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My 914 has developed MAP fluctuations at 115% throttle position. The MAP gauge fluctuates between 38.5 and 40. After the flight I put the computer on the TCU and with the engine not running and using the monitor function saw the setpoint airbox pressure was at 1370 ( full throttle) and 1500 ( idle).

In flight the airbox pressure is exceeding 1370 at WOT which may or may not be causing the wastegate to open and close. I don’t really understand how it all works and would appreciate some assistance.

  • Re: MAP fluctuations

    by » 4 weeks ago


    Hi Rick

    I don't know if you have a fixed or inflight adjustable however the TCU is a fitted with 7 sensors and it must match the data to an internal map.  You can see this described in a number of Rotax manuals. 

    At take off, any position on the throttle over 108 % of movement on the throttle position sensor, TPS (the one on the carb over cylinder number 4) it is going to closed the waste gate for max boost.  This has a limit internally and if you do not back off WOT then after 5 mins it will automatically open the waste gate to save the engine from detonation.  it may open earlier if the airbox temperature, another sensor, sees high values that can also lead to detonation.  Hot inlet air can also lead to detonation.  

    Be sure that your throttle has a throttle position indication at the 100% value of opening.  When you have finished your take off climb always pull the throttle back to that position or lower.  You can see the reason why in the internal map program, the difference from maximum continuous to WOT is very dramatic and should only be used for take off.  

    It is possible you may have some contamination on the waste gate or not adjusted correctly on the servo control cable.  Sort these out from the manuals. 

    Cheers


  • Re: MAP fluctuations

    by » 4 weeks ago


    Rick

    This is the chart from the operators manual, current issue.  Note the almost vertical climb at 108% and up setting.  This is to be used for take-off only.  

    Cheers


  • Re: MAP fluctuations

    by » 4 weeks ago


    sorry, file did not attach.  here you go. 

    39525_2_TPS map 914.jpg (You do not have access to download this file.)

  • Re: MAP fluctuations

    by » 4 weeks ago


    Hi Rick,

    As RW mentioned, have a look at the waste gate and cable as per the maintenance manuals. I’d also check the electrical connector at/near the servo motor, maybe disconnect it and apply some electrical contact cleaner then reconnect. If there are other electrical connectors you can identify as being associated with the TCU, clean them too. Attached is a page from the Line MM with a reference for waste gate lubrication. My thoughts are to clean, lube & adjust the easy stuff before looking for deeper problems.

    Cheers, Des.

    39531_2_IMG_2400.png (You do not have access to download this file.)

  • Re: MAP fluctuations

    by » 2 weeks ago


    Thanks for your help, I’ll keep investigating.

     


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