more and it won't throw a "charge pressure positive deviation code"but as soon as it starts getting inconsistent with the boost pressures it sets a code and goes into limp mode ..... Why would the sensor make a difference when I change it, for any amount of time, if it wasn't the problem? Just clearing codes doesn't have the same effect.

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  6. #6
    Senior Member Three Rings LJS's Avatar
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    Pete- I'm confused---are you talking about the pressure sensor for a boost gauge or the boost control valve (N75). The issue you described is an indication of a malfunctioning N75 valve as noted by DieselElectric.
    As an aside- you should learn about how to data log your vehicle as noted by S4JAMES---the TECH section has excellent information on this.The last post on that sticky,written by some putz, details how to use a Ross-Tech cable to log data. I would collect some data before adding a manual boost controller---identify the problem verses the application of a band-aid. For a sorted stage-II you should see data as follows-this is my 00 S4.
    PS- NICE engine compartment !
    [IMG][/IMG]

  7. #7
    Registered Member One Ring Pete6641's Avatar
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    Quote Originally Posted by LJS View Post
    Pete- I'm confused---are you talking about the pressure sensor for a boost gauge or the boost control valve (N75). The issue you described is an indication of a malfunctioning N75 valve as noted by DieselElectric.
    As an aside- you should learn about how to data log your vehicle as noted by S4JAMES---the TECH section has excellent information on this.The last post on that sticky,written by some putz, details how to use a Ross-Tech cable to log data. I would collect some data before adding a manual boost controller---identify the problem verses the application of a band-aid. For a sorted stage-II you should see data as follows-this is my 00 S4.
    PS- NICE engine compartment !
    [IMG][/IMG]
    I'll look into data logging on the Ross tech on Monday when I get back to work... The boost pressure sensor at the top of the bipipe right before the throttle body is what ive been replacing and what has made a difference in the way the car runs... And thank you I do try to keep it clean



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  8. #8
    Registered Member One Ring Pete6641's Avatar
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    This is the sensor I'm talking about (the one to the right) some call it the map sensor even though it's not in the manifold and is before the throttle body... My boost gauge seems to be accurate and it's not a cheap eBay gauge or anythingScreenshot_20190112-231930_Google.jpeg

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  9. #9
    Senior Member Three Rings LJS's Avatar
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    15 VW GTI S-Very WHITE
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    OK- that's the problem----Wrong sensor----It's the N75 valve----pull the pass side engine cover off and you'll see it
    [IMG][/IMG]

  10. #10
    Senior Member Three Rings LJS's Avatar
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    15 VW GTI S-Very WHITE
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    FWIW- not to sound like a smart-ass----You need to look through this-
    https://www.amb.org/ti/audi/2.7Biturbo-SelfStudy.pdf

  11. #11
    Registered Member One Ring Pete6641's Avatar
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    Quote Originally Posted by LJS View Post
    OK- that's the problem----Wrong sensor----It's the N75 valve----pull the pass side engine cover off and you'll see it
    [IMG][/IMG]
    My only concern is that I change the charge pressure sensor (g31 on your diagram) and the car behaves perfect for a week or two driving it every day why would this make any difference with the n75 valve for that time? I'll see if the GTIs n75 will fit...

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  12. #12
    Registered Member One Ring Pete6641's Avatar
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    Quote Originally Posted by LJS View Post
    FWIW- not to sound like a smart-ass----You need to look through this-
    https://www.amb.org/ti/audi/2.7Biturbo-SelfStudy.pdf
    I'm never against learning shoot any relevant info my way but the link you posted isn't workingScreenshot_20190112-235220.jpeg

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  13. #13
    Senior Member Three Rings LJS's Avatar
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    The link opens a PDF which you can save or open with a PDF reader.....Works for me w/ FF on my laptop.
    Regarding using a GTI N75----I don't know what to say about that ......period.

  14. #14
    Registered Member One Ring Pete6641's Avatar
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    Quote Originally Posted by LJS View Post
    The link opens a PDF which you can save or open with a PDF reader.....Works for me w/ FF on my laptop.
    Regarding using a GTI N75----I don't know what to say about that ......period.
    Lol I'm pretty sure they are the same.. just to see if it makes a difference, I'll check the part numbers before swapping worse case I waste 5 min swapping around n75s I know new ones are pretty pricey ... Got the PDF to work I'll read through it thanks

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  15. #15
    Registered Member One Ring Pete6641's Avatar
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    I'm thinking that I'm having less of a boost control problem and more of ,my ecu doesn't know what my boost pressure is, problemScreenshot_20190113-011806_Drive.jpeg

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  16. #16
    Registered Member One Ring Pete6641's Avatar
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    I believe this is what's happening.. maybe the ECU is putting out too much reference voltage and burning the sensor out ...I'll check that also on Monday at work Screenshot_20190113-012656_Drive.jpeg

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  17. #17
    Veteran Member Four Rings MacDaddy's Avatar
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    When i was overboosting it was the n75, if you want to be meticulous, you can use a vacuum tester to see if the n75 is air tight, my bad n75 wouldnt hold vacuum from my handheld tester and the new one did.

  18. #18
    Senior Member Three Rings ReggieNoble's Avatar
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    Check the grounds on the firewall! Make sure they are clean and tight. I would try to refrain from using engine dressing. I'm also assuming you pressure washed it too in recent time?

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  19. #19
    Registered Member One Ring Pete6641's Avatar
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    Quote Originally Posted by MacDaddy View Post
    When i was overboosting it was the n75, if you want to be meticulous, you can use a vacuum tester to see if the n75 is air tight, my bad n75 wouldnt hold vacuum from my handheld tester and the new one did.
    Ok I'll definitely try that thanks!

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  20. #20
    Registered Member One Ring Pete6641's Avatar
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    Quote Originally Posted by ReggieNoble View Post
    Check the grounds on the firewall! Make sure they are clean and tight. I would try to refrain from using engine dressing. I'm also assuming you pressure washed it too in recent time?

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    No I never pressure washed it I cleaned it very carefully with Windex and fantastic and used an aerosol interior satin shine on the plastic and rubber parts..... I have checked those grounds before but I will check them again thanks!.....


    I've been pretty much in and out on this entire car all the work done has been done by me I've been working on cars for more than half my life with my own car especially the s4 I am meticulous and try to make everything to perfection ... This problem is not something recent I've been making my way towards finding out what is wrong I've come to the conclusion that the "charge pressure sensor" is the problem...I replaced it and the symptoms were gone, for about 2 weeks, and then returned, then I replaced it again thinking it was defective again symptoms disappeared for a few weeks then returned.... I will mess with the n75 valve even though I believe it is not the problem hopefully it is I'm tired of fighting this over boost code... Tomorrow I will get as much info as I can from the car I'll try to get some data logs and I'll post everything I can I'll get a new n75 tommorow they have them in stock at our part suppliers wearhouse so I can have one tommorow by 10:30 I'll post how everything works out ... thanks for the suggestions and info!

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  21. #21
    Veteran Member Four Rings zillarob's Avatar
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    Quote Originally Posted by DieselElectric View Post
    I would be curious to try a different n75 valve.

    I'm running 30+ psi on the stock MAP sensor (pegging it, but that's OK for my setup. I'm not using the n75 to control boost).

    Stock map pegs at 2.5 bar (including atmospheric, so roughly 21.5psi boost.)
    Why no 5180 and bigger charge pressure sensor?
    How are you controlling the fuel?
    There are only 2 things needed to make an Audi work properly - Duct tape and WD40. If it moves and it shouldn't - Duct tape. If it doesn’t move and it should - WD40.

  22. #22
    Veteran Member Four Rings
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    06 9-3 Aero 2.8T, 96 Miata
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    Quote Originally Posted by zillarob View Post
    Why no 5180 and bigger charge pressure sensor?
    How are you controlling the fuel?
    Just haven't gotten around to it.

    I have the MAF underscaled a bit. Haven't had any issues so far.

  23. #23
    Registered Member One Ring Pete6641's Avatar
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    I changed the n75 valve and the MAF (because I had a "MAF signal to high -intermittent" code when I checked today) and it seems to be working fine, although the new n75 didn't hold any more boost than the original one , I only got to test it briefly before sitting in traffic I ordered another n75 that should be here tomorrow morning I'll test to see if it holds vacuum better than the other 2... My friend has a 1.8 and it has almost the same n75 besides the orientation of the plug to the hose connections is different, but it held vacuum very well .... I will follow up in a week or so to say all is good or as soon as there is an issue .... Thanks to all for the advice and information I really appreciate the help !!!

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  24. #24
    Registered Member One Ring Pete6641's Avatar
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    Another update: the n75 valve seems to have done the trick! No codes at all ! So far so good anyway... I did a 0-130 pull on the highway and no limp mode no warning lights seems much faster now actually even though it seems to boost at about 17 or 18 instead of 19 or 20 now.... I'm glad I did post on here u guys were a huge help!... still one thing is bugging me though, why did the car run so good for a week or so after replacement of the boost pressure sensor?

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