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  1. #1

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    2000 STX DI detonation damage to Mag Piston HELP NEEDED!!

    So I had my number 1(mag) piston fail due to detonation. I had the cylinder oversized and replaced the piston and head. Ran for a short time on an hose. I then removed the head to inspect and found a little detonation damage. I am stumped to find what is causing this damage only on the mag piston. The other pistons look great.

    This ski has an recently rebuild EMM by DFI, and a new stator. No other engine codes, and it ran great prior to the failure.

    I have tested the following:
    Injector ohms in spec.
    Voltage and grounds at injector are good.
    Voltage to injectors cranking are normal.
    Fuel pressure was high, so the regulator has been replaced.
    Fuel flow was within spec.
    The filters on the injectors were clean.

    I still need to check.
    Injector voltage during running, and do an compression test.

    If it was bad gas, It would effect all cylinders.
    If it was a crank leak, it would affect all cylinders.

    I am planing on changing the gas to the higher octane fuel, and thinking about changing the fuel map to +5% to help correct it, but would prefer to find the problem not just patching it.

    This is the original piston that was replaced.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  2. #2

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    Me again, could a bad reed cause this?

  3. #3
    DavidR's Avatar
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    If the front seal was leaking, wouldn't that cause the #1 cylinder to lean out?

  4. #4
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    Quote Originally Posted by DavidR View Post
    If the front seal was leaking, wouldn't that cause the #1 cylinder to lean out?
    Yes

  5. #5
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    Your crank could be out of index. Check that. I would also check for air leaks or possibly an injector not working properly.

  6. #6
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by keithg1964 View Post
    So I had my number 1(mag) piston fail due to detonation. I had the cylinder oversized and replaced the piston and head. Ran for a short time on an hose. I then removed the head to inspect and found a little detonation damage. I am stumped to find what is causing this damage only on the mag piston. The other pistons look great.

    This ski has an recently rebuild EMM by DFI, and a new stator. No other engine codes, and it ran great prior to the failure.

    I have tested the following:
    Injector ohms in spec.
    Voltage and grounds at injector are good.
    Voltage to injectors cranking are normal.
    Fuel pressure was high, so the regulator has been replaced.
    Fuel flow was within spec.
    The filters on the injectors were clean.

    I still need to check.
    Injector voltage during running, and do an compression test.

    If it was bad gas, It would effect all cylinders.
    If it was a crank leak, it would affect all cylinders.
    ...
    A leaking front crank shaft seal would only affect the front cylinder.

    The MAG piston alone cannot get out of time from the flywheel, since there is a direct mechanical link between the flywheel and the mag crank web.

    Detonation damage while running on the garden hose, with no load on the impeller, seems strange.

    Check the flywheel for damage or small metal debris. Also check the CPS sensor.

    The EMM may have been repaired, but that does not guarantee that the EMM is actually now good.

    What are the cylinder compression numbers?

    Did you inspect the crank shaft for excess connecting rod bearing play?

    The fuel injector could be failing. If you swap injectors between cylinders, does the problem move?

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