Re: OMC 5.8L 351 Overheating..need help
As for the hose, on a boat like this that has overheated from lack of water they will crack and with age, eventually become unsound. Also, when you remove your ex. manifold you have to loosen them. If you dont get them back on good and snug, and they are not in great shape, the dry rot from heat and old age will make them blow. (Trust me, I have been 7 miles out in the Gulf of Mexico and now I double wrap mine using a larger size hose I got from Ace hardware. In big water you can never be too careful.)
Back to the list of items it could be causing the over heat.
NOTE - Over heating is almost impossible to diagnose on muffs. In the water under load is your true measure. Don't say you haven't been told. This comes from years of 800 stringer experience.
1. Check the sending unit and the temp guage. (When in doubt, replace it for 15 bucks with one from the auto store. Even better go get an infra red temp guage for 30 bucks you can point at different objects and get temp readings.)
2. The swivel bearing housing HAS TO BE GREASED. (Lack of grease causes exhaust fumes to mix with water and jacks the temp up.) A cracked swivel bearing seal will do you in.
2A. Clear hoses from the intermediate to the thermostat will show bubbles if you dont have a good seal in the upper/lower connection.
3. The exhaust manifold and risers have to be clear of scale. Rust is allowable, scale will do you in...scale holds heat.
4. The thermostat must be good.
5. The timing must be set correctly. Premature ignition will make them run hot.
6. Get an empty gallon jug and pull the hose off of that comes from the intermediate houseing and disconnect where it goes into the thermostat. With the engine on, and the lower unit under water IN THE LAKE, crank your boat and see how long it takes to fill that gallon bucket. A good pump will push out a gallon in less than 30 seconds at idle.
7. Water pump on engine can be bad. It usually leaks.
8. Blown head gasket. Sometimes tough to diagnose but the heads can warp at real high heat.
9. Don't forget most big boats have a hot water heater that circulates water from the engine water pump back to the intake manifold to heat the water in the heater cheaply.
There is your list. Good luck.