There have been many posts on mercruiser small block stern drives With raw water cooling and dry joint manifolds overheating. One scenario is that the engine overheats at idle but not at speed. Many owners have replaced the raw water impellers, thermostats and water circ pumps without fixing the problem.
If the engine cools at speed but overheats at idle, it is unlikely any of the above are the problem.
The 6.2 (and some others) have a check/bypass valve at the bottom of the manifolds, where cooling water enters the manifolds. When the input gets clogged with rust etc., and the engine is at low speed, the path of least resistance is for cooling water to just go out the bypass. At high speeds the pressure is sufficient to force cooling water into the manifolds. This scenario might occur with single hose inputs into the lower manifolds.
The obstruction may not be visible at the riser/manifold joint. You have to pull the T/check valve, Which is most easily accomplished after pulling the manifold. Be sure to drain the manifold first to prevent water from entering the cylinders.
solution:
Replace the manifolds
If the engine cools at speed but overheats at idle, it is unlikely any of the above are the problem.
The 6.2 (and some others) have a check/bypass valve at the bottom of the manifolds, where cooling water enters the manifolds. When the input gets clogged with rust etc., and the engine is at low speed, the path of least resistance is for cooling water to just go out the bypass. At high speeds the pressure is sufficient to force cooling water into the manifolds. This scenario might occur with single hose inputs into the lower manifolds.
The obstruction may not be visible at the riser/manifold joint. You have to pull the T/check valve, Which is most easily accomplished after pulling the manifold. Be sure to drain the manifold first to prevent water from entering the cylinders.
solution:
Replace the manifolds
