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post #16 of Old 02-06-2013
casey1999
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Re: Engine over temp at idle

The suspense of what was causing the overheat made me work on the boat till dark yesterday. Found the problem. it was a single yanmar engine cooling bypass hose (about 4 inches long) that runs from the cooling water inlet to the engine, then to the themostat housing.

This hose will bypass all the raw sea water so that it does not enter the engine block. Once at the thermostat housing, the water is directed to the exhaust manifold then to the mix elbow. This hose (about 5/8 inch ID) was packed solid with material. I think the material was oxidized zinc anode from the engine.

I think what happened is that the first thing I did was removed and cleaned the engine block zincs. When I removed these there is a crusty deposit on them and some drops into the engine block- not much you can do to prevent, maybe could be sucked out with a wet/dry vac. After cleaning and replacing zincs, I went to the raw water pump to change the impeller. I think when I took the discharge hose off the pump (needed to remove to open it up as access is tight), the water from the block, along with anode oxidation drained to the pump discharge hose. After impeller replacement, I started engine. The zinc deposits were then pumped into the bypass line and clogged it. Since on engine start up, the thermostat is closed, all cooling water goes to the bypass line. Once the engine is warm, the thermostat opens, and water would go through the engine.

To prevent this, in the future I will first change the water pump impeller. Run the engine, then check the zincs, then open the thermostat and check for deposits they may have gotten trapped.

I still plan to install new impeller and open up thermostat housing to inspect.
But for now, she is purring like a kitten.
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