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Thread: Compression testing, what is normal?

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    Compression testing, what is normal?

    Since the FSM says nothing, I wonder how much compression is typically measured on a new engine, and what can we expect as normal, after say 100 000 miles?



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    Senior Member Cobrajet's Avatar
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    Quote Originally Posted by foama View Post
    Since the FSM says nothing, I wonder how much compression is typically measured on a new engine, and what can we expect as normal, after say 100 000 miles?
    https://mirageforum.com/forum/showth...g-replacement!

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    Thanks Cobrajet, already read that thread, but I was looking for something more precise.
    Where does it say what pressure is normal for a new engine? What about the 3A90?
    Maybe Fummins can report from his experience with 3A92's?

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    EDIT:

    Found it, finally...
    The FSM has a section in chapter 11A-16 called "compression pressure check"

    COMPRESSION PRESSURE CHECK

    1. Before inspection, set the vehicle to the pre-inspection
    condition.
    2. Disconnect the connector of the ignition coil, and then
    remove all of the ignition coils and spark plugs (Refer to
    GROUP 11B  Generator and Ignition System P.11B-10).
    3. Disconnect the all of the injector connectors.
    CAUTION
    Keep away from the spark plug hole when cranking.
    If compression is measured with water, oil, fuel, etc.,
    that has come from cracks inside the cylinder, these
    materials will become heated and will gush out from the
    spark plug hole, which is dangerous.
    4. Cover the spark plug hole with a shop towel etc., and after
    the engine has been cranked, check that no foreign material
    is adhering to the shop towel.
    5. Set compression gauge to one of the spark plug holes.
    6. Cranking the engine with the accelerator pedal fully
    depressed, measure the compression pressure.
    Standard value [at engine speed of 250 r/min]: 1,410 kPa (205 psi)
    Limit [at engine speed of 250 r/min]: Minimum 950 kPa (138 psi)
    7. Measure the compression pressure for all the cylinders, and
    check that the pressure differences of the cylinders are
    below the limit.
    Limit: Maximum 98 kPa (14 psi)

    8. If there is a cylinder with compression or a compression
    difference that is outside the limit, pour a small amount of
    engine oil through the spark plug hole, and repeat the
    operations in steps from 5 to 7.

    (1) If the compression increases after oil is added, the cause
    of the malfunction is a worn or damaged piston ring
    and/or cylinder inner surface.

    (2) If the compression does not rise after oil is added, the
    cause is a burnt or defective valve seat, or pressure is
    leaking from the gasket.

    9. Connect all of the injector connectors.
    Last edited by foama; 07-03-2019 at 07:26 AM.

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    Senior Member Cobrajet's Avatar
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    Quote Originally Posted by foama View Post
    EDIT:

    Found it, finally...
    The FSM has a section in chapter 11A-16 called "compression pressure check"

    COMPRESSION PRESSURE CHECK

    1. Before inspection, set the vehicle to the pre-inspection
    condition.
    2. Disconnect the connector of the ignition coil, and then
    remove all of the ignition coils and spark plugs (Refer to
    GROUP 11B  Generator and Ignition System P.11B-10).
    3. Disconnect the all of the injector connectors.
    CAUTION
    Keep away from the spark plug hole when cranking.
    If compression is measured with water, oil, fuel, etc.,
    that has come from cracks inside the cylinder, these
    materials will become heated and will gush out from the
    spark plug hole, which is dangerous.
    4. Cover the spark plug hole with a shop towel etc., and after
    the engine has been cranked, check that no foreign material
    is adhering to the shop towel.
    5. Set compression gauge to one of the spark plug holes.
    6. Cranking the engine with the accelerator pedal fully
    depressed, measure the compression pressure.
    Standard value [at engine speed of 250 r/min]: 1,410 kPa (205 psi)
    Limit [at engine speed of 250 r/min]: Minimum 950 kPa (138 psi)
    7. Measure the compression pressure for all the cylinders, and
    check that the pressure differences of the cylinders are
    below the limit.
    Limit: Maximum 98 kPa (14 psi)

    8. If there is a cylinder with compression or a compression
    difference that is outside the limit, pour a small amount of
    engine oil through the spark plug hole, and repeat the
    operations in steps from 5 to 7.

    (1) If the compression increases after oil is added, the cause
    of the malfunction is a worn or damaged piston ring
    and/or cylinder inner surface.

    (2) If the compression does not rise after oil is added, the
    cause is a burnt or defective valve seat, or pressure is
    leaking from the gasket.

    9. Connect all of the injector connectors.
    That is pretty much the procedure I followed. I mentioned 205 psi as the factory compression pressure, but it is also worth noting that Mitsubishi lists the 'service limit pressure'...or minimum allowable compression...on this engine as 138 psi.

  7. #6
    What's compression?


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