Re: OVERHEATING 302 Mercruiser....Please Help!
If you believe you have good pickup pump water flow (test on the muffs and see how much water flows out of the side holes in the transom assembly), you may have crud build up in your exhaust elbows. That was the case with my 1976 model 888.
I unbolted the elbows, pulled the hose barbs and plugs, and gave them a good clean out with wire rods, pressure washing, etc. If they look like toast, replace them. They go before the manifolds, so bear that in mind when you examine them.
To do this, it takes some elbow grease (work, not a gasket dressing) and new elbow to manifold gaskets. Rather than try to save the stainless plates that are sandwiched between the gaskets, I replaced them too.
It takes some elbow grease to remove the stock gasket material, so be prepared to stuff the exhaust hole in each log manifold with some rags and scrape the gasket surfaces clean.
If there is a lot of rust packed into the water passages of the exhaust manifolds, pull them and clean / inspect them as well.
Overheating is caused by poor water circulation - usually traced to either a worn or damaged pump, or clogged exhaust passages.
