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Thread: 1991 BMW 850i V12

  1. #1
    Join Date
    Sep 2018

    Default 1991 BMW 850i V12


    I have resurrected an old BMW 850i 5.0 V12 automobile. Runs great now after replacing most of the fuel system components from inside tank to injectors, belts, tensioners, and secondary ignition components including caps, rotors, plugs, wires, crank and cylinder(cam)identification sensors.

    But, with the aftermarket wires and the included CIS sensors (cylinder identification pulse generator donuts on wires 6 and 12) engine would cut out occasionally at highway speeds. I could not determine the cause of this fault so I replaced wires 6 and 12 with the old OEM wires and sensors. These old wires and sensors corrected the problem with engine going into a 'limp home mode' so it runs fine now. I have not determined why the new aftermarket sensors are not working properly and mfr, Kingsborne, does not know what could be wrong with the wire set or sensors.

    I suspect that the low output OEM Bosch coils, 10k, could be the cause of issue. The OEM secondary ignition system requires low resistance wires and plugs to function properly probably because of the low output coils. I am currently using the recommended low resistance NGK plug BRK6EK which may be the only low R plug on the market for this engine application. This plug has two electrodes on it.

    I thought I would try the MSD SS Blaster coils and see if the higher output coils would improve the function of the aftermarket CIS donut sensors. I am also wondering if the aftermarket wires might have better insulation preventing the new sensors from detecting the pulse signal from the weak OEM coils to plugs. I installed two MSD SS Blaster Coils 8207 last weekend. Car runs fine with the new coils, probably a bit smoother, but I have not had a chance to replace OEM CIS sensors with the new aftermarket sensors to see if the new coils help. I have not re-gapped the plugs to see if I notice a slight mileage or performance improvement.

    Another reason I replaced the stock coils is because there is a much larger variety of plugs I can use with the engine. Would the iridium or platinum plugs be okay to use with the MSD coils? I think the stock gap is .032". The NGK iridium or platinum plugs come pre-gapped at .040 or .044 depending on type. I think I would prefer the better quality plugs since they should last longer and may also provide a smoother idle and acceleration. This engine is only 8.8:1.

    Last edited by JDP; 09-08-2018 at 02:05 PM.


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