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Hi - I desperately need some advice please.

I bought a Zodiac fitted with a 2003 912ULS with just over 500 hours on the engine a couple of months back.

The main symptom of the problem is when you turn over the prop by hand (when you gurgle to check oil), the prop just turns too easily like there is low compression. It does not "kick" like normal 912ULS engines when you hand turn it over where you clearly feel the compression cycles build up with increased resistance and then accelerate as the compression peaks and then declines before the next "kick" over (apologies for crude description!).

I've check the compressions on each cylinder a few times per the instructions with engine warm, all good per manual. Checked the blow-bys and also seem normal by manual's specs.

The plane stood unused for 4 years and it seems the problems started after that when I bought it (according to the old owner but without witnessing that I can't be 100% sure). The engine seized at 30 hours after an oil pipe broker and was rebuilt, not by approved Rotax agent however.

The only other possible symptom I can give is it seems to use quite a lot of oil (over 100ml / hour as best I can tell), although I've done very few hours in it trying to resolve this issue.

Help!
  • Re: 912ULS Compression Issue

    by » 10 months ago


    I had a high oil burn that was due to stuck oil control rings. The compressions measured OK. Replaced the rings and the oil consumption increased until they bedded in. Looking at the old rings is was obvious that they were worn from the groves made by the spings in the back. Only way of discovering this is to remove the cylinders and take a look. It's not too difficult to do.

  • Re: 912ULS Compression Issue

    by » 10 months ago


    Thanks for your response. Did you notice any change in the "pressure" felt while turning the engine over by hand on the prop before/after your replacement?

  • Re: 912ULS Compression Issue

    by » 10 months ago


    Yes, before I had the oil burn problem the engine had good compression when pulling the prop through by hand. I cannot remember what it was like after the oil burn started but I think that it was not as good. It happened gradually over a period of time and so it was more difficult to detect. After the rings were replaced it definitely didn't feel as tight at TDC. To be honest, now that the oil burn has stopped it still doesn't feel as tight as it used to but the leak-down pressures are all very good.

  • Re: 912ULS Compression Issue

    by » 9 months ago


    Hi - just an update - turns out the original blow-by tests were completely wrong (likely an issue with the testing equipment) and when re-tested the blow-by were well out of spec. Opening the engine showed a number of problems including seized rings on the pistons. Lesson learned is the blow-by test on a 912ULS is a critical inspection and make sure you have the history of test results as a comparison!

    Thank you said by: RotaxOwner Admin

  • Re: 912ULS Compression Issue

    by » 2 weeks ago


    You could also use a bore scope to look for marks on the cylinder walls.  A negative inspection, however, probably doesn't help ensure the rings are OK.  A positive one would confirm the need to do something.

    During the annual CI on our RV-12 with a 2016 200-hour Rotax 912 ULS, Cylinder #2 had rapid leak-down to near nothing, all of the air leaking into the crankcase.  The engine was not as hot as I would have wished, but the other three cylinders had no leak down at all.

    The air leakage past the rings was a big surprise to me. Oil consumption and engine function had been quite normal.  I sought a lot of advice, and in the end decided to go with the diagnosis most gave of the ring gaps being lined up in that cylinder.  The leakage went to zero after I flew it for an hour and redid the test, and as many on this forum suggested, "promised",  I saw no pressure leak-down at all. 


    Thank you said by: RotaxOwner Admin

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