Yamaha 250 OX66 Scorched Cylinders

[jfbetz01]

As a follow up to an earlier thread looking for a replacement engine, I thought I would try and determine the exact cause of my cylinder failure.

During my first time in the water this season, one of my 2 engines seized. I did several service related items over the winter including change water pump, changing remote oil reservoir oil pump, changed remote and on board fuel filter. I was initially concerned the failure was related to some of this service work, but confirmed all installations as correct. The only variable I cant account for is the injector(s).

The attached picture shows top port cylinder seized, and looks like the beginnings of something in middle cylinder.


Any insight would be appreciated

[99yam40]

I am posting one of the pics in a larger format so Rodbolt or others can see it easier

[jfbetz01]

Much appreciated. I should also note this failure came about with little/no warning about 10 minutes after launching. I was paying particular attention to the temperature gauge at the time. The temp needle rose and stopped in the normal range, as did the other engine temp gauge, and stayed there. Then, all hell broke loose, alarm went off, engine seized.

[robert graham]

Almost burned the top right off that piston!....how many RPM's were you running when it seized? My guess is a failure of the oiling system, but Rodbolt will hopefully chime in here with his thoughts....

[jfbetz01]

3,000 RMP +/-. The engine was started at the dock, 5-10 minutes through no wake zone, then up on plane for half mile or so, and...blam

[boscoe99]

Looks to be text book detonation damage IMO.

Usually seen in the upper most cylinders. They run a bit hotter. In a big block HPDI it is usually (more than 50% of the time) the top left cylinder when looking at the motor from behind.

Usually seen on the exhaust side of the piston. Where temperatures are a bit hotter.

Yamaha has not said (that I know of) what kind of pistons they use. Hypereutectic pistons are known to be susceptible to detonation damage. I suspect that is what they are. They don't fare as well as old style forged pistons. Over on the bass boats boards they all use aftermarket pistons if and when a motor needs to be repaired.

Over propping, poor quality gasoline, an injector leaning out just a tad, inadequate cooling, etc. can all lead to the perfect storm of detonation.

All three pistons on the side shown appear to have detonation damage. You might as well remove the starboard side head and let us take a look at those pistons as well. I suspect some will show some damage.

[boscoe99]

[boscoe99]

[jfbetz01]

Starboard head comes off tonight; no visible damage on those plugs, but will be curious to peek inside. the top plug in particular here was ugly, and middle plug had beginnings of damage, bottom plug ok. Plugs on starboard bank all looked normal,

Nonetheless, with a strong tell-tale stream, is this likely a lean out issue?

btw, this is OX66, not HPDI

[boscoe99]

Starboard head comes off tonight; no visible damage on those plugs, but will be curious to peek inside. the top plug in particular here was ugly, and middle plug had beginnings of damage, bottom plug ok. Plugs on starboard bank all looked normal,

Nonetheless, with a strong tell-tale stream, is this likely a lean out issue?

btw, this is OX66, not HPDI

HPDI was just cited for reference.

With respect to the injector question, if three (or more) cylinders are damaged what is the probability of three (or more) injectors all leaning out at the same time and at more/less the same rate?

[pstephens46]

It might be the picture. The bottom of that fuel filter cup looks funky. Could be the plastic cup is aged and not as clear as it used to be. Like my eyeballs.

[jfbetz01]

Valid point on the multiple cylinder failure, although the lower cylinder appearance is a little misleading. The crud in the bottom fell from the upper cylinder when the head came off. Still, two cylinders with failure is curious.

As for the fuel filter, yes, it's just a darker Amber because of its age. That filter was changed the day before, and the vessel was cleaned out.

[boscoe99]

It might be the picture. The bottom of that fuel filter cup looks funky. Could be the plastic cup is aged and not as clear as it used to be. Like my eyeballs.

Some stuff I no longer see as well as I used to. Some stuff (floaters) that I used to not see, I now see.

Like plastic cups, I have not aged well. Dammit. If I had known I would live this long I would have taken better care of myself.

You can't tell a young man squat. They all have to learn it on their own. Via the school of hard knocks.

So said the blind man.

[pstephens46]

Yep. Should have eaten more fiber 30 years ago...

[99yam40]

Not sure eating more fiber would have helped keeping the floaters away.

boscoe ,ever been driving down the road and end up with a blind spot when looking at road signs?

Did not last long but had me concerned.
Eye Dr said floater probably got in the way.
that was years ago, and has not happened again