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Thread: VNT overboost

  1. #1
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    VNT overboost

    Hope you lot made the most of the long weekend.

    I went away with the TDI and started to get overboost from the alisport/turbotecnics VNT. I figure vanes have been obstructed or actuator has jammed. Ive been unable to move the actuator lever by hand, access is very difficult, but it seems to be jammed.

    I was getting up to 29psi of boost, which I beleive is the limit for these turbos. Way too high for my liking, so I had to baby it home. Cruising on 20psi near constant!

    To make things worse, my rear exhaust section disconnected at the weld during some offroad abuse. After I removed it overboost became even harder to avoid.

    Question is in regards to the actuator. It is fed from the turbo outlet, so i assume as boost pressure increases the vanes open up and decrease boost potential to avoid overspeed.

    If thats correct then my actuator must be stuck in the at rest position, failing to actuate on boost. Does that sound right for these vnts?

    I will remove the actuator and see whats what. I'm hoping I dont have to open up the turbo to clean the vanes - I've barely used the bloody thing!

  2. #2
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    You are correct, at rest vanes are closed down to the set screw limit. As boost increases the vanes are opened to reduce boost and redirect the exhaust gasses away from the turbine blades.
    The balancing act for a well setup vnt is to maintain boost without overly restricting the exhaust gas flow.

    which is why they are setup in a flow rig at the factory.

    If you remove the actuator don’t touch the locked nut on the actuator rod.
    This position is critical to the VNT operation as is the set screw.
    Once the position is lost it is virtually impossible to reset it without the above mentioned flow rig.

    Cheers,
    Paul.
    Paul.

    77 series3 (sold)
    95 300Tdi Ute (sold)
    2003 XTREME Td5

    I thought I was wrong once, but I was mistaken.

  3. #3
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    Taken from lo-fi on another forums thread < IRB Developments Td5 Dual/Two Channel Turbo Booster Box adjustable tuning instructions? - Discovery Forum - LR4x4 - The Land Rover Forum >:

    You've got two options really:
    Pull off the retaining clip holding the actuator onto the arm, pop the actuator rod off the arm and see if the arm moves freely.
    Use a vacuum tester plugged straight onto the actuator, dial up some vacuum and see if it moves. This is a better test as you can check whether the actuator holds pressure too. If not, you have a punctured diaphragm and that needs replacing.
    I'm either case, if the arm isn't moving, you've found your problem and no amount of electronic trickery is going to fix it. Sometimes it takes just a little more force than the actuator can provide to get it moving, but don't force it! If it's really stuck, dismantling the turbo hot side and clearing the gunk is the best course, though I've seen people use oven cleaner in situ (!)
    VNT turbos can get stuck quite readily, the black diesel nastiness gums the vanes right up. Worse on vehicles that are driven gently most of the time and don't have the vanes moved much. Same goes for the standard TD5 wastegate. It leads to the same faults as you're getting, I put a thread up about fixing my mates a couple of months ago.
    Whilst it is from a discussion on the Td5 motor and some issues experienced that were being dealt with by electronic trickery, the majority relates to the waste gate getting stick on the VNT which is relevant to you.

  4. #4
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    Might seem like semantics but no Wastegate on a VNT!
    Paul.

    77 series3 (sold)
    95 300Tdi Ute (sold)
    2003 XTREME Td5

    I thought I was wrong once, but I was mistaken.

  5. #5
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    I had a quick look at it and managed to get a long screwdriver onto the actuator arm. Takes quite a bit of force but the arm can be pushed down. The actuator springs it back up very quickly on release.

    Soon as I find a bike pump with a gauge I'll hook it up to the actuator, see what 20psi does.

    For the last 50km home I found that flooring fifth gear to load up the engine did a good job of keeping the boost stable at 26psi. So I was hard on the accelerator, then off the pedal to coast - repeat. Wish I had found that out earlier, much fun!

    Its probably not sustainable to be running 25-30psi but with all that air I was running clean as a whistle. Tailpipe may have gone from euro 0 to euro 6! Another bonus, egts never went over 600, even when flat out storming up hill.

    Hopefully no damage done. VNT overboost

  6. #6
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    Pressure gauge on the actuator: starts moving at 12psi, stops at 28psi

    This suggests 28psi is the calibrated upper limit. But 20-24psi was my usual max. Can't see how the actuator calibration can change unless dirt has added resistance.

    Or maybe I released a stuck actuator when I went at it with a screw driver.

    Will have to test drive to find out if problem persists - another day.

    I suppose I better check the pressure sender too!

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