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S4 boost issues?

Discussion in 'A4/S4 forum(B5 Chassis)' started by benh999, Dec 17, 2010.

  1. benh999
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    benh999 Member

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    thought I'd throw this one out there. My S4 is restricting boost under heavy load. If the accelerator is gradually pressed she boosts perfect and acceleration is rapid, however, if the accelerator is fully depressed, the car will start to boost and then suddenly cut all boost. I think the fault code is pressure control? cannot remember what VAG-COM said but possibly a vacuum leak somewhere?

    Car is at RSD being looked at but thought I'd throw it out there for any ideas.?:blackrs4:

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

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    Pressure test is your first port of call. Its probably a boost leak, F hose and thottle body boots are the most common culprits
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  3. benh999
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    benh999 Member

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    F hose? what does the F stand for? I would have agreed boost leak but how can it be a leak if when the accelerator is gentally pressed throughout the range it maintains boost right across the range?

    Regards

    Ben
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  4. s4best4
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    s4best4 Member

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    I Had a similar problem with my S2. it turned out to be a throttle body pipe had split causing lack of pressure and therefore losing boost.
    Also had a more dramatic problem when the intercooler pipe came off and car was undriveable as car would start but no power at all and would cut out.
    I would go with Aragorn's suggestion first.
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  5. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    Because the car has electronic boost control, I'll just demand more from the turbo until it gets the desired pressure, or the turbo runs out of steam. When you floor it though, its possible for the boost to overshoot the target by enough

    The F hose is so called because its shaped like an F. Its at the front of the engine,near the throttle body boot but down and a little to the drivers side.
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  6. benh999
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    benh999 Member

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    Aha so the easiest way then would be to carry out a pressure test on the system to eliminate any leaks. Then start looking at monitoring the boost levels. I think we've got readings off a similiar spec s4 to compare boost readings with to see if mine is spiking and causing engine managment to cut in.
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  7. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    Yep check for leaks first off.

    If thats fine i'd look at changing the N75 valve (the boost control valve).
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  8. benh999
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    benh999 Member

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    Well this weather is proving a right royal pain in the arse. Seems there are no vacuum or boost leaks but he is struggling to live read the data on the roads as they are sooo bad with ice and snow that the ESP kicks in all the time. Seems for now I am just going to have to drive her as she is until the weather picks up. Real shame as the power I experienced before the ECU limits boost was phenomenal and I just did not realise how quick a mapped s4 is :-(
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  9. benh999
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    benh999 Member

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    I've only just picked up on the fact that MRC are 2.7T specialists and are just down the road from me in banbury. they have a rolling road and could get me out of a pickle in this ***** weather as hopefully they could diagnose the problem for me??????
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  10. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    yup, if your going to take it anywhere, i'd be going to MRC
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