Buy Yamaha Outboard Parts


No announcement yet.

F100 fuel in oil

  • Filter
  • Time
  • Show
Clear All
new posts

  • F100 fuel in oil

    When i changed the crankcase oil on the last two occasions, the old oil contained fuel. The motor seemed to be firing on a few cylinders. When oil was changed the motor ran great for the first two hours and then starting missing again.

    Can anyone explain this?

  • #2
    Change your mechanical (diaphragm) fuel pump. It's bad


    • #3
      fuel in oil

      Check the thermostat. It is stuck open. Motor is running too cool not burning the fuel. Not the Fuel pump. It is driven off the camshaft and a ripped diaphram will not cause that issue.


      • #4
        I have a 2003 F115. even thought the fuel pump is camshaft driven it still has to be connected to the crankcase somehow I think. So my question is what prevents fuel from entering the crank case if the diaphragm is bad??? If it even is truly connected??? it appeared to be connected in the diagram i looked at.


        • #5
          if the diaphram is bad it will dump fuel into the crankcase.
          if the T stat is hung open it runs cold and the excess fuel can get past the rings and contaminate the oil.
          we call it "making oil".


          • #6
            I did not know that about the t-stats..that is a good piece of info to have.


            • #7
              would be interested to know what fixed your problem in the end? i have had similar problem with engines making oil. i have changed plug type, engine oil type and thermostats in an effort to sort it.


              • #8
                overpreopping or lugging will make them make oil as well.
                worse on EFI motors.
                engine must be propped to run at the top of its RPM range with the load thats on it that day.
                yesterday with no load isnt worth much.