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Thread: Sequential turbo problems

  1. #1
    I'm part of the furniture Moderator Ian C's Avatar
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    Post Sequential turbo problems

    Here is a look at the control systems of the sequential turbo system and the problems that may occur. Hopefully it will help out with a common set of problems.

    Actuators and VSVs:

    There are four VSV/actuator assemblies that control the sequential turbo operation:
    Intake Air Control Valve (IACV)
    Exhaust Gas Control Valve (EGCV)
    Exhaust Gas Bypass Valve (EGBV)
    Wastegate

    VSVs switch manifold pressure (boost) to and away from actuators. The actuators open/shut valve butterfly flaps.

    IACV
    When this switches, boost generated by #2 turbo is allowed to join the intake stream
    When it isn't switched, there is a small flap in the IACV assembly that allows any boost generated by #2 turbo as it prespools to join the intake stream, preventing turbo stall.

    EGCV
    This allows exhaust gas to flow through #2 turbo, causing it to spin up. When it's shut, there is no gas flow through the impeller of #2 turbo and therefore it doesn't spin.

    EGBV
    This allows some exhaust gas to bypass the EGCV and join the output of the first turbo. It serves two purposes - one, it prespools the second turbo by allowing some exhaust gas flow through it. Two, it controls the amount of exhaust gas going through the first turbo in much the same way as a wastegate - therefore controlling the maximum boost pressure generated by #1 turbo.

    Wastegate
    This bypasses exhaust gas from the first turbo, lowering the overall amount of exhaust gas going through *both* turbos and controlling the maximum boost pressure generated by both turbos in parallel.

    What it should be like:
    You get 0.7bar of boost on turbo 1, a slight dip in power between 3500 and 4000rpm, and then 0.8bar of boost with both turbos online. This should be a smooth and linear power delivery.

    When modified with a boost controller/decat, you'll see an increase in boost after 4000rpm when the second turbo comes online. You may also get 0.8bar of boost on the first turbo with a decat. You'll feel the same power dip at 3500rpm but then a noticeable surge in power at 4000rpm when the second turbo comes online and hits over 1 bar of boost. Full boost is acheived before 4500rpm.

    When things go wrong:
    Problem: I don't get a 2nd turbo rush, it just seems to sit at 0.8bar.
    The second turbo isn't kicking in. This usually means your IACV isn't opening - the turbo is trying to spin up but can't flow any air, so #1 turbo does all the hard work.

    By the time #1 turbo is producing 0.8bar at 5000rpm it's way beyond it's efficiency map and is overheating the air and overspeeding the turbine. Try not to do this too often.


    Problem: I don't get a 2nd turbo rush, and as soon as I should it dumps all boost instead.
    The IACV is opening but the second turbo isn't spinning. All the air boosted by #1 turbo now has an easy path back to the non-pressurised side of the turbos via a non-spinning #2 turbo. In effect, it all goes 'backwards' through #2 instead of going into the engine. You'll maybe see .2 or .3 of a bar of boost but that's about it.

    This usually means the EGCV isn't opening. The turbo can't spin as it has no exhaust gas flow path. Although annoying, this won't do long-term damage as #1 turbo won't generate much boost because the engine loses a lot of power, so no overspeeding or overheating.


    Problem: The second turbo comes in late.
    It seems like, instead of 4000rpm, your second turbo cuts in at 5000rpm or later. Usually this is caused by no prespool taking place - your IACV opens, your EGCV opens, but the second turbo is sat on it's ass doing nothing. So, not only has it got to suddenly get up to speed, but it's fighting against #1's output trying to go backwards through it. Hence, the looong lag.

    This is usually caused by the EGBV not opening. You may notice more than 0.8bar of boost being generated by #1 turbo because of this, as the EGBV also acts as #1's wastegate.


    Troubleshooting:
    The usual cause of these problems is hose related. Either a vacuum hose has popped off or it's split. These hoses can get hard and brittle with age as they are subjected to continued engine bay heat cycling. It's usually a slightly split hose if the problem is intermittent, and a popped off or completely holed/severed hose if it's permanent.

    To troubleshoot your problem, look at the pressure map of the system and trace the hoses/pipes between the suspect components. Check they are on and intact - removal of the hoses may be necessary to fully check for splits.

    Edit I deleted the images off my personal webspace so I've uploaded them direct to the thread. Pressure map first then all the component locations.

    See also:
    http://www.mkivsupra.net/resources/turbopipes/left.html
    For images and descriptions of the pipework

    -Ian
    Attached Images Attached Images
    Last edited by Ian C; 19-06-08 at 17:45. Reason: Uploading pictures
    I love my job. It pays for my Supra.

  2. #2
    Hoping for sunshine Club Member RICHARDA's Avatar
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    Ian, what a fantastic post. Well done.

    Rich
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    retired Club Member Dragonball's Avatar
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    Just something you knocked up in 5 mins eh Ian?

    Fantastic info there

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    Work... what work? MEMBERSHIP EXPIRED nic_f's Avatar
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    Top post mate, I have been looking for somthing like this for ages.

    Well Done.

    Nic

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    I'm part of the furniture Authorised Trader Chris Wilson's Avatar
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    Gets my vote for best technical assistance post of the year, needs putting somewhere permanent. Congratulations Ian, well written and illustrated.
    Regards, Chris Wilson. No PM's PLEASE, use e-mail!
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    Asian Dude Club Member Soop Dogg's Avatar
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    Excellent post Ian. This will be of use to many,many people on the BBS.

    Good Man!!

    (Have I seen that engine before somewhere....?)

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    Posting is my life! Club Member MONKEYmark's Avatar
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    great post ian

    good to see close up pics with descriptions of what each part is.

    on pipe marked up number 10 if you follow it round there is a part with a butterfly inside, what is that part called?
    is it IAVC?
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  8. #8
    I'm part of the furniture MEMBERSHIP EXPIRED
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    Like it a lot, well written Ian

    Regards,

    Gaz.

  9. #9
    I'm part of the furniture Moderator Ian C's Avatar
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    Originally posted by MONKEYmark

    on pipe marked up number 10 if you follow it round there is a part with a butterfly inside, what is that part called?
    is it IAVC?
    If you mean the big diecast piping, yeah, that's the IACV

    I'm glad this appears to have been worth the effort, thanks all

    -Ian
    I love my job. It pays for my Supra.

  10. #10
    Work... what work? MEMBERSHIP EXPIRED
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    Ian,

    You are hereby nominated for post of the year

    What a fantastic article, especially as I am currently suffering from 2nd Turbo cutting in late

  11. #11
    I'm part of the furniture Moderator Ian C's Avatar
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    I've re-done this post in the Articles section of the site, sadly I can't quite bring myself to move all the nice comments by everyone as that would be just too big headed


    So go and post new ones hahahahaha




    No really, off you go. :flame Dev

    -Ian
    I love my job. It pays for my Supra.

  12. #12
    Work... what work? MEMBERSHIP EXPIRED chris_bramley's Avatar
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    Quote Originally Posted by Ian C
    Here is a look at the control systems of the sequential turbo system and the problems that may occur. Hopefully it will help out with a common set of problems.
    Ian - The Man

    This post - The Legend


    Thanks dude!

  13. #13
    Supree mo fo Club Member suprattgaz's Avatar
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    I can't see the diagrams, are they still there????
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  14. #14
    Work... what work? MEMBERSHIP EXPIRED
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    Nice one ian

  15. #15
    I'm part of the furniture Moderator Ian C's Avatar
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    Erk - I mucked out my personal webspace and killed off the images. I'll sort this asap.

    -Ian

    Edit - sorted
    Last edited by Ian C; 19-06-08 at 17:52.
    I love my job. It pays for my Supra.

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