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  • Changing afr at idle

    Have installed split sec maf a while ago and have set afr over most of map but one thing that is puzzling me is the fact that I can set the idle at say around 13.5 and idles happily there. switch off car take it for short drive look again at meter and its changed to around 15.5 and ticking over real rough. any ideas?

    These things have done already:
    set idle speed ie short out all contacts on throttle switch and adjust on big screw
    coolant temp switch tests ok
    wiring from switch ok
    air temp switch measures good and wiring new
    tried disconnecting baro sensor
    another ecu (same results)

    Im thinking could it be faulty maf sensor or signal converter?

    :fishin: Help

  • #2
    Are you running with the stock O2 sensor connected? If so, try disconnecting the sensor from the DME. See if the idle AFR holds steady. If it does, retune the idle region for 14.7:1 and reconnect the O2 sensor.

    Initial calibration should always be done with the O2 sensor disconnected from the DME. You should adjust the idle AFR for 14.7:1. After tuning, reconnect the O2 sensor. In closed loop, the AFR should cycle around 14.7:1. If it doesn't the most likely cause is your O2 sensor.
    Mark Amarandos
    89 M3 w/MAF 4.0
    90 M3 DM w/MAF 4.50
    07 335i w/Turbo Tuner

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    • #3
      Yes stock 02 sensor connected, but have both black and blue connectors grounded using euro map in DME which believe ignores the sensors output? but as you say have tried disconnecting the sensor with same results.

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      • #4
        Just a little update in case anyone does a search in future. Used voltmeter and measured output of MAF which seemed to rise and fall quite a lot, disconnected it and fed 850mv with a voltage reg I made up into SS box, result a steady voltage reading into DME from SS box. So got me thinking about the airflow going into MAF why havent I fitted a short trumpet on entrance to sensor?? makes sense really you cannot expect air to enter nicely into MAF with it just bolted on. so off to supermarket and found a stainless steel cake ringey thing, 3 1/2" dia 2" deep tapped sharp end with small hammer and made a nice trumpet shape welded on four legs to bolt it inside the airbox, result steady idle mixture all the time 14.5, the only problem have found is the afrs have all changed across the whole map so will have to go through that all again but the readings are one hell of a lot steadier than before. one thing am wondering is could I use a larger 4" MAF?

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        • #5
          The way that you feed air into the MAF sensor is critical. When running a cone filter on the front of an MAF sensor I try to use at least 9" of straight tube between the filter and sensor. If you bolt the MAF to a stock airbox, the airbox does a decent job of smoothing out the air flow.

          You can see some examples of larger MAF sensors at www.splitsec.com. Go to Project Cars and the E30 M3 DM race car. You will see the MAF 4.0 used with a CF version of the stock plenum and the MAF 4.5 used with a CF plenum normally used on alpha-n setups. The MAF 4.5 was used to make 248 hp to the wheels.
          Mark Amarandos
          89 M3 w/MAF 4.0
          90 M3 DM w/MAF 4.50
          07 335i w/Turbo Tuner

          Comment


          • #6
            Quite impressive outputs from those cars eh? Do you think there would be any negatives if I used a 4" maf but moved it closer to the engine then used a metal 4" pipe with flanges welded on each end to connect to the airbox with short trumpet inside airbox as before. that way there would be a short piece of pipe before the maf sensor. Am running 284/276 shrick cams and was wondering if any pressure waves coming from engine could affect the sensor if it was moved closer?

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            • #7
              You can run the MAF 4.0 sensor bolted directly to the stock airbox. The MAF 4.0 sensor is almost the same diameter as the top of the airbox so there isn't much room to put a horn inside there. A short run of tubing between the airbox and the MAF sensor would help to smooth out the air flow. That tube can be no more than about 3 inches long. If it is any longer you won't be able to make the turn into the plenum.

              I don't think that the pulsations would be a problem if you only reposition the MAF by 2 or 3 inches.
              Mark Amarandos
              89 M3 w/MAF 4.0
              90 M3 DM w/MAF 4.50
              07 335i w/Turbo Tuner

              Comment

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