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Cylinder leakdown test?

Discussion in 'The Garage' started by Thumper, Jun 26, 2005.

  1. Thumper

    Thumper 1/2 ton status

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    Hey guys,
    I am in the process of replacing the valve stem seals on my 454, using the air fitting to hold the valves up. I put about 80-90psi on it, and it works like a charm, however there is a constant hiss of air escaping... its not a gush, but it runs the compressor about once every 15 mins or so. Should it be a tight seal? The motor hasnt run for prolly a week and a half. Its coming from the oil passages, thru the pan, so its getting past the rings. Should there be air getting past the rings? I imagine the rings are dry since it hasnt run for a while... would that make the difference? I know there is a leakdown test you can do, but I cant find a procedure anywhere. I suspect that you have to squirt oil in the cyl, but does anyone have the actual way to do it?
    My issue with the valve stem seals is that since my little stuck on May long, its now burning oil... not bad at idle, and not bad under slow accelleration, but bad when you let off the gas or rev it several times in a row. I was told that stem seals may help it for a little while. The plan is to run it for the summer, and prolly pull it out and rebuild it over the winter.
    Thanks
    Mike
     
  2. jarheadk5

    jarheadk5 1/2 ton status

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    Unless you're running gapless rings, the air is working it's way past the ring endgaps and down into the sump. Pretty normal on a cold engine.



    Here's the tester used by most aircraft mechanics to perform a "differential cylinder pressure test" on a piston engine:

    Tester

    The engine manufacturer specifies the exact procedure IAW the FAA's guidelines, but here's a general rundown:

    Starting with the engine at normal operating temp -
    1. Close the fill valve on the tester and hook up shop air at about 100psi line pressure.
    2. Set the regulator on the tester for 80psi (Lycoming and Teledyne-Continental) on the left gauge (looking at the pic of the tester), then open the fill valve for a couple seconds to blow out any debris in the tester-to-cylinder hose.
    3. Disconnect all sparkplug leads, remove #1 upper sparkplug (aircraft engines run 2 plugs per cylinder) and set the piston at TDC of the compression stroke.
    4. Screw the tester-to-cylinder hose into the upper spark plug hole fingertight.
    5. While holding the crank stationary (using an assistant!), slowly open the fill valve.
    6. With pressure in the cylinder, very carefully rotate the crank slightly before and after TDC, watching the right gauge for the highest pressure indication. Anything lower than 60psi is grounds for cylinder disassembly and inspection. Listen to the carb intake (intake valve), exhaust pipe (exhaust valve), and oil sump breather/oil dipstick (rings) to tell what's leaking.
    7. Repeat steps 3-7 in the engine's firing order for the remaining cylinders.

    Now the pressures I gave are the FAA's bare minimum standards. Manufacturers give tighter limits; e.g. Lycoming says 80psi regulator setting, and 65psi cylinder pressure is mandatory cylinder removal and inspection. They've determined that 15psi of pressure differential is an unacceptable power loss for an aircraft engine.
    For an automotive engine, the "standard" compression test with a single gauge is a lot less time-consuming to get an idea of what cylinders may have problems. Then you could do a differential check on the bad cylinder to find the source of the leak. Or just hook up shop air to the cylinder and listen...
     
  3. Thumper

    Thumper 1/2 ton status

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    Ok, thats quite the procedure. It doesnt mention anything about adding oil to the cylinder... but it does say start with a engine thats at normal operating temp.. does anyone know if it would seal tight normally after its warmed up? Id think that it would always let some air past... wouldnt it?
    So, I guess I wont worry about the air thing right now. I am going to change the easy side seals first, and see if the smoke out of the pipe lessens... hopefully! :)
    Thanks
    Mike
     
  4. Iron_Weasel

    Iron_Weasel Registered Member

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    Yes, a warm engine seals better than a cold engine.
    As metal heats up, it expands. So when your rings heat up, they expand and seal against the cylinder walls better.
    However, there will still be a small amount of air leakage (1% - 3%) no matter what.
     
  5. jarheadk5

    jarheadk5 1/2 ton status

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    Precisely.
    Plus, an engine that was run up to normal operating temp will have a very thin film of oil on the cylinder walls (thin like only a couple microns thick). It's not much, but it's enough to fill the microscopic gaps between rings and cyl. walls where air might slip through.
    Even gapless rings will leak a small amount of air; less than standard rings, but still some.
    Unless your engine is super gummed up, the rings will slowly rotate on the piston while it's running. It's possible that instead of the ring end gaps being staggered, they're lined up. This will exaggerate the leakdown rate and make a problem appear worse than it really is. Solution - run the engine for a few minutes at varying RPM to re-stagger the rings, then re-test, BEFORE tearing it down. You might be pleasantly surprised...
     

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