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  1. #1
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    HELP - Can't pass smog - High HC at Idle

    Hi,

    I've got a 85 535 and this year I can't seem to pass smog for the life of me (took it in 3 times now). The reason - HC is measuring 500+ at idle (needs to be 200 Max). I do know that as soon as it comes off idle HC drop to 50ppm. They won't tell me the other gasses unless I go to an "approved" repair shop and pay $75 for an "inspection". Other than this the car runs & idles fine (~750rpm) and pulls strong across the entire RPM range.

    I seem to have tried everything in the Bentley book and searched online forums and articles on the topic.

    Specifically:
    *Performed valve adjustment
    *Measured fuel pressure with or w/o idle hose - in spec.
    *Replaced spark plugs, cap rotor & wires
    *Pulled the fuel rail and the cold start injector - works fine and no drips under pressure with fuel pump running
    *Measured voltage coming from the temp sensor - in spec (cold & hot engine)
    *Checked for vacuum leaks - none found and pulling dipstick drops engine RPM at idle
    *Checked for binding/stuck AFM flap - all good there
    *Checked 02 sensor - all good (don't know if this is measured at idle anyway, but RPM did drop when I disconnected it completely)
    *Checked purge valve & hoses
    *Changed oil & filter (getting desperate now)


    It seems that there's something that's telling the ECU to pour in extra fuel but only at idle and I can't figure out what it is. I don't have another ECU to swap in to see if it's ECU itself.

    If you have any ideas, or hunches please send them my way.

    Thank you in advance,

  2. #2
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    Re: HELP - Can't pass smog - High HC at Idle

    Is the engine getting up to proper temp? middle or a little higher on a properly working gauge setup.
    The most common reason for a high HC at idle is becasue of the air box (a.k.a Mass air flow meter) There is an adjustment from out side the box, its under the box down a long hole/tube where there is a allen head adjustment screw. Or the box is "bad"

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    Re: HELP - Can't pass smog - High HC at Idle

    Yes, the engine is coming to mid-gauge temp, especially when idle for a few minutes. The meter's CO screw is about 2 turns away from being full out. I don't have another meter to swap in, at the moment either, but was thinking of getting one anyway.

    I also forgot to mention before that the air air filter has been swapped.

  4. #4
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    Re: HELP - Can't pass smog - High HC at Idle

    Well the adjustment does not work in the way you would think the slightest turn can make a huge difference. I don't remember what way is lean and what way is rich from memory. I have an old rebuilt aix box with a sticker on it telling me what way is rich and what way is lean for reference.

  5. #5
    Ray7er6ers5er3er
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    Right is lean when looking into the hole. On the

    notched wheel under the black cover, you turn the wheel right for rich, left for lean...the exact opposite. From turned all the way closed...the the allen screw factory setting is 2 1/2 turns open.

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    Re: Right is lean when looking into the hole. On the

    Messing with the wheel is not going to make a difference.

  7. #7
    TJ
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    tried loosening that CO screw?

    Although you have only two turns of adjustment left for leaning it out, you might be surprised how sensitive that adjustment is. On my 3er I recently set the idle mixture using a voltmeter connected to a 2nd O2 sensor in the exhaust. The difference between a reading in the proper range and one that was off the scale was only a half-turn of the screw.

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    PASSED, after adjusting AFM!

    Thank you all for your advice. I finally opened the AFM and moved the wheel a little clockwise (i.e. harder to open and leaner) to the point where the engine started to stumble. I then moved it a few clicks back, so it'd idle normally again and left it. Still leaves me curious how it passed last year without needing this, but I'm not going to lose sleep over it.

    Again, thank you all.

  9. #9
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    Re: PASSED, after adjusting AFM!

    Would have passed if you just used the little adjustment. Its way to much to explain why and even if I did EVERYBODY here would attack me anyways.

  10. #10
    Ray7er6ers5er3er
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    Congrats! Now iron out whats REALLY wrong

    with it. LOL

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