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  1. #1

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    Please Help 2007 FX HO MR1 Piston Keeps Melting

    Good day to all. This is my first post. Hopefully in the correct section. Iím in desperate need of some advice and guidance. I picked up a 2007 Yamaha FX HO Cruiser back around April of this year. It came with a remanufactured engine that the previous owner had not yet installed. To avoid any confusion In my story, I will call the previous owner, PO2. (This is for previous owner 2). PO2 bought the waverunner from previous owner 1 as is with a knocking engine. PO2 purchased a remanufactured engine for it, but as stated above never installed it. I Purchased the Waverunner, and installled the engine. I took my time on the install. New motor mounts, new exhaust gaskets, exhaust seal. I cleaned out the oil cooler and installed new gaskets. I did the same with the oil pump. Flushed out every coolant hose, vent hose, drain hose, you name it. I removed the gas tank and cleaned out the tank. New spark plugs and thermostat. I did notice that PO2 included the used spark plugs that he removed out of the original engine. I glanced at the plugs and noticed one of the electrodes was smashed. So obviously something came apart in the original engine. So moving forward I started up my project and it sounded good. The ski only had 54 hours on it. I thought that was pretty low for the age and was surprised the engine already went bad. I took it out for a ride. I was breaking the engine in. Not dogging it. Nothing stupid. I put a total of about two hours between idling and riding it. The ski died on me and there was oil up in the air filter area. NO I did not overfill it. I towed it in with my boat. Did a compression test in the garage and cylinder one and cylinder four had zero compression. I removed engine and piston number 4 on the intake side was melted. Also there was some piston break down on the exhaust side of cylinder number 4. Cylinder number 1 piston was also breaking apart the same way just not as bad a cylinder 1. I received some advice indicating that it look like the cylinders were washed out with gasoline. I did a lot of reading and learned that Yamaha had some fuel injector output on their ECUs go bad. I ended up ordering another brand new ECU for it. In the meantime I had to get the block bored out. So now Iím running WSM .25 larger pistons. I had a Machinist go through the head and replace three intake valves because they were damaged from the piston coming apart. I reassemble the engine using brand new oem rods and bearings. I also took some carb cleaner and a 9 volt battery and made sure my injectors opened with voltage applied and confirmed I could spray through them. I replaced the injector orings and seals. I also checked the ohms on the injectors. Everything checked out OK. However I was not able to confirm that they were spraying the correct pattern. I was sold on the ECU being bad and thatís what caused my issue. So my engine was reinstalled and again it ran great. I took it with me on my vacation to a lake. Again I was breaking it in not dogging it or abusing it. The ski ran great with light used for maybe a total time of an hour and a half. I Took the ski out the next day. Again it ran great then I got on the throttle and it started to miss. I could smell raw fuel and I knew I had a cylinder not firing. I cut it off and pulled the spark plug. Cylinder number 4 had a smashed electrode. You could hear it knocking. Yesterday I pulled the engine and cylinder head. Piston number four has melted again on the intake side. ZERO damage to cylinders 1-3. Iím lost on what to do next. I canít keep guessing. Iíve read about Detonation and also preignition. Preignition is actually more damaging and happens faster than Detonation. I also read about a slight chance of a combination of fuel injector and cam positioning sensor possibly causing this. I know what spark knock sounds like. It didnít have any. This is the second time this is happened within four hours please help.
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  2. #2
    ptscon's Avatar
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    It's too late now but you really should have had the injectors flow tested before the new motor went in. Not knowing what caused a meltdown that's the first item I would question on a non tuned ski.

    Injector opening when voltage is applied really doesn't mean much. Does it flow the correct amount at a given pulse width is the question.

  3. #3

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    That is my next plan of action. I’m even scared if it comes back that they “pass”. I just don’t trust anything anymore.

  4. #4
    BLASTER 1's Avatar
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    I agree on testing the injectors and having them cleaned. My son out of nowhere melted a piston, so he had his injectors checked. 2 were flowing like they should 1 was around 75% I think and the melted piston was around 55-60 if I remember correctly
    Now this is a stage 3 ski and it’s never ran better after having his injectors taken care of

    I wish you Luck finding the culprit please post when found
    Last edited by BLASTER 1; 08-26-2021 at 12:28 AM.

  5. #5

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    Quote Originally Posted by BLASTER 1 View Post
    I agree on testing the injectors and having them cleaned. My son out of nowhere melted a piston and his injectors checked. 2 were flowing like they should 1 was around 75% I think and the melted piston was around 55-60 if I remember correctly
    Now this is a stage 3 ski and it’s never ran better

    I wish you Luck finding the culprit please post when found
    Thank you Blaster1. It’s been a frustrating summer for me. The scary thing is, if it is an injector, this could happen to anyone of us. Ski ran great and sounded great both times.

  6. #6
    Myself's Avatar
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    On a computer controlled fuel injected engine.......and only one piston melts down......always suspect an injector issue.
    I have a GP1300 fi apart right now...melted piston.....injector sputters at best.

  7. #7

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    Quote Originally Posted by Myself View Post
    On a computer controlled fuel injected engine.......and only one piston melts down......always suspect an injector issue.
    I have a GP1300 fi apart right now...melted piston.....injector sputters at best.
    Bad news for myself. I just got a call from the shop who tested the injectors. All 4 injectors are flowing correctly and are passing all of his test. I don’t have any of the numbers right now. That report will be mailed to me with my injectors. I am at a total loss on what to do next.

  8. #8

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    I have an update to this thread. I believe the cause of my issues are to high of a compression ratio. I have run regular gas as suggested by Yamaha. I had a machinist compare my block to another used original block. My block has been decked .014 of an inch and my head is .008 of an inch. That’s a total of .022 of an inch. I also believe that my head has been welded in the combustion area and is possibly causing cylinder 4 to have an even higher compression ratio than the other cylinders. Anyone have any thoughts or experiences with this? Thank you

  9. #9
    butterbean_29512's Avatar
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    .008 isn't that much, a badly warped deck could possibly need that much to true up. But the head being shaved .022 in combo with the block shave could cause problems. Probably the only way to solve it would be to replace the head unfortunately. Where did the reman engine come from? SBT? Or somewhere else? You could possibly have a custom head gasket made for it, just throwing ideas out.

  10. #10

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    Have you installed a fuel pressure gauge to see if it is dropping when you get on it? That piston looks really rich like you have an injector or ecu issue on that cylinder. I have burnt more stuff up being too rich than being too lean. Also I'm a fan of more compression.


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