I have a 1992 mercruiser 7.4 litre in a celebrity 230 bowrider. It has only been used in fresh water. It has standard thru the hub exhaust. I have a mechanic winterize it each year, but do not believe he did so last year. I started and ran it with muffs on and everything was o.k. 4 hours into my first boat trip this year everything was perfect until the alarm went off. The engine was at 200 degrees, and I immediately shut it off. Up to this point the engine ran perfectly. I pulled the boat back to the dock at 5mph. Ten minutes after I had shut it off, I attempted to crank the engine and it would not turn at all. The next day I pulled all 8 plugs and found cylinders 3-4 hydrolocked. I blew out all the water, fogged the intake and cylinders. Two plugs were dry 6[/LIST]were wet. I cleaned and reinstalled the plugs. After cranking for a while the engine started and ran fine but I couldn't get any water to go thru the engine. I found that the seawater fuel pump had destroyed the impeller. I replaced it and found all the old pieces in the pump output hose and th p.s. heat exchanger. Started the engine all was o.k. except the oil was coffee with cream colored---i.e. full of water. I drained it and refilled it ran it for 10min. again coffee with cream for oil. I drained and refilled again (the only way I can do it is thru the dipstick tube) and this time the oil stayed pretty clean. The color was still a tad off so I changed it again. I am looking for suggestions as to what caused the hydrolock. The engine was off when it occured. Additionally, the original h2o pump lasted 8 years and 400 hours, the second 3 years and 40 hours. I saw no evidence of dryrot on the impeller blade remains. The flappers in the exhaust have been replaced 3 times, and I guess they are still working. So, how did cylenders 3-4 (which are on opposite banks but in sequential firing order get full of water and what weird chain of events caused this? If the block was cracked or something along those lines wouldn't it be misfiring on a cylinder or two?
overheated engines often diesel when shut down. sometimes backwards sucking water up tailpipe. second scenario is 3 and 4 exhaust valves were open and as engine cooled down it pulled enough vacumn to pull some water up the pipe. vp and merc are putting vacumn breaks on some of thier engines. third is you just hauled back throttle real fast and shut down engine when alarm went off. with enough headway your own wake could have hit the stern and forced water past the flappers. or the leak thing.