why did it overheat

[sgreen5]

this is a 88 bayliner 305 chevy omc out drive and it has a heat exchanger
friday i ran my boat for the 1st time since last sumer useing muffs in my driveway and it ran fine. saterday i took it to the river ran fine threw a long no wake and fine going down the river up an plane. hers were it went wrong. my freind hooked a fish and was loosing all his line so i fired up the boat to run it down in reverse and the boat overheated real fast and boiled over. let it cool down for a couple hours and added water to the heat excanger. stared it up and got on plane comeing back to the dock when we ran agound. as soon as i backed the boat up the heat started riseing fast so i shut it down again. let it cool and when i tried to start again the motor was hydrolocked.
today i pulled the outdrive off and in the exhuast bellow the flaper was melted and stuck in there.
what would your gusse be as to what hapened?

 

[jpwdesigns]

Re: why did it overheat

Ok, my guess is that you're loosing prime. I had this issue with my boat last summer. I'm sure there is a reason, like bad impellor or a leak in the sender tube between the water intake holes and the impeller...which i believe is my issue too. The way mine behaved is that I would not get prime unless I went forward real quick after starting the boat in the water. the forward motion imo, forced some water up into the intake and got the prime going so that water would circulate through the engine. If there isn't prime you suck air and the impellor can burn up/ melt because its spinning dry. SO you might want to break open your impeller housing to take a look at the condition. Because you went in reverse, if you had lost prime, then water may not of been getting forced up to the impeller. I could be COMPLETELY off base because i'm a bit of a noob at this but based on my experience and research last summer I tracked down my issue to this sender tube. http://www.crowleymarine.com/johnson-evinrude/parts/55196.cfm look at part 79. The gaskets on that thing can go bad from what I read in other posts.

 

[PaJon]

Re: why did it overheat

Almost sounds as if when running in reverse the water flowed past the damaged flap and entered the combustion chamber through an exhaust valve. If it did it could have done damage to the engine. Water doesn't compress and can cause a connecting rod to bend.