Dear fellow pilots and Rotax experts,

I am reaching out because I have a strange problem, which is becoming very difficult to diagnose. I have an X-air ultralight with Rotax 582 engine.  It has two fuel tanks that are mounted low. I have two fuel pumps, a Mikuni mechanical pump and an electric pump. 

My trouble started over a month ago when I lost fuel pressure mid flight and had to do an emergency landing. Luckily the engine was sputtering but did not quit fully and I just about made it to the airfield. I first thought I had experienced carb ice and after cleaning the filters and carb jets I went for another flight. This time again around the 15 minutes mark I lost fuel pressure again. On this occasion I had to do an emergency landing with little to no power from the engine. 

As I have had this issue twice, I was not going to take further risk. I therefore changed the fuel pipes from the tank to the fuel pumps + all new clips + new Mikuni pulse pump and new electric pump + two new filters (Glass ones with nylon filter - no paper). I then tied the plane to the ground and filled the plane with 25 litres of fresh fuel. I kept the engine running for just under an hour with approx 5000 rpm. I could not throttle to the cruise speed due to the fear that it might damage the airframe. I had no loss of power and felt that the problem might have been solved. I felt it must have been the old fuel pumps.

Two days ago I flew the aircraft for a permit renewal. I had no issue flying to and back from the nearby airfield, which is about a 5 minutes flight. 

I left the aircraft for about an hour and thought to fly again for local flight just to test the system again. Around 13 minutes mark I could see that I was losing the fuel pressure again despite the fact that I had the electric pump on. I rushed back to the airfield and made it just in time. 

The interesting thing is that on all three occasions, the fuel pressure returns to normal as soon as I land. it is almost like there never were any issues with the system.

I have checked everything and cannot find a way to replicate this on the ground, hence I am reaching out to the community for some help. I am hoping someone could guide me. I have spoken to local pilots and everyone has run out of ideas including local mechanics.

Please share any thoughts on what this could be. Your help would  be much appreciated. If solved I would like to post a solution on the forum so that someone else in the future can benefit.

  • Re: Rotax 582 low fuel pressure after 10 to 15 minutes in flight

    by » 11 months ago

    Diagnosing this issue solely through the forum is not a straightforward task. Do you have any photos, videos, or diagrams of your fuel system?

    Regarding the process of switching between fuel tanks, could you provide more information? There is a chance that there might be an intermittent blockage occurring at the pickup tube in your fuel tank. Additionally, I'm curious about the proximity of your fuel tanks to the Facent fuel pump.


  • Re: Rotax 582 low fuel pressure after 10 to 15 minutes in flight

    by » 11 months ago

    Thank you very much for the above message. I believe I have found out the cause of my trouble. I will post conclusive results once it has has been verified.

    I believe the issue is to do with vapour lock in the fuel system. I am currently using E5 mogas on the Rotax 582 engine. 

    Could you please advise if a fuel return line can be introduced for the Rotax 582 fuel system. I have found extensive information for the Rotax 9 series but have not found anywhere information about the 582 engine. 

    Kindly provide some guidance. Thank you. 

  • Re: Rotax 582 low fuel pressure after 10 to 15 minutes in flight

    by » 11 months ago

    A return fuel line is not permitted for the Rotax 2-stroke engine. While we haven't received reports of vapor occurrence in Rotax 2-stroke powered aircraft, it doesn't necessarily mean it's impossible.

    To investigate further, one approach is to run 100LL fuel and observe if the problem persists. Compared to automotive fuel, 100LL fuel has lower vapor pressures and is less prone to vapor lock. Operating a Rotax 2-stroke engine with 100LL fuel for a short period of time should not cause any issues.

    If you determine that vapor lock is indeed the problem, it is crucial to maintain a cool fuel system and to use the appropriate fuel type. The specific methods for achieving this will depend on factors such as the type of fuel lines, fuel system design, operating environment, aircraft type you have, and fuel that is available in your area. 

    Regarding the Rotax 900 series 4-stroke engine, it's important to note that having a return line is not a definitive solution for preventing vapor lock. There are several other measures that need to be taken into account. The primary purpose of the return line in the fuel system of the Rotax 900 series 4-stroke engines is to aid in priming the mechanical pump and fuel system. However, it's worth noting that vapor lock can still occur even with a fuel return line in place.

    If you're interested in learning more about vapor lock specifically in the context of the Rotax 900 series 4-stroke engines, I would recommend referring to the article provided. It's essential to keep in mind that this information specifically applies to Rotax 4-stroke engines and may not necessarily be applicable to Rotax 2-stroke engines.



  • Re: Rotax 582 low fuel pressure after 10 to 15 minutes in flight

    by » 11 months ago


    Many thanks for your further help. Can you please advise why the fuel return line is not permitted on the Rotax 582? It seems to be a viable solution to maintain safety.

    I have also found out that I am not alone in terms of my aircraft with 582 affected experiencing vapour lock. I have included a link to an accident report of the same aircraft as mine with 582. The investigator suggested the cause was vapour lock. 

    The mikuni pump seems to be well documented to experience a pneumatic lock once the vapour is produced. This means unless there was considerable pressure difference the pump will stop the fuel delivery, creating a very dangerous situation.

    Please see this accident report


    The video of the accident is also available online. I have the exact same aircraft.


    The pilot in the video got very little warning. I  too have experienced the vapour lock three times in similar situation. On the first occasion I thought it was carb ice but after the subsequent two failures I am confident that I am having problems due to the vapour lock.

    you have suggested to keep the fuel lines cool. The installation is far away from the exhaust. The ambient temperature is all that is affecting the E5 fuel. 

    Although not directly relevant, I have found countless automotive videos for vapour lock online. They all seem to be either fitting a filtre with the a fuel return line connection or a fuel return line with a restrictor in place.

    My concern is that overcoming vapour lock may not be possible during the flight. There has to be a solution applied to prevent the vapour lock in the first place. The only feasible solutions I have come across is aux pump or lowering the altitude along with return fuel line.

    I feel a fuel return line like with a good restrictor will at least allow the vapours to be flushed out. Why then is this not considered for Rotax 582. Can you please shed some light on this? 

    Does it mean that Rotax has not considered any real solutions to prevent the problem in the first place? Vas majority of Rotax 582 users are using E5 fuel in the UK.

    It seems to be problem mentioned and experienced by too many pilots. I have personally come across three pilots in the last two weeks with the same issue. One of whom had to land away from the airfield.

    I look forward to your reply. Thank you.



  • Re: Rotax 582 low fuel pressure after 10 to 15 minutes in flight

    by » 11 months ago

    If you are using ultralight style fuel shutoff valves, check them for leakage. I had a similar problem with an aircraft using a Rotax 618. I found that the fuel valve was leaking air. The valve did not leak fuel, just air. If you are running E85 fuel you are exposing your fuel system to very high levels of ethanol which will deteriorate the seals in the valves. My theory was that the fuel pump was pulling air past the valve and reducing fuel pressure. I rebuilt the fuel system with certified aircraft parts and never had the problem again. I don't use ethanol fuel now. The Rotax 618 needed at least 91 octane and that was the reason I was using ethanol fuel.

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