Originally Posted by Minnewaska
If my silicon grease, you mean the sticky stuff in a tube, that may not be a great idea. It should certainly help prime the pump, but I would worry what happens to it as globs move through the heat exchanger or block. Glycerin or dish soap, which are less viscous, are the norms.
Running the impeller dry will do damage, even if you can't see it with the naked eye. The vanes could let go on the next run. Best to always replace an impeller that you know you ran dry for more than a few seconds to prime.
Could be a blocked intake, but it would be very unusual. Blockages in the exchanger are more common. Or, as I said, if the seawater strainer has a leak, it may not allow for the suction necessary to draw the raw water.
This engine is direct sea water cooled- no heat exchanger- sea water goes right throught the block. I smeared the grease but no globs. One of the Yanmar manuals states to grease the surfaces of the impeller and pump housing. You are probably right to replace the impeller after a dry run. I'll let you know what I find. The way I attach the fresh water bucket I would actually have water pressure throug the raw water strainer all the way to the sea water pump. A leak should have shown up as at least a drip- but I had nothing. One thing my yanmar has a Kanzaki V-drive (that has an oil transmission cooler), what happens is the raw sea water suction side of the pump pulls the water through this cooler then into the raw water pump. The oil cooler looks to be an aluminum housing- it does not leak, but could be clogged. I will know more once I start tracing lines for cloggs. Figure I'll just start pulling hoses and doing a blow test till I find a restriction.