2002gtx di piston damage

[carlos_fig]

Hello there. so last year I purchased this seadoo for a very low price knowing it had some engine problems. so being that the craft was in very good condition and I'm pretty savvy with small engines I took on this project . I did a compression test and found it to be low so i removed the top piston cylinder cover and seen damage on the pistons so i went ahead and removed the engine knowing that at the very least i will be doing a top end rebuild. so correct me if I'm wrong but isn't piston scoring usually caused by not enough oil? because i checked the main oil line from the tank and there are no obstructions . the oil flows no problem . i also manually checked the oil pump by turning it with a drill and it seem to be working just fine. so my question is what could of caused this problem? i aready have a oversize piston and cylinder kit i had purchased from sbt a coupled of months ago. I'm trying to figure out the root of the problem the last thing i want is to trash a new piston and cylinder. any help would be appreciated thanks.

 

[mikidymac]

First of all something hit the top of the piston, probably a piece of broken ring. There is a good chance some made it down into the crank.

A 951 bottom end typically will not survive a new top end and most blow the crank out the cases so on a toasted 951 like yours you should be doing the crank and balancer also.

Finally, it is typically a lack of fuel that causes these to run lean and seize not just oil. The oil needs a carrier (fuel) to distribute it around the pistons.

Water ingestion can also cause this on a 951 and isn't uncommon. IF it was me I would suspect the fuel pressure and injectors for your failure.

 

[carlos_fig]

Since I have to check the entire fuel system i went ahead and removed the injector block. I pulled the fuel and air injectors and found some corrosion on the fuel injectors. they will be sent out to be cleaned and tested hopefully they can be saved.

 

[DooWacka]

I agree with miki for the most part.
The oil is injected into the case and works up to the pistons. Remember, the fuel in a DI is only injected a bit before the spark and therefore I would think has little to do with the oil distribution.

Miki is right that this looks like a lean condition that caused melting of the pistons and movement of the rings. These DI pistons always melt on the exhaust port side, in my experience.
I would suspect low fuel pressure or output (fuel pump). Maybe, not using "premium" 91 octane fuel which the DI are designed for. The knock sensor would normally cause the MPEM to change the spark timing if only 85 octane was used, but if that fails then it can cause this problem. Or, possibly not warming the engine a bit before running at WOT. This problem with lack of warming is especially common in the SkiDoos (snowcraft).
BTW, I have never had an injector go bad.
However, I have done 6 top end rebuilds between my four DI, and have never had a bottom end fail afterwards (knock on wood). The ring fragments (there aren't many) seem to go out the exhaust.

 

[carlos_fig]

Thanks for your input DooWacka i plan on taking apart the crankcase in a few days to check for debris and check the crank bearings for play and pitting and if all looks good I'm not going to bother replacing the crankshaft . its $600+ that i really don't want to spend at the moment. i mean in all reality i probably wont put no more than 20hours a year on it. being that i live in la county and getting to the lake is a job itself. i will be sure to replace fuel pump filters and the inline fuel filter and check the fuel pressure before i would even feel comfortable taking it to the lake . i found a video on YouTube that shows how to make the pressure tester and check the pressure's air and fuel. btw what is the highest amount for hours you have on a oem crankshaft ? i know everybody's experience is different and i read more often than not if you rebuild the top end your crank and balance shaft wont survive too long .