Overheating

[Carl Thunberg]

Last week, my engine started overheating and there was noticeable white smoke. Knowing that white smoke is generally caused by steam and insufficient water flow, I immediately suspected the raw water pump impeller. Upon removal, there were some broken vanes, but none of them had broken off completely. See photos 1 and 2 below. Feeling very proud of my diagnostic skills, we went out for a ride. Within 20 minutes, the engine was overheating again. Thinking the problem could be scaling inside the heat exchanger, I bought 3 gallons of barnacle buster and let it sit for 4 hours before flushing with fresh water through an in-and-out bucket. The water flow was greatly improved and we needed to keep the hose going full blast in the bucket to keep up with the water pump. Again, feeling very proud of my diagnostic skills, we went out again with the same overheating. I then suspected the front end of the raw water cooling system. I pulled the hose off the seacock and it was clear. I opened the seacock and it was clear. I then took the intake hose off the raw water strainer and Voila! It was plugged with seaweed. See photo no. 3. It may not look like much in the bottom of a bucket, but inside a 1-1/2 inch hose and it was packed.

The moral of the story is, look for the obvious stuff first, not last. Granted, the water pump impeller was damaged and it's good to replace it before a vane breaks off and finds its way into the heat exchanger, and de-scaling was probably good preventive maintenance, but they didn't get me to the solution.

[Jim Walsh]

Nice detective work. That impeller was an accident waiting to happen. Thankfully you’re not fishing pieces out of your heat exchanger.

[gonesail]

the impeller may have disintegrated due to the plugged intake hose .. but it looked pretty old. did you notice a problem with exhaust flow when the overheating occurred? good diagnosis with the white smoke.

[Carl Thunberg]

I knew by the white smoke that I had reduced water flow, so I didn't look over the stern to verify what I already knew. I was more concerned about reducing speed to the point where the operating temperature dropped down to normal and getting myself home where I could start working on it. I can't say one way or another whether the vanes broke due to running at reduced water flow or if they were broken beforehand. I will say that the engine runs cooler now than before, so I suspect the vanes may have been broken for a while. This is my first full season with Loon, so I have no idea when the impeller was changed last. It certainly does look old.

[gonesail]

i had white smoke once. turned out to be the infamous yanmar mixing elbow. easy fix.