flow is the most critical item to go after first starting with the impeller. If blades are missing you need to disassemble all the pipes and hoses between the raw water pump
and the heat exchanger
to find and remove all the broken vanes which can lodge in corners and bends in the hoses and pipes.
- - Of course sea water strainer and inlet/through-hull/seacock are next along with reaming out the tubes in the heat exchanger. The big deal about raw water flow is that your exhaust system is probably made of rubber exhaust hose. Insufficient raw water flow into the exhaust hose and you will cook the hose and it will crack and fail. Now you have a very serious mess and danger
as raw exhaust gases and salt water
is being sprayed around inside the vessel.
- - After all that switch to the fresh water coolant system. Check to be sure there is no coolant leaking down into the oil pan. Also fill the coolant reservoir and run the engine but not to the overheat point. check to see if you have a lower coolant level in the reservoir. You may be pumping coolant through broken or worn out tubes in the heat exchanger and into the raw water system. Heat Exchangers do wear out especially if not zinc'd properly.
- - Next likely culprit is the fresh water engine coolant pump. Take off the belt and grab the pulley on the pump and see if there is a lot of play in the bearings. If so replace the pump. As mentioned by others normally you may see coolant leaking down the face of the engine into the bilge
is you have bad bearings.
- - In very old engines it is common that the coolant galleries and thermostat are clogged with "mud" (corrosion product of the coolant flowing through the cylinder head
galleries.). Remove the thermostat and check it for "mud" and/or not operating - staying closed. If there is a lot of "mud" on the thermostat then the cylinder head
galleries need to be flushed and cleaned. Normally there are cover plates on the front and/or rear of the cylinder head that can be removed for cleaning
out the galleries.
- - If all that is okay and you are still overheating - try buying
a new temperature sender and install that. You can borrow an infrared temperature "wand" instrument and point it at the various parts
of the engine to see if they are really getting that hot.
- - Finally, obstructions and clogs in your exhaust system from exhaust manifold to riser/water injector to the hose and overboard
exhaust through-hull can create excessive back pressure and that will also cause over-heating.
- - There are probably a dozen other things that can cause the overheating from improper propeller
size to dragging prop shaft but the above are the most common things to look after first.