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  1. #1
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    What can knock a knock sensor?

    Doing some tests with the VAG-COM. I'm trying to find reasons for some poor performance and misfires I have been suffering, along with poor fuel consumption (27MPG average).

    Anyway, I have monitored the voltage output of the knock sensors and found a strange phenomenon.

    When I let the car idle 'til it's warm (I know I shouldn't really but it was for a test), it ticks over nice and smooth and the knock sensor readings are all very similar. However, when I drive the car to warm it up and repeat the test, the knock sensor for cylinder 1 & 2 shows a voltage rise of approx. 0.4 to 0.5 volts, and the engine is not smooth - in fact it seems to 'chug'.

    If the sensor is faulty, surely the difference would be apparent all the time? Why should it change after driving the car? Could it be vibration?

    Question based on the above: Would noisy valve gear have an effect on the knock sensor readings? I know that one cause for knock retard problems can be loose ancillaries, so do valves count on this score?

    Thanks in advance all you tech-heads [img]/ubbthreads/images/graemlins/wink.gif[/img]


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  3. #2
    Mo-S3's Avatar
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    Re: What can knock a knock sensor?

    Erm, i dont know too much about these knock sensors but from what your said about loose ancilleries its very concievable that it is the case. After all the knock sensor i believe is specically designed to monitor 'knock' or noise in the engine in that region but if there is external noise from other parts around it that its not meant to monitor and it hears them then of coarse its gonna take action. Whether its right or wrong it wont know.
    Have you done a search on the net for 'what is a knock sensor' ?
    Oh and what about this Lamda probe sensor cleaning thing?? Have you checked yours ?? I know for a fact that if you have a dodgy one it really fecks your fuel consumption badly. I know i have 2 sensors on my S3, on yours.... im not sure. Some one with ETKA caould confirm that for you.
    Anyway thats my input. Keep us posted.

  4. #3
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    Re: What can knock a knock sensor?

    [ QUOTE ]
    MO-S3 said:

    Oh and what about this Lamda probe sensor cleaning thing?? Have you checked yours ??

    [/ QUOTE ]

    Doing it at the weekend if I can source a tool to remove it.

    Some info about knock sensors (and other ignition parts, quite interesting) here:
    Forparts
    And here:
    Knock Retard

    Good old VAG-COM, I'm learning loads. Suddenly it doesn't seem such a daunting task to do my own servicing.

    [img]/ubbthreads/images/graemlins/groovy.gif[/img]

  5. #4
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    Re: What can knock a knock sensor?

    I'm tempted to buy the VAG-COM for my A3, as long as it doesn't turn me into a hypochondriac [img]/ubbthreads/images/graemlins/grin.gif[/img]

  6. #5
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    Re: What can knock a knock sensor?

    Khufu - if you want one - let me know!

    http://www.vag-com.co.uk/products.htm

    Best wishes,

    Ram

  7. #6
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    Re: What can knock a knock sensor?

    Knock is basically any noise at all.
    Yes noisy valve gear ( if very noisy ) could pick up on the knock sensor.
    Hitting a metallic bit of the engine with a spanner ( not too hard! ) will do the same.

    Background noise (like noisy valve gear etc) is one of the biggest problems with knock sensors - it can hurt performance of the car for no good reason.

    You are on the right track logging whats going on but a higher knock reading should not cause misfires. 27 mpg isnt too bad from what I read of other peoples' averages.

    Log ignition advance, you might find that idling the ignition doesn't get advanced very far, but driving it and then idling means the ignition is further advanced, hence higher knock readings *maybe* not 100% on this.

    I think you might want to try a new MAF, lambda sensor..

    Let us know what happens. [img]/ubbthreads/images/graemlins/smile.gif[/img]

  8. #7
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    Re: What can knock a knock sensor?

    [ QUOTE ]
    Crafty said:
    <font color="orange">&gt;a higher knock reading should not cause misfires.
    </font>
    Well I haven't described that very well, really - it's more the lack of power and general uneveness that I'm moaning about
    <font color="orange">
    &gt;27 mpg isnt too bad from what I read of other peoples' averages.
    </font>
    I want more! My last A6 could do much better.
    <font color="orange">
    &gt;Log ignition advance, you might find that idling the ignition doesn't get advanced very far, but driving it and then idling means the ignition is further advanced, hence higher knock readings *maybe* not 100% on this.
    </font>
    I can't get any reading on advance/retard for any cylinder. Don't quite know why.
    <font color="orange">
    &gt;I think you might want to try a new MAF, lambda sensor..
    </font>
    I'm going to look at the Lambda, I'll have to save up for a MAF!!

    [img]/ubbthreads/images/graemlins/blush.gif[/img]



    [/ QUOTE ]

  9. #8
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    Re: What can knock a knock sensor?

    I have now re-tightened the knock sensor and the readings have improved slightly, but the bad running remains.

    I think I will have to bite the bullet and have the Motronic system fully checked.

    ...................... .......................

    [img]/ubbthreads/images/graemlins/frown.gif[/img]

  10. #9
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    Re: What can knock a knock sensor?

    Dont rule out the the Lamda sensors either !! They are crucial in the emissions and economy setting for the car.

  11. #10
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    Re: What can knock a knock sensor?

    A breakthrough today. It seems from my readings on the charge pressure that there could be one of these things wrong:

    1)Solenoid Valve for charge pressure control to -N75 defective
    2)Wiring to solenoid valve for charge pressure control defective
    3)Charge pressure regulating valve jammed in open position
    4)Leaks between turbocharger and intake manifold
    5)Turbocharger defective

    anyone wanna guess which one it's going to be?
    [img]/ubbthreads/images/graemlins/idea.gif[/img]

    I'll try to sort out a graph for you all to see.


  12. #11
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    Re: What can knock a knock sensor?

    This is a bit crap, but I'm tired and I have been working all night and haven't been to bed yet....etc...

    Basically, the chart shows four periods of accelaration at full throttle in 3rd from 2k revs to 3k revs.
    Where the spikes are (when I put my foot down!) the two values should be within 0.3ms of each other.
    Sorry, I'm not an excel user! Apologies also for the dodgy screen capture! [img]/ubbthreads/images/graemlins/blush2.gif[/img]

  13. #12
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    Re: What can knock a knock sensor?

    Here's some interesting stuff to help us understand charge pressure:

    The solenoid valve for charge pressure control N75 changes the opening time to atmospheric pressure according to the signals it receives from the engine control unit (duty cycle).

    Thus, a control pressure is produced by modulating the charge pressure and atmospheric pressure. This pressure acts on the pressure unit for the wastegate.

    The wastegate is kept closed in a depressurised state by a spring inside the pressure unit . The entire exhaust gas flow is routed via the turbine, and a charge pressure is built up.

    The control pressure counteracts this spring force and opens the wastegate. Part of the exhaust gas flow is fed from the wastegate past the turbine, and the charge pressure stops rising.




    If there is no flow, N75 is closed and the charge pressure acts directly on the pressure unit. The waste gate opens even if the charge pressure is low.

    If the charge pressure control fails, the charge pressure is thus limited to a "basic charge pressure" in order to prevent the maximum permissible charge pressure being exceeded. This results in a loss of performance.

    The "basic charge pressure" is the charge pressure (approx. 300 - 400 mbar) which is achieved without regulation (mechanical charge pressure).







  14. #13
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    Re: What can knock a knock sensor?

    boost leaks or a faulty amal (N75) valve don't often cause rough running mate. If you have a bit of a missfire check your leads or plugs as some of these may be breaking down under load.

  15. #14
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    Re: What can knock a knock sensor?

    Cheers, Dude.

    It does seem like a misfire in feel. The 'breathing' noise at low revs under load is sounding very 'spitty' though. That's what made me think; 'valve'.

    The car is just out of warranty, but I've been complaining of this problem since July of last year. Feeling like the dealer has made a bit of a c**t out of me, to be honest.

  16. #15
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    Re: What can knock a knock sensor?

    Seeing as you logged the problem with the dealer before, then I'd say that you should still be covered by warranty.. Worth trying.

  17. #16
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    Re: What can knock a knock sensor?

    [ QUOTE ]
    MisterD said:
    It does seem like a misfire in feel. The 'breathing' noise at low revs under load is sounding very 'spitty' though. That's what made me think; 'valve'.

    [/ QUOTE ]

    What sort of noise exactly mate? Is it a hissing noise? If you've got a leak somewhere after the MAF then this often causes poor low down driving characteristics (similar to the effects of fitting a cone filter directly to the maf). During warm up you'd more than likely notice a lumpy 'pick up'.

    Best thing to do at this stage is check all the hoses and make sure the plugs are ok.

    Another thought i've had - coil packs! Are your's the updated version? [img]/ubbthreads/images/graemlins/beerchug.gif[/img]

 

 

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