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Fuel in oil

Category: 912 / 914 Technical Questions one month ago

Hello everybody,

I recently did a pressure check after disconnecting fuel lines from carb (912ULS) using electric pump. The fuel pressure (engine not running) was right on upper limit. (No leaks) left overnight. 
anyway then went flying next day.

After 45 minute flight I found 25ml of fuel/oil (mostly fuel) mix in the oil tank vent (I catch it in a little cup) Emptied the cup. 

I flew 30 minutes return trip and there was nothing in cup on my return. 
Possibility? When I did pressure test, fuel flowed into intake manifold and cylinders then overnight drained into sump then was “burped” into oil tank on pre flight and then boiled off into vent, catching in cup. All was evaporated out so none on second trip.

does this sound reasonable? If so, oil should be ok with such a small amount of fuel?

I am getting the oil tested for fuel rather than change oil (which was just changed)

any thoughts appreciated.

  • Re: Fuel in oil

    by » one month ago


    That sounds quite reasonable.  The Oil Should be just fine.

    When the oil Approached 200°F, the Fuel would be boiled out of the oil just like a liquor-still boils the alcohol out of water.

    I am surprised the fuel would recondense and was not just blown to the wind. 

    Are you sure it wasn't water?  Did it smell like fuel?


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Bill.Hertzel@Yahoo.com
    Clicking the "Thank You" is Always Appreciated.


    Thank you said by: Neil Cooper

  • Re: Fuel in oil

    by » one month ago


    thanks Bill,
    It catches in a little cup inside the cowl which doesn’t get a lot of airflow. Three of us felt and smelt it and it was definitely fuel. 


  • Re: Fuel in oil

    by » 2 weeks ago


    Old or contaminated fuel can get into the oil but most likely the fuel pump is leaking into the engine via the gearbox. Just fit a new pump.


    Thank you said by: Neil Cooper

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