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    Re: BMW X5 / Mixture Lean Archived Message

    Posted by Mark Carter on May 15, 2010, 9:21 am, in reply to "Re: BMW X5 / Mixture Lean"

    Hi Ian, no trick questions here,Scopes are a very important tool in the modern workshop, but as technicians we need to understand what is generating the voltage trace across the screen.

    In your case, and without teaching you too suck eggs, we need to understand that the speed of a petrol engine is controlled by the throttling effect generated by the throttle butterfly, and therefore at idle we have a large pressure differential on either side of the butterfly.(it is this that gives us our MAP reading and the vacuum for our servo)

    Now, when you plant your pedal into the shag pile the first thing to happen is a pressure equalisation phase within the manifold and this can be seen as the first voltage peak on your graph (you could plot this against a MAP output if you wanted to). The down turn at the end of the first peak is when the pressure is equal(ish).

    The next stage is where the engine speed picks up and therefore uses more air (WOT)

    The last stage (the bit you are questioning) is when you come off the loud pedal.

    Now, more questions,
    1) do we have direct control of the loud pedal nowadays?
    2) if not, what does and how fast does it close the butterfly?
    3) could you scope the accelerator output, the butterfly feedback output and the AMM output and look for a relationship?

    At the end of all that we need to ask how does the ECU react to this input in a trailing throttle condition?

    I hope I have not bored you but I find that the more I know how something works, the more I understand how that part fits in with the bigger picture.

    Regards Mark.

    PS. once we understand all of that, we can start to look at the important parts of the signal and think of the ECU's responce to those inputs.


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