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Thread: K75 oil pressure feature??

  1. #1
    Rally Rat paulsibek's Avatar
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    Question K75 oil pressure feature??

    I changed my oil the other day and couldn't get the bike to start. It would momentarily run and turn off.

    Somewhat baffled I noticed that I forgot to replace the oil filler cap.

    Once I replaced it the bike started and ran normally.

    Do I presume that this is an undocumented feature? If it is it's nice to know that there is oil pressure protection.

  2. #2
    Registered User kioolt's Avatar
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    My 91 K100LT will start with the oil filler cap off but it does run poorly. I have never been able to figure out why. Maybe someone can enlighten us.
    2004 R1150RT 145,700 miles
    1991 K100LT 126,700 miles
    1982 R100RT 106,900 miles
    Total 379,300 BMW miles

  3. #3
    A bozo on the bus deilenberger's Avatar
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    Originally posted by kioolt
    My 91 K100LT will start with the oil filler cap off but it does run poorly. I have never been able to figure out why. Maybe someone can enlighten us.
    Sure - happy to.

    The oil fill cap being off is the same as having a large hole in your intake between the throttle-bodies and the intake valves.

    It makes the engine run super lean, so it's difficult to impossible to start and runs very poorly.

    The crankcase normally vents into the intake plenum after the air-flow-meter (AFM) in the air-filter housing. The AFM tells the FI brain how much fuel to inject. If you have the filler cap off - you're venting directly into the plenum bypassing the AFM - so the bike doesn't know that extra air is getting into the engine and it runs lean.

    Anyway - it's not exactly a feature - it's a consequence of normal engine design. You'll find the same thing with lots of FI car engines, although they can compensate somewhat using the O2 sensor to richen the mixture back up.

    Oh - Paul - having the cap off doesn't effect oil PRESSURE at all.. that is strictly effected by the oil pump, oil in the sump and the engine clearances on lubricated components. Your oil pressure light will stay on for about 10 seconds after an oil change while the oil pump fills the new oil filter you installed. That's normal.

    Best,
    Don Eilenberger http://www.eilenberger.net
    Spring Lk Heights NJ NJ Shore BMW Riders New Sweden BMW Riders
    '07 R1200R (current ride) and some bimmers.. and a Porsche

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