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Thread: Do these VCDS traces explain my limp mode?

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  1. Do these VCDS traces explain my limp mode? 
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    Apr 2014
    SW London / Surrey
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    Turbo on our BKD has been dropping into limp mode pretty reliably on motorway trips for a few months. I've been underneath and dropped the turbo actuator from its mount so that I could wiggle the VNT lever around, and it seems to shift pretty easily. I've also put a hand vacuum pump on the actuator and watched the action from along the exhaust tunnel. It seems to run smoothly to the stop and back in the expected vacuum range. Also popped the vacuum manifold / distribution plate off and all the lines that I'd expect to do appear to hold a good vacuum.

    I did a bit of VCDS logging and noticed that when the actual / specified on the boost control vary significantly from each other there is often a similar variance on the EGR control, such as in the areas I've marked with a red rectangle below. Is there likely to be a connection between these, such as vacuum system losing (negative) pressure? Could EGR action be dragging down turbo response and triggering limp mode?

    BTW, the car went into limp mode towards the end of this trace. Looking at the traces I'm a bit surprised that the area in the first rectangle didn't do this as well. Is there a grace period of some kind before it will actually trip?

    [Hmmm. I'm struggling to make image large enough to see easily. Top trace is boost pressure (blue=specified, red=actual) and lower is EGR (green / purple respectively).]

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    Last edited by VAG-Abound; 15-04-2019 at 08:43 AM. Reason: Trying to make image a bit bigger :(
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  2. Re: Do these VCDS traces explain my limp mode? 
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    Jan 2007
    Nottingham, England
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    You could do with extending the time base as it helps see what is going on but you have clear overboost flat lining on the sensors max measuring pressure so it is pretty certain the turbo needs a rebuild.
    Dark was that day when Diesel conceived his grim engine that begot you, vile invention, more vicious, more criminal than the camera even, metallic monstrosity, bale and bane of our culture, chief woe of our Commonweal. How dare the Law prohibit hashish and heroin yet license your use, who inflate all weak inferior egos? Their addicts only do harm to their own lives: you poison the lungs of the innocent, your din dithers the peaceful, and on choked roads hundreds must daily die by chance-medley. Nimble technicians, surely you should hang your heads in shame. Your wit works mighty wonders, has landed men on the Moon, replaced brains by computers, and can smithy a "smart" bomb. It is a crying scandal that you cannot take the time or be bothered to build us, what sanity knows we need, an odorless and noiseless staid little electric brougham.

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