After steam started coming out of the exhaust, followed by a high temp engine alarm, It's time to start troubleshooting the raw water side of a Yanmar 2GM20F engine. I take a look at the raw water strainer, raw water pump, heat exhanger, antisiphon valve, and the exhaust mixing elbow. I don't think the problem was just one thing, but a few things compounded together. One of the splines was missing on the impellor, another was cracked, and the heat exchanger was heavily scaled and partially blocked. See more detail in our blog. diysailor.com