Join Date: Jan 2008
Location: Lake Ontario
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In the class, the instructor recommended that you let a professional do the flush and refill so you get a good pressurized flush out (particularly if its been many years since you've changed your coolant) and so there's no chance of an air void remaining in the cooling system. The Yanmars are a little less prone to this then other engines (those with bolt on heat exchangers), but it can happen (e.g., in the lines / plumbing for the hot water heater). You may want to consider letting a pro do it the first time, if you're unsure of what you're doing.
While you're at it, if your engine's getting a little older and it's never been done before, consider if you want to replace the pressure cap(s) and maybe even the thermostat. If you have a higher remote tank with a cap (due to a hot water heater installation) and a pressure cap on your heat exchanger, be sure the pressure cap on the (lower) heat exchanger is rated for higher pressure than the cap on the (higher) remote tank, which should be as specified for the engine.
The heat exchanger and oil cooler (if you have one) should be serviced at least every 1000 hours as well (raw water tubes are prone to clogging with impeller blades, bio garbage, etc.), and of course monitor your exhaust output routinely to make sure there's no raw water restriction due to carbon build-up in the exhaust elbow.
Goes without saying, check the specific gravity of your coolant as part of our winterization routine (or once a year) using a meter appropriate for the coolant you're using.