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912 ULS Weak Cyinder

in 912 / 914 Technical Questions 8 years ago
I hate to even post this, but I have to start somewhere. This is a 100hp engine in my RV12 First, I noticed that my oil was extremely dirty after only 25 hours on it. Total time of the engine is 152 hours, and each oil change has found very clean oil until this one. The oil seems loaded with carbon. Pulling the engine through with a totally cold engine exposed 3 nice compressions, and one soggy one. The next step will be a compression test to locate the weak cylinder, but then it will be exploratory from there. My guess is some broken rings are a bad piston. Does anyone have any experience with the above symptoms? I think that I am going to really need some help, and support with this one.

Thanks in advance.................Tom

  • Re: 912 ULS Weak Cyinder

    by » 8 years ago

    Do you run 100LL with out TCP? Maybe it is just a stuck valve.
      
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  • Re: 912 ULS Weak Cyinder

    by » 8 years ago

    Hi Thomas,
    I'm very familiar with the RV12 and service several of them on a regular basis.
    You may be jumping to conclusions here and thinking something is wrong when it isn't.

    Is this your first Rotax 912 and have you been to Rotax school (Service and Line Maint)?.

    The black color in oil is from burning fuel and yes it has carbon. Most things that burn produce carbon. There is no time specification at which oil should turn black. It could be black in just a couple of hours. It could be because you left a little more older oil in at an oil change to contaminate the new oil. It could be because you are running to wide a gap on the plugs causing incomplete burning and cold running conditions. It could be from running too low an rpm at cruise and long idle times creating more carbon and colder running conditions. Engine temps during its oil cycle will definitely play a part in this color change. As your engine gets more time on it it will have more internal carbon/contaminants as they build up. It's the oil's job to help remove some of these and suspend them until you change oil. The other thing will that will make some slight difference is how long between oil changes.

    The color of oil has nothing to do with its lubricity. This is why some fleet centers for automotive vehicles send samples in to have them tested. There is no other way to tell. You can't look at oil and just say well at 5000 miles it looks good and at 7000 miles it looks bad so it must be bad.


    If these two items that you brought up are your only two symptoms then I'd bet money that your engine is fine since you aren't having any other symptoms like mag drop issues and instrumentation changes.

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

      
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  • Re: 912 ULS Weak Cyinder

    by » 8 years ago

    Thanks Roger. This is my first 912, and I did take the first 2 engine classes at Lockwood. I've ordered a couple of oil analysis kits, and plan on performing a compression test.
    Also, I always run ethanol free 93 octane auto fuel, with the exception of one tank of 100LL while on a longer cross country.
    I'll continue to post my findings........Tom
      
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  • Re: 912 ULS Weak Cyinder

    by » 8 years ago

    Always use ethanol free 93 octane auto fuel, except for 1 tank of 100LL while on a trip
    Plus, I don't think that a stuck valve would account for this very black oil in just 25 hours using Aeroshell Sport exclusively..........Tom
      
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  • Re: 912 ULS Weak Cyinder

    by » 8 years ago

    Black oil in 25 hrs. is perfectly normal.

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

      
    Report
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