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2.0 TDI Mystery... can you solve?

Discussion in 'Audi S4/A4/A4 Cab (B7 Chassis)' started by mattjarv, Oct 21, 2013.

  1. mattjarv
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    mattjarv Member

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    [Oct 21, 2013]
    Hi guys,

    Wondering if you can help me with a problem which nobody else has been able to so far. I have an a4 Cab 2.0 TDI and between 2000-3000rpm on a slow accelerate, the engine jerks a little and starts making a hissing/sucking sound. If I floor it, this doesn't happen but only does on slow-med accelaration.

    I've taken this to Audi and several other garages and they have run a scan and nothing. Someone said it was the fly-wheel, but it wasn't, others the EGR Valve, it wasn't and the problem continues!

    SO this is just a post to see if anyone has had this before with the same symptoms and could possible shed light on to the problem?

    Thanks!
    #1
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  3. harmless
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    harmless Member

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    [Oct 21, 2013]
    i had a similar issue where it whistles had the egr cooler pipe replaced and fixed it.. didnt happen at full speed just on slow acceleration or revving the engine stood still it would happen occasionally
    #2
  4. ChrisShepB7
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    ChrisShepB7 Member

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    [Oct 21, 2013]
    Mine does the same thing I'm thinking it's either throttle body is sticking, turbo vanes sticking or injector wiring harness.
    #3
  5. Paul B7
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    Paul B7 Active Member

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    [Oct 21, 2013]

    My money would be on an EGR related issue....................
    #4
  6. Springer*
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    Springer* Member

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    [Oct 21, 2013]
    Had something very similar to this on an old 02 plate Golf GT TDi PD 150.

    The car would jerk on acceleration, usually around the point where the turbo would kick in and you could hear a loud(ish) sucking/hissing sound in the cabin that was much louder outside.

    Turned out to be a kn4ckered intercooler pipe.

    J:)
    #5
  7. Alexthesparky
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    Alexthesparky New Member

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    [Oct 23, 2013]
    Have you checked the vacume actuator that does the flaps (long to short air intake) which i believe opperate 1800 rpm.
    Just a thought.
    #6
  8. Tolak
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    Tolak Member

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    [Oct 24, 2013]
    Two big suspects at this stage.
    First is the EGR, which is active at this speed, and for which a dirty EGR causes hesitation at this rev range.
    Note that dirt in the EGR is one thing, but that the pipes around the EGR can also get dirty, and are not so easily checked.
    Follow-on from the EGR is that there is (apparently) a cooler for the EGR flow, and there is a habit of these cracking. (See http://www.audi-sport.net/vb/audi-s...600-b7-2-0-tdi-egr-cooler-pipe-blown-off.html)

    Second option, which would make less hissing, is that the turbo VVT actuator is leaking. The VVT is actuated by a tennis-ball sized vacuum actuator on top of the turbo, which has a modulated vacuum "flow" from a control trembler valve on the rear bulkhead of the engine bay. If the hose between the trembler valve and the VVT actuator has come off or damaged, then it would hiss (but probably not very loud), but the VVT (and hence the turbo) would not work properly. Check this assembly with a cold, non-running engine, and grip/squeeze the actuator to pull its shaft, which should move easily but make no hissing noise.
    #7
  9. mattjarv
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    mattjarv Member

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    [Oct 25, 2013]
    Thanks guys all sounds rather expenseive but lots to check!
    #8
  10. mm707
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    mm707 Member

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    [Oct 26, 2013]
    That fixed mine hesitating and part is only about £10
    #9
  11. Tolak
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    Tolak Member

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    [Nov 6, 2013]
    Adding a third; since the diesel system does not have vacuum in the manifold at any stage, it requires a vacuum pump, which tends to have a reservoir. Cracks in the hoses or the reservoir will hiss, but will also decrease the servo "muscle" and hence degrade the operation of the turbo, and hence the performance.
    I'm investigating this on the daily hack; at the start of the journey, the turbo is less effective, but become more effective once the engine has run for a while. Might be the new turbo being tight (but 2k miles done now), but worth checking as the vacuum system had to be disturbed to fit the new turbo. I will report back once proper tests are done.
    #10
  12. mattjarv
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    mattjarv Member

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    [Nov 11, 2013]
    Hey Tolak,

    My car seems to make this nice and do the jerking more when the engine is heated up. Does it in most gears and even 70 on the motorway in 6th gear (where the rpm is between 2 - 3000rpm is was doing it but always more when the engine is hot.
    #11
  13. Tolak
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    Tolak Member

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    [Nov 14, 2013]
    Working with old-skool logic (rather than actual knowledge), but it seems that if the engine is jerking it is a control that is changing a state quickly.
    So the EGR is unlikely to do this (and would not be relevant on the motorway).
    It might be a sensor that has a backup (some Lambda systems have two, and can run with one) so you have one miss-reading, and this drops in and out of being acceptable.
    Similarly, the flap (mentioned above) might have a marginal actuator, so it sucks from one position to the other, and the transition causes a pulse.

    What it might remove from consideration is the broken component mechanism; if something is broken, it will not cause a changing system.

    With the modern systems, if a sensor goes bad, a CEL will be set, and so you will have a fault that can be followed. If not CEL is set, then you may have an actuator that is not working correctly, or a secondary actuator that is broken/intermittent. (eg turbo is actuated by vacuum, which the ECU cannot see, compared with the electrical modulating valve, which it can "see" and run Built in test (BIT) to report a fault.

    Generically, and excuse me for stating what may already be obvious to you, you are either misleading the controller (eg leaky system giving different environment to that which the controller perceives) or interfering with the controls (eg stopping the controller from achieving what it thinks it is doing). And to stop the CEL being lit, these errors are beyond the electrical interface for the controller.
    But another less obvious input is the communication from one sub-system to another; if the gearbox controller reports something, it may cause the engine controller to change its "attitude". But this would also have to be a non-fault condition, and somehow intermittent, so is most unlikely.
    #12
  14. mattjarv
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    mattjarv Member

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    [Nov 21, 2013]
    I'm taking the car into Audi tomorrow who have agreed that if what they think it is, isn't, they wont charge me for it and will keep looking unti they find out what it is! I was worried they would change something and then charge me anyway! Will let you know the outcome!
    #13
  15. Wrath
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    Wrath Well-Known Member

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    [Nov 21, 2013]
    Unlikely but I did get juddering as if the car was going to stall at med-high revs and it was the fuel filter.
    #14
  16. mattjarv
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    mattjarv Member

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    [Dec 19, 2013]
    In the end it was the ECU that didn't marry up with the remapping I had done at Top Gear in St Albans. They said they did a bad job and I got it re-done (somewhere else) and now it's perfect. Think it messed with the boost pipe as well.
    #15
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