V8 overheating problem ( 5.7 mercruiser)

[spareparts]

if it closed cooling, that opens up more posibilities and options, first, if the manifolds are included in the closed cooling( full cooled system vs half cooled), your maniolds are probably OK. If it uses the belt driven water pump, just take the end cap off the heat exchanger or pull the raw water hoes off the exchanger, you should see pieces of impeller in there, if you replace the impeller, use the late style impeller kit that uses a one piece housing, it pumps more water and is easier to install. Verify that the raw water is pumping to the heat exchanger, you may still have a problem with the risers, you'll get that burning rubber smell. While you have the end cap off the exchanger, take the one off the other end and shine a lighht thru the tubes, if you see any blockage, blast some water thru there, or use a rifle cleaning rod to clean the passages. Once you get water going to it, let the engine run with the radiator cap off the heat exchanger,if you have a blown head gasket, you should get a lot of air bubbles in the exchanger, check the oil and see if it has gotten milky( an indication of water leaking past the head gasket). If it checks out, you may want to keep going with this boat, not many engines in boats this size come with cloosed cooling, it is an expensive option when new, and really extends the life of the engine and manifolds( if included in the system) form corrosion. Those motors are pretty tough, if a 5.7 blew its headgasket everytime it ran hot, i would'nt have to work on anything else but putting heads on these motors for a living.

PS, have the thing checked out by a mechanic that is familure with Mercruisers, there is still a lot of other things you could get burned on with an boat engine( I/O or OB)