Bad turbo? Boost leak?
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This is a discussion on Bad turbo? Boost leak? within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; Hey guys, So I have noticed that while driving when boost kicks in and I throw in the clutch my ...

  1. #1
    Registered User jraney2008's Avatar
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    Bad turbo? Boost leak?

    Hey guys,

    So I have noticed that while driving when boost kicks in and I throw in the clutch my engine revs a little. When more boost kicks in around 10-14 psi it revs higher, like 1000 rpms. I just had the clutch replaced last week hoping it was just the throwout bearing causing this but sure enough problem is still there. I have looked around and cant find anyone with similar problem. Car runs great but I just cant quite figure out what this is. Any input would be great!

    Modifications:

    Cobb AP Stage 2
    HKS catback exhaust
    3 inch catless down pipe (not sure what brand)
    Uppipe
    K&N short ram intake

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  3. #2
    Registered User Skreekin's Avatar
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    I'm pretty sure this has been discussed here before Jon. Did you search for "boost creep"? I'm not entirely sure what the issue might be, but I'll be happy to take a look. FB me or PM me if you want.

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    Subaru Newb MainFrame's Avatar
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    Quote Originally Posted by jraney2008 View Post
    Hey guys,

    So I have noticed that while driving when boost kicks in and I throw in the clutch my engine revs a little. When more boost kicks in around 10-14 psi it revs higher, like 1000 rpms. I just had the clutch replaced last week hoping it was just the throwout bearing causing this but sure enough problem is still there. I have looked around and cant find anyone with similar problem. Car runs great but I just cant quite figure out what this is. Any input would be great!

    Modifications:

    Cobb AP Stage 2
    HKS catback exhaust
    3 inch catless down pipe (not sure what brand)
    Uppipe
    K&N short ram intake

    Are you saying that when you press the clutch the rpms blip up? Because that is normal.

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    Registered User jraney2008's Avatar
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    Ya but only when boost kicks in. This is my first turbo car so I am still learning. But it just seems strange that rpms would spike when I throw in the clutch when turbo kicks in and not when it isn't. But rpms stay normal when I let off the gas without engaging clutch with boost.

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    Subaru Newb MainFrame's Avatar
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    That's because when you're making boost the intake manifold is pressurized. When you depress the clutch it opens the BPV to release that air. The stock BPV on an Impreza recirculates that air back into the intake, and in turn that charge of air rushes back into the engine causing the RPMs to rise briefly. Or something to that effect..


    If you were running an aftermarket BOV that vents to atmosphere this wouldn't happen, of course then you're dumping air that has already been metered by the MAF, so your engine still dumps the fuel for the air that has just been released causing your engine to run extremely rich between shifts. Which is a bad thing.

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    Admiral Ackbar mycologist's Avatar
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    Some folks in the past have suggested this is an emissions feature.
    "From a little spark may burst a mighty flame." - Dante
    "The stitch is lost unless the thread is knotted." - Italian proverb

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    Subaru Newb MainFrame's Avatar
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    Quote Originally Posted by mycologist View Post
    Some folks in the past have suggested this is an emissions feature.

    Care to expand on that? Last I read on it was in 2003 and people were saying it's caused by what I described..

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    Admiral Ackbar mycologist's Avatar
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    Quote Originally Posted by MainFrame View Post
    Care to expand on that? Last I read on it was in 2003 and people were saying it's caused by what I described..
    This makes it sound pretty much as you described still.

    Quote Originally Posted by GV27 View Post
    Bingo! You beat me to it. I was thinking about it last night and came to the same conclusion. Those who want to know FOR SURE might want to check this with the regulars in "Engine Performance" as they know a heck of a lot more about it than I do, but here goes:

    All modern engines continue to spark after you let off the throttle in an attempt to burn off as much hydrocarbons as possible for emmisions reasons as OldGuy stated. Now in a N/A motor this doesn't produce much energy at all and you don't notice it. In a Turbo things are a bit different.

    The BOV is on the intake side of the throttle plate, so it bleeds off pressure in the intake. However, this leaves a considerable amount of pressurised air on the manifold side of the throttle plate. Just like the pressure helps create a bigger explosion on throttle, so it also creates a bigger explosion during this hydrocarbon burn-off phase. This doesn't produce all that much energy, but enough to accelerate a free-running motor slightly.

    I'm not sure if the recirc BOV has anything to do with it or not - again it's on the intake side. But the throttle plate never closes completely - the idle screw sticks it open slighltly - so some of that air could make it's way back to the manifold. I guess it would help keep pressure in the manifold rather than backing out around the cracked-open throttle plate.



    I'm sure that's part of it to - if your timing is off a bit it's going to exagerate this as the throttle plate will still be open a bit and the BOV wouldn't have gone off so the turbo would still be feeding pressure to the motor. You'll always have the throttle open slightly when you push in the clutch if you're getting a smooth shift - since neither clutch actuation or throttle closing is truly instantaneous, if you waited to actuate the clutch 'till the throttle was completely closed you'd get a big jerk.

    C
    "From a little spark may burst a mighty flame." - Dante
    "The stitch is lost unless the thread is knotted." - Italian proverb

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    Registered User jraney2008's Avatar
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    Alright. So is there anything I should do about it? Or just leave it as it is?

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    Registered User Skreekin's Avatar
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    I had forgotten about the BOV, but now that you've gone back to the OEM part, I think I know what you're seeing. Still, I don't recall quite the surge that you're getting. My '05 doesn't really do that. PM me if you're in town. I'm off today.

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    Subaru Newb MainFrame's Avatar
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    Quote Originally Posted by jraney2008 View Post
    Alright. So is there anything I should do about it? Or just leave it as it is?
    As it is it's not hurting anything, and anything you could do about it will be bad for your motor. So I would just leave it alone.

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    Admiral Ackbar mycologist's Avatar
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    Quote Originally Posted by MainFrame View Post
    As it is it's not hurting anything, and anything you could do about it will be bad for your motor. So I would just leave it alone.
    You can use it as a mini anti-lag
    "From a little spark may burst a mighty flame." - Dante
    "The stitch is lost unless the thread is knotted." - Italian proverb

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    Subaru Newb MainFrame's Avatar
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    Quote Originally Posted by mycologist View Post
    You can use it as a mini anti-lag

    That charge of air rushing back in does help keep the turbo spinning between shifts.

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    Good news, everyone! xxxxxxxAnub1s's Avatar
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    It's a nature of any turbocharged car to blip a little between shifts. It's due to latent pressurized air coming into the engine even after the BPV has opened. Don't worry about it, you can change your driving style to pretty much alleviate any high increases in RPM. When I shift anymore I hardly see a 250RPM blip at all. You just have to know how to time your throttle and clutch work.
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    Registered User jraney2008's Avatar
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    Yeah I have been driving a lot the past couple days and have altered my timing on shifts. It all feels great now especially with the new clutch. Thanks for all the info and help everyone!

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