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Damaged valve seats with perfect compression tests

in 912 / 914 Technical Questions 13 years ago
After much troubling and time spent to identify the source of a big power loss (around 8 hp) after an overspeed (>6200 rpm) my rotax mechanic dismantled the engine heads and found ALL valve seats damaged , one burned exhaust valve and two hydraulic tappets in bad condition.

But, before of this disassembling, two leak down test (with cold and warm engine) showed perfect values (85-86 psi) and also old style compression test was well inside rotax limits. ( max cylindr. press 11.1 bar min 10.5 bar). I know that these tests can't identify tappet problems but, nevertheless, are perfect suited to valve problen diagnosis.

Moreover this perfect test values delayed problem identification because even rotax experts can't believe in a so damaged engine heads showing perfect compression.

It is common experience of anyone to have this kind of damage with normal compression tests or my it is a very unusual case?

Thanks


Nicola di Biase

  • Re: Damaged valve seats with perfect compression tests

    by » 13 years ago

    Hi Nicola,

    You would think that would have shown up on a differential compression test, but things being bent every where else doesn't mean a valve won't seal. At a 6200 rpm over speed I would have automatically pulled the heads and checked for bent rods and valves.

    p.s.
    How did your engine get to such an over speed rpm?

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

      
    Report

  • Re: Damaged valve seats with perfect compression tests

    by » 13 years ago

    Hi Nicola,

    You would think that would have shown up on a differential compression test, but things being bent every where else doesn't mean a valve won't seal. At a 6200 rpm over speed I would have automatically pulled the heads and checked for bent rods and valves.

    p.s.
    How did your engine get to such an over speed rpm?


    I know that with a so high value overspeed it is mandatory to dismantle the heads and inspect whole valvetrain.

    But problem is : witch is it the reason to use a leak down compression test on the rotax 91X if it do not shown any problems even in an engine with ALL valve seats damaged and a burned exaust valve ?

    Even spinning propeller by hand the feel is of riduced compression but, i repeat (and think it is very unusual) the leak down (done by TWO different Rotax auth mechanics) was perfect.

    i'd like to know from your experience if compression tests are a valid diagnostic tools or False negative fault indications are common.

    p.s. overspeed was done because in my plane Ivoprop propeller (with in flight variable pitch but no governor) was set at minimum pitch (landing/Go around phase) when a sudden traffic conflict in landing pattern (a trike on the base leg) forced me to an escape manouvre at high velocity and i had no time to change pitch. overspeed was in a 2-3 second range but >6200 rpm. At the next take off performance was very degraded (around 8-10 Hp less).

    thanks

    Nicola di Biase
      
    Report

  • Re: Damaged valve seats with perfect compression tests

    by » 13 years ago

    The differential compression test is only one tool to use in checking this problem. You may or may not have seen a leak show up. We try to use more than one tool or idea to solve and diagnose a problem and use the sum of all the checks to come to a focused conclusion. Some minor problems can be solved with only one diagnostic idea or tool and some require more.

    Think of it like a doctor checking a patient. If they just used one test or only ask the patient a few questions they would miss a lot of diagnosis, but they try to use several diagnostic tools and ideas to narrow down the possible issues with the patient. This is all we are doing.
    This is why we must have more than one idea or tool to use in checking our problems with our engines to narrow down the possibilities and rule out other possible issue.

    I don't have an in flight adjustable prop, but from the incident you described it would seem to me to set the prop up on a landing approach to a point that if an immediate go around happened again then the setting at WOT would be already set and you would have to even worry about making a change.

    Your incident could have happened to anyone and it was very unfortunate. Hopefully this posting may help someone else from the same situation.

    Leak down test are always a good tool, but just that a single tool. I would have been a little surprised just like you that at least something didn't show up.

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

      
    Report

  • Re: Damaged valve seats with perfect compression tests

    by » 13 years ago

    Hi Nicola

    Curious, did you use the orifice recommended by Rotax for your differential compression tester? It is smaller than that used by Lycoming or Continenetials.

    If you did a differential compression test on a Cox .010 with the piston missing while using a compression tester made for use with a diesel motor with a 3 foot bore, it would probably pass!

    Each cylinder on Rotax 4 stroke is ~18 cubic inches. An O-360 cylinder is ~90 cubic inches and even an O-200 cylinder is ~50 cubic inches.

    Ron Parigoris
      
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  • Re: Damaged valve seats with perfect compression tests

    by » 13 years ago

    My rotax mechanic (head of one of the two heavy maintemance and overhaul center here in italy) use a leak down tester specific for rotax 91x engine. http://www.aircraftspruce.com/catalog/topages/diffpress12mm.php

    he says that even in his experience it is first time to see damaged valvle seatsv with perfect leak down tests values.

    In my opinion the pressure used in test is too low to inspect subtle damage. Mean pressure in combuston chambers in whole cycle is around 12-13 bar with very high value during combustion phases. Remember all valve seats in my engine are damaged and so the 7-8 hp loss was spread in four.

    It is very difficult to search for a even loss of compression in cylinders resulting a 2 hp loss. With high probability If the loss was in only one cylinder leak down would have shown the problem.

    I will search for someone with same experience.

    Nicola di biase

      
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