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Why won’t my overheating alarm stop?

2009 July 1
Dear Boat Doctor,
I have a 1980 C&C 36, powered by a Yanmar 3QM30H (raw-water -cooled) engine.?I use a Martec folding propeller size RH18D x 12P.?My problem is the "overheating alarm" of the engine.?This has been ongoing for over 10 years now and I have learned to live with it, but would like your opinion as to the cause.

I have checked to ensure that the raw water flow is adequate, no obstructions in the line. I have changed the pump impeller and even the brass cam within the pump.?I have played with various scenarios with the thermostat.?There are two openings for a thermostat within the engine.?The Yanmar shop manual shows only one thermostat being used and its placement.?I have run the engine with two thermostats, ran it with one as shown in the manual, ran it with the thermostat in the other location, and also ran it without any thermostats.?All to no avail; still the overheating alarm will sound.?Here is the best part, we live in the south and this situation only occurs when the ocean water temperature is over 80 degrees.?Below this temperature the alarm will not sound.

When the overheating alarm sounds and the idiot light comes on, I do not know what the temperature of the engine is, as I do not have a gauge.?The manual states that the high temperature alarm (water temp switch) is set to go off at 140 degrees +/- 35 degrees.?I have been running the engine with the alarm sounding and it does not seem to be a real problem.?I have good water flow out of the engine exhaust, it is very warm but you can hold your hand in the stream.?And there is a slight amount of steam from the exhaust.

Gary Klaybor
Florence, South Carolina

Dear Gary,
It sounds like you have thoroughly researched this problem, but I have a few other ideas for you. Your cooling problem is right at the edge of the specifications, so you'll need to look closely for a problem.

First off, raw-cooled engines run cooler than those with a heat exchanger. The thermostat and overheat switch are matched to that lower temperature. If you run the engine at a higher temperature, the minerals in the water will have a tendency to precipitate out and clog the cooling system.

You mentioned that you serviced the water pump, have you checked that the seawater strainer and seacocks are clean and free? I'd also check the exhaust mixing elbow, as it could be partially clogged.

Your hoses are likely almost 30 years old and they could be getting clogged or even delaminating on the inner wall. I'd inspect a few of them.

Finally, the engine could be scaled with lime and salt, and these deposits act as insulators. There is great product that will clean the deposits from the engine, it is called Rydlyme Marine (www.rydlymemarine.com, 800-451-6291). You just need to remove the thermostat, remove both the intake and output hoses (and extend them above the engine if necessary), and fill the cooling system with Rydlyme per the instructions. A few hours later your engine will be clean and will likely cool better.

You can add a temperature sender and temperature gauge to your engine. The simplest way to install the sender is to tee off of the port where the over temperature switch is now installed, with bronze plumbing parts.

Hopefully some of these suggestions will solve your problem and can provide some respite from that "buzzing" in your ears.