1998 OX 66 Powerhead Failure

[jack'd up]

If I am repeating a previous post I apologize, but I have yet to find an answer.
Last year I did a compression check on both motors and came up with around 98 psi. After my second time out I had one motor quit on me at idle. Appeared to be the same symptoms I have seen many times over the past 7 years ( diaphragm pumps ) After getting the boat back to the house I realized my flywheel would not make a full rotation. I removed the flywheel believing (hoping ) it was a stater that came apart. This was not the case! I then removed the head cover and realized I had a piston splinter on me and was caught up causing it to not make a full revolution.
My questions are:
1. What would make just one piston fail on a EFI motor? Is this due to normal wear? I bought the boat in 2004 with approximately 300 hrs. on it. I have approximately 1000 on it now.
2. Is it possible ( or recommended ) to repair just the one bad piston and re-ring the whole motor?
3. Should I rebuild? Buy a Remanufactured?
4. Can I bore out that motor instead of re sleeving it? If I bored it out would all the sensors function properly??

Please help

 

[99yam40]

Re: 1998 OX 66 Powerhead Failure

Post some pics of piston and cylinders so some one may be able to comment on what caused the problem.
And a model # may help

 

[jack'd up]

Re: 1998 OX 66 Powerhead Failure

The model number is a S250TXRV

 

[robert graham]

Re: 1998 OX 66 Powerhead Failure

Looks like the upper piston ring snagged in that port on the way down and broke the top land off that piston. That ring may have been carbon-stuck in it's groove, causing it. A Yamaha Dealer can repair/replace that piston/cylinder for a price$$$, but maybe there's carbon on the other pistons also? Maybe a remanufactured powerhead would be something to consider. Good Luck!

 

[multimech]

Re: 1998 OX 66 Powerhead Failure

From the scoring and aluminium transfer all around the cylinder I would say you had a heat problem on that cylinder, which would follow since it is one of the highest pistons. The piston expanded out causing the failure. This could have been caused by a lean injector or failed ignition, lack of water in that area. Whatever, you need to really determine exactly why this happened or you will put the same crap that caused the problem on a block rebuilt by you or a rebuilder. Unless you can confidently do this, I would not go it alone. Looking carefully again, those pistons look really dry. Look carefully at the break in the piston, does it look burned? Does the plug have aluminum throw off on it. This could also have been caused by detonation, which is always at the exhaust port.

I work in an outboard rebuild shop. We get engines back with exactly the same failure as the one's we replaced. Because the technicians didn't find the problem that caused the failure in the first place. Be very careful.

 

[jack'd up]

Re: 1998 OX 66 Powerhead Failure

From the scoring and aluminium transfer all around the cylinder I would say you had a heat problem on that cylinder, which would follow since it is one of the highest pistons. The piston expanded out causing the failure. This could have been caused by a lean injector or failed ignition, lack of water in that area. Whatever, you need to really determine exactly why this happened or you will put the same crap that caused the problem on a block rebuilt by you or a rebuilder. Unless you can confidently do this, I would not go it alone. Looking carefully again, those pistons look really dry. Look carefully at the break in the piston, does it look burned? Does the plug have aluminum throw off on it. This could also have been caused by detonation, which is always at the exhaust port.

I work in an outboard rebuild shop. We get engines back with exactly the same failure as the one's we replaced. Because the technicians didn't find the problem that caused the failure in the first place. Be very careful.

I was looking in my mechanics manual and noticed that the block for the the 200 hp,225 hp and 250 hp were the same stroke and bore???? I have a same year model 225 that is in good shape. Would this block work on mine?? If so what makes up the difference in the 3 size motors? Is it made up in the fuel systems?

 

[multimech]

Re: 1998 OX 66 Powerhead Failure

Ill check with our machinist. Just saw your post.