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Thread: Jet ski -- possible ways for the water to get in theengine?
www.piclist.com/techref/index.htm?key=jet+ski+possible
face picon face BY : email (remove spam text)(Olin Lathrop)



Carl Denk wrote:
> I still believe the valve failed first, and caused all the other
> damage,

Maybe, but remember the first symptom was overheating with no other apparent
engine problems.  The valve head breaking off would have caused immediate
and sudden obvious symptoms.  The engine would have sounded clearly
different, not to mention the rattling, bangs, and knocking as the piston
smashed the loose valve around and occasionally jammed it into the head, as
the pictures clearly show happened.

A head gasket failure could certainly have messed with where coolant went
and caused overheating, perhaps even draining of the coolant enough to cause
damaging overheating.  It's possible that a already weak valve stem cracked
from the additional stress caused by excessive heat and water getting into
the wrong places.

Vitaliy also mentioned steam coming out of somewhere when the engine was
stopped.  I wonder if that was possibly some crankcase vent.  There would
have to be a connection between the coolant channels and the crankcase for
that to make sense, something a broken valve alone doesn't explain but a
head gasket failure does.

Another thing to consider: If there was enough water someplace to boil and
come out as steam near the craft's water line, could the lake water have
been sucked back in when the steam eruption was over?  I think this is a
reasonable possibility if the place where the steam came out was under
water.  The steam would have ensured that whatever cavity it was coming from
was filled almost entirely by vapor phase water.  All it takes is a little
cold water getting back in, which cools some steam, which reduces the
pressure, which sucks in more cold water, etc.  I've seen this happen (not
on a engine), and it's rather spectacular and happens very quickly.  It's
essentially a runaway process that ends with the cavity completely filled
with liquid water, and can take less than a second depending on the size of
the cavity and the size of the opening.


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