What can be the consequences of turning over/ starting a 912ULS engine with oil present in the compression chambers (hydraulic lock)? I have an engine that has been started in this way that now seems to be presenting intermittent power losses at rpms from around 4500rpm to max rpm. I am trying to deduce if this is due to mechanical damage sustained as a result of hydraulic lock, or coincidentally a totally different issue for example fuel starvation. All other t's and p's are normal and the engine runs smooth and in balance.

Just for background information the engine has a high mounted oil tank that sits above the crankcase and a non-standard oil system installation that is prone to allowing oil to siphon into the crankcase and past the pistons. The owner wasn't aware of this characteristic. He had gurgled the tank pre-start but was not aware of the need to remove the lower plugs to allow pooled oil to drain from the chambers if the engine had been standing for a long enough period.


  • Re: 912ULS hydraulic lock

    by » 11 months ago

    I would think the crankshaft distortion inspection needs to be done if the engine experienced a hydraulic lock. It is made up of multiple pieces press fit together and will twist if the maximum torque is exceeded. See this video.


  • Re: 912ULS hydraulic lock

    by » 11 months ago

    If he was able to pull the cylinders through by hand to "Burp" The Oil Tank, and then subsequently start the engine with the starter, then this is not a Hydraulic lock condition.

    The Rotax starter is not known for being over-spec'd.  It gets the job done, but just barely.  It is unlikely it has the power to even come close to bending a connecting rod.

    If he engaged the starter and was not able to get the engine to turn without removing plugs to drain Oil; that would have been a Hydraulic Lock condition.

    Pulling the prop through for at least one full turn before starting verifies that it is not locked up.

    A small cloud of oil smoke due to your nonstandard installation is mostly harmless and impresses the little folks on the other side of the fence.

    Any damage done would have been noticed immediately and would be persistent.

    Intermittent power loss at high RPMs is a different issue.

    The Fuel starvation theory is worth pursuing.


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

  • Re: 912ULS hydraulic lock

    by » 11 months ago

    Hi there

    Prior to engine start the owner said that he pulled the prop through and gurgled the tank. He is a student but he is used to doing this task so I believe the tank was indeed burped. The engine was then turned over on the starter motor alone (with ignitions off) but would not turn over normally. It would attempt to turn for maybe a few blades but would then stop. The assumption in the heat of the moment was that the battery was weak so start up was attempted with the ignitions. The engine fired first time but a significant amount of oil was seen to fly out of the exhaust so the engine was shut down again. On inspection a lot of oil had pooled in the exhaust and also in the exhaust chamber (after the valve) of at least one of the cylinder heads.

    Subsequent runs have been carried out and, provided the lower plugs have been removed to drain any pooled oil and the oil tank burped all operations have functioned normally save for this high rpm power loss. 

    I am thinking that there was indeed some oil pooled in the compression chambers before that first start up that hadn't been cleared by burping the oil tank. This made it harder for the engine to turn over on the starter alone but when it was fired up this residual oil was instantly flushed out through the exhaust. 

    The student would not then have been sensitive to any higher resistance felt whilst turning the prop through to burp the tank because he had never experienced any issues before this event.

    When people talk of hydraulic lock is this the engine going 'solid' and not being able to be turned through at all? Perhaps this engine was close to being locked up but got away with it? 

    Thanks for your thoughts

  • Re: 912ULS hydraulic lock

    by » 11 months ago

    On a related note is it possible to achieve an oil tank 'burp' before all the oil has been pumped from the crankcase to the tank if air is trapped in the oil circuit on the positive pressure side after the oil pump? In this situation an unsuspecting operator would be led to believe that all oil has been pumped through to the tank but in actual fact a residual amount is still 'hiding' in the crankcase (Ie, an incomplete 'burp').


    This installation is as follows; 

    Oil is fed from a high mounted oil tank direct to the pump. Oil then passes direct from the pump to the filter via a Mocal oil sandwich plate fitted with a thermostat. When the oil temp rises above 80 degrees C the thermostat opens on the sandwich plate preventing flow direct across the plate to filter and instead forces oil up into a high mounted and upside down oil cooler. The oil cooler is effectively installed in a parallel circuit.  It is possible for air to be trapped in the high mounted upside down cooler if air is not fully purged from the system following an oil change. Not until the engine is run at temps above 80 deg C will the full oil flow pass through the cooler and fully purge the system. 

  • Re: 912ULS hydraulic lock

    by » 11 months ago

     Check out the Rotax 912 install manual. Bringing the installation within the Rotax installation guidelines may help resolve the issues you described.


    Or download the latest installation manual from Rotax.


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