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Thread: Assessment of engine condition ...

  1. #1

    Assessment of engine condition ...

    I realize that getting a PPI from a reputable source is important, but I am looking to see if my initial observations hold weight with Lycoming/Continental aircraft engines, as they would with most other automotive engines. I would not want to do a PPI on unworthy candidates that I can eliminate myself.

    I recently was interested in a local homebuilt airplane and the seller represented that the engine (IO-540) was 750 hours since new. The engine looked clean, seemed to run well enough, and he shared the results from itís last inspection. Compressions were in the upper 60ís and oil use was claimed at a quart every 4 hours. He then showed me a few images of his engine monitoring panel, while in flight, to illustrate his claims of speed and fuel burn. What surprised me most was that the engine was level cruising at 2400 rpm, at about 70% power, with the oil at 150 degrees and the oil pressure was just over 60psi. EGT and CHT both looked normal.

    None of these items, individually, are disqualifying but when taken as a whole they seem to indicate that the engine does not have as much service life to offer before needing a major overhaul, as itís hours might imply.

    Does this sound reasonable, or am I being too critical?

  2. #2
    CarlOrton's Avatar
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    I'm not that familiar with an IO-540, and am not an A&P, so take this with a grain of salt.

    What is your concern? Looking at the data provided, about the only item I'd be concerned about is the relatively low oil temperature, but without further data, that might not be too bad.

    You didn't mention the altitude at which the plane was flying. Any normally aspirated engine will lose power as the altitude increases, so even if fully firewalled, you will see 65% or so at altitude.

    You also didn't mention if it had a fixed-pitch or constant speed prop. Given the types of planes a 540 is run in, I'd assume constant speed, so again, I'm not seeing anything I'd be concerned about. What was the manifold pressure?

    Did you check the logs to ensure that a "new" engine was installed, or was it built-up from a used core?

    Carl Orton
    Sonex #1170 / Zenith 750 Cruzer
    http://mykitlog.com/corton

  3. #3
    Let's start at the oil pressure, Lycoming states that 60psi is the minimum acceptable oil pressure under load. Given this same engine on a warm day, or within a climb, where the oil temperature is 200+ degrees, that pressure would be below the 60psi minimum. Most people report 70-80psi on warm cruise at moderate RPM, and considerably higher with cold oil.

    In this case the prop was a constant speed prop and the manifold was at 20 inches.

    The compression seems low for a "fresh" engine, Lycoming lists 65psi or below as cause for increased monitoring or investigation. Then we have the 1/4qt /hr oil usage, within spec but on the higher end for a fresh engine.

    If the oil pressure had been higher, and the oil usage and compression were as they are, I would assume the engine might need new cylinders sometime but was otherwise solid. The cylinders should resolve the compression and oil usage at the same time, leaving you with a solid engine. I just see the combination of factors as a red flag that this engine has seen a hard life, or the hours are not adding up.

    That is the reason I am asking if you believe my logic is faulty.
    Last edited by Velocity26; 06-25-2018 at 11:50 AM.

  4. #4
    cub builder's Avatar
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    The IO-540 has an adjustable oil pressure relief valve. A normal oil pressure of 60 psi only indicates to me that you might want to adjust the oil pressure relief setting up a bit by adding a couple of washers behind the relief spring. The compressions do seem a bit off, but that doesn't really tell you the story. While doing a compression test, where is it leaking? Rings or Exhaust valves? So the engine might need to be topped eventually. But I would want a really solid logbook history of the engine, in particular, a parts list from the last overhaul indicating what all was oversize or undersize and by what amounts, as well as whether these are overhauled cylinders or new installs at the last overhaul. I would guess they were probably service limit cylinders that were honed with new ex valves and guides, which is probably why the compressions are off a bit and it's using some oil 700 hrs later. For cylinders, I've used a lot of refurb service limit cylinders over the years, but there's nothing like new if you want low oil consumption and good compressions. I don't go with refurb cylinders anymore if I'm going to do a complete top or a major overhaul. They either meet new spec, or I'll buy new. Makes for a much happier engine and owner in the long run.

    IO-540 oil pressure.pdf

    In the attached.pdf, you'll see the drawing of the oil pressure relief valve. Note in the parts it calls for a maximum of 9 washers behind the spring. Typically, 3 washers is about right. The relief valve is easily removed without doing anything else to the engine. Item #6 is an optional adjustable oil pressure relief. The engine could be equipped either way.

    -Cub Builder
    Last edited by cub builder; 06-25-2018 at 01:49 PM.

  5. #5
    You are likely right, assuming the the relief valve is the cause of the low oil pressure. If the problem is bearings, the relief valve may only open on cold start and adding preload does not change the hot pressure. I have seen that go both ways.

    Thanks for the replies!

  6. #6

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    How old since new? 750 hrs on a Homebuilt is quite a bit. If the engine is 20 years old it might not make TBO.
    But a new engine is better than one with 5000 hrs when you go through it and replace any parts. It might just need be to flown more to wear in the valves. Or do a valve job.
    As mentioned, compression test indicates if valves or rings or both.

  7. #7
    Bill,

    Your suspicions are correct on all counts, the airplane has just over 300 hours since completion and the engine already had 450 when it was installed. The engine is over 20 years old. I did not pursue the engine's prior history because I had already seen enough to dissuade me from negotiating a deal.

  8. #8

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    Then again, it could just be gauge calibration.

    Velocity, just out of curiosity, what will you be looking for on a PPI?

  9. #9
    CarlOrton's Avatar
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    Velocity, you probably made the right choice. As I wrote in my original post, it was unknown if it was *new* as written, or overhauled. If it was new, I'd say that you could probably have negotiated a lower price on the assumption you'd have to do at least a top overhaul, but without knowing the bearing status, whether the crank had already been reground, etc., it's just too much of a gamble. While the oil pressure didn't seem to be an alarm to me (didn't know Lyc said min was 60), one thing I found on my own engine (O-200) was that the cavity for the oil pump gears was damaged, necessitating a new accessory case. The engine was running fine, but there was evidence of attempts to raise the oil pressure by placing washers under the relief spring.

    Carl Orton
    Sonex #1170 / Zenith 750 Cruzer
    http://mykitlog.com/corton

  10. #10
    Quote Originally Posted by martymayes View Post
    Then again, it could just be gauge calibration.

    Velocity, just out of curiosity, what will you be looking for on a PPI?
    On a homebuilt, I cannot assume that basic structure is sound, or that the materials used are suitable for the task. My plan is to have someone with brand/model experience do the PPI and check out the major structural items, along with a mechanical exam.

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