Diagnosing sudden rough idle

Discussion in 'Maintenance Bay' started by Geoffrey William Rutledge, Mar 29, 2021.

  1. Geoffrey William Rutledge

    Geoffrey William Rutledge Filing Flight Plan

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    Have an IO-360, at 1715 hours in my Glasair IIsFT.
    This weekend, after startup, developed rough running at idle, with a low temp on CHT #3
    On run up, the #3 cylinder temp rose to equal the others.
    Cruise flight was smooth, with equal CHT on all cylinders.
    Then on power back, and at idle, the engine is very rough again. It's not subtle, feels like trying to run on 2 or 3 cylinders.

    Fuel controller was replaced ~300 hours ago.
    Mag checks are fine (dual p-mags).
    After power down, compressions on warm/hot engine all 72-76 lbs.
    The upper plugs all look fine too.

    Any ideas for source of the problem? Could it be an induction leak??
    Could it be a failing exhaust valve that's not leaking when the engine is warm?

    How best to diagnose the problem?
    Geoff
     
  2. Jdm

    Jdm Pre-takeoff checklist

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    Intake gaskets? Check for blue stains.
     
  3. Dan Thomas

    Dan Thomas Final Approach

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    Yup. Or a slipped intake runner hose.
     
  4. Jdm

    Jdm Pre-takeoff checklist

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    I’ve never found a slipped hose. Cracked yes, but not slipped. I typically change out the rubber hoses when I do the intake gaskets anyway. While you have all of that apart also check the tubes that go into the manifold. Not terribly likely, but they can become loose. If so they will need to be reseated with a special tool.
     
  5. Geoffrey William Rutledge

    Geoffrey William Rutledge Filing Flight Plan

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    Thanks very much for your helpful responses. I found a significant intake leak at the #3 cylinder (sprayed soapy water, used the compression tester to pressurize the cylinder, and backed the propellor off from TDC by 1/4 turn to open the intake valve and ‘burp’ the intake pipe)
     
  6. RyanB

    RyanB Administrator Management Council Member PoA Supporter

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    Did you have any luck resolving the issue?
     
  7. Geoffrey William Rutledge

    Geoffrey William Rutledge Filing Flight Plan

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    I'm waiting on ACS to send me new intake gaskets, but I'm highly confident that stopping the leaking intake will have my engine purring at idle once again!
    The source of the leak wasn't a bad gasket, it was a loosened bolt holding the intake pipe on the cylinder. The existing gasket was actually in good condition.
    I suspect a mechanic failed to put a new star washer on the bolt the last time it was tightened.
    Cheers, Geoff
     
  8. Greg Ellis

    Greg Ellis Filing Flight Plan

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    I have seen this procedure on a video on Youtube. Seems like an excellent way to look for leaks. Were you able to do this by yourself or did you have help? It seems like a two person job since pressurizing a cylinder will cause the prop to move. I wanted to do this on my airplane. Trying to track down a possible leak. I was just seeing if it is definitely a two person job. Thanks.
     
  9. Geoffrey William Rutledge

    Geoffrey William Rutledge Filing Flight Plan

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    Indeed, I suspect I saw the same Youtube video. I did use a helper. I found that the best way to get the "burp" was to find TDC, rotate backwards 1/4 turn, then turn on the air. At that position the intake valve is open, so no pressure builds at that point. Then I moved the prop toward TDC, feeling the pressure build (and the gauge shows pressure rising in the cylinder). Then release the prop and get a loud burp. If you can stand by the prop and see the cylinder head it's possible to do this as a one-person operation, but much easier with a helper!
     
  10. Greg Ellis

    Greg Ellis Filing Flight Plan

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    Thanks for the information. I appreciate it.