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Had two instances where it was very hot, I went to full power and after a while my 912ULS engine started missing. I had low fuel pressure about 1.7 PSI so I turned on my aux fuel pump and the fuel pressure went to 2.2 and cleared it up immediately. My fuel pressure stayed at 2.2 (or so) as I cautiously flew back to the airport. Got over the runway, looked down and my pressure was back up to 4 to 5 PSI as it normally is.
This was an intermittent problem happening twice in a 10 hour period. Hot day full power, low fuel pressure. Otherwise everything is running great. I cleaned out my gascolator/filter and nothing has happened in the last 10 hours and flying in the same conditions. It does not make sense that a clogged filter would cause this because it ran fine MOST of the time.
Running autogas I suspect it is vapor lock in the hot conditions. I am now running 10 degrees cooler with changing oils and coolant assist.
Is this a case of vapor lock? Any other similar situations?
  • Re: Engine missing low fuel P must be vapor lock?

    by » 8 years ago


    Hi Paul

    That does sound stressful not knowing what it is . I have head of a guy near us having a few intermittent engine outs with a rebuilt 912 ul it would be ok one day and engine stop or loss of power the next day . His fault ended up being the mechanical rotax fuel pump . You could check the age of your pump first they have a life of 5 year and had some bad batches over the years the latest one is alloy all over . Do you have the .33 mm fuel bleed back to the fuel tank as drawn out in the manual . If in any doubt don,t fly it sort the problem tie to a car on the ground not worth the risk .


    Cheers Daniel

  • Re: Engine missing low fuel P must be vapor lock?

    by » 8 years ago


    Hi Paul,

    It does sound like vapor lock and should at least be ruled in or out. Are you running an old fuel pump? If you are it's time to get a new one. Do you have a fuel re-circulation line? If not you really need one. Are your fuel hoses in fire sleeve? Are they routed away from all radiating heat sources? Some types of hose seem to absorb heat more readily than others, just consider this in the problem solving. You can try reducing heat with some header wrap cloth on the exhaust pipes. Start at the exhaust outlet port on the head and wrap down and around the knuckles at the muffler. Do not cover the springs. Does you engine get enough air flow while flying?

    These are just a couple of ideas to consider in your diagnoses.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


  • Re: Engine missing low fuel P must be vapor lock?

    by » 8 years ago


    This is a 100 hour new 912S so the fuel pump is a new one. Since I changed oils and running coolent boost I am getting 10 degrees less CHT and oil temp. Additionally I cleaned out my gascolator filter.

    No problems now. I will say there is not allot of circulation in the engine compartment. My fuel lines to the pump are fire shielded but my lines going to the carbs are not.

    What I will do is get some of those temp measuring patches to see what I am getting.

    I was wondering if the symptoms of low fuel pressure/missing was typical of vapor lock. I was told that vapor lock can only happen after the engine is shut down and trying to restart it and that the low fuel pressure reading would not be an indication of vapor lock, both of which i question.

  • Re: Engine missing low fuel P must be vapor lock?

    by » 8 years ago


    I had low fuel pressure about 1.7 PSI so I turned on my aux fuel pump and the fuel pressure went to 2.2 and cleared it up immediately.


    I use to have similar problem in my Rans S12 XL. When I leave the airplane under the hot sun for around 2 or 3 hours and then go to fly I has engine missfire as if the engine was not getting fuel.

    Then I installed a Aux fuel pump and when I experience the same problem I turn on the aux and the problem goes away............ So I was sure I have Fuel Vapor Lock.

    After checking and checking, I decide to replace all hoses and install all again by the Rotax instrucction, I found out that my engine does not have a fuel return line to the fuel tank and thats why the vapors can not get out of the fuel system.

    Since I bought a used Rans its look like the guy that install the engine does not installed a fuel return line to the tank.

    So, exactly were you hook up your fuel presure sensor? Are you sure you are not using the top hose male outlet that is suppose for the fuel line return to tank to conect your fuel presure sensor?

    Check this rotax diagram were you see the fuel return line set up:

    http://forums.matronics.com/files/912_fuel_fig_54_941.jpg

    Here is the configuration for use the top hose male outlet for fuel return line or a double hose male outlet for use fuel return line and presure sensor/gauge

    http://forums.matronics.com/files/912_fuel_fig_55_249.jpg

    Hope this info help you,

    Isaac

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