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Where is coolant leaking from my engine?

2009 January 2
Dear Boat Doctor,
I have a 2003 Jeanneau 45.2 powered by a 76-horsepower Yanmar. It has leaked coolant from somewhere on the left side at a rate of quarter-to half-teaspoon an hour, since the engine was new.

I replaced the pressure cap and tightened all the hose clamps. The impeller is checked during every service and the engine has 1,100 hours on the original impeller.

Next spring I will change the coolant. I will also change the impeller whether it needs it or not. Going forward I willnot checkevery service butchange the impeller every five years and keep a watchful eye on the engine alarms and exhaust raw water.
Do you have any suggestions or ideas where my coolant may be leaking from?

Ed Life
Vancouver, British Columbia

Dear Ed,
I have a couple ideas and some advice for you. Cooling problems can be frustrating, but the system overall is pretty simple.
There are great test tools available to help you find your leak. Any auto parts store can supply you with a cooling system pressure test kit, and many locations will allow you to use the tools free of charge. The kit consists of a pressure pump and gauge, and adapters to attach the pump to the pressure cap or the engine itself.

Start by attaching the pump to the engine and pump it up to maybe 15 pounds. I'd leave the engine pressurized for a few hours, and I am confident that you'll see the gauge drop down in pressure after that amount of time. This does not directly give you the answer, but it will confirm that you have a leak somewhere in the cooling system.

If you saw pressure loss, I would pump the engine back up to 15 pounds or so and start to look for the leak. I'd look closely around any hose connections and at the drain petcocks. You should see a drip of coolant after a short time based on the leak you describe.
If you do not see a pressure drop, you may have a temperature-related leak (the engine only leaks when it is hot) or something vibration related. It is possible that you could have a leak in the pressure cap assembly or the overflow system.

While you have the test kit, you can also test the pressure cap. Yanmar caps are 13 pound, meaning the cap opens at 13 pounds of pressure. You will pump up the cap and it should release pressure at 13 pounds.

I think it is wise to replace your impeller every year or two; maybe every 100 to 200 hours. If an impeller fails in use, it can be a complicated problem. This problem hardly ever happens at the dock, so you'll be resolving it in a seaway. When the impeller fails, parts of it may get into the heat exchanger or passages to it. You need to retrieve all these little pieces or risk a cooling system blockage or restriction. Being able to change the impeller on your own terms and while it is still intact is well worth the cost of the parts and the 20 minutes that it takes to perform.

I would recommend an extended-life coolant designed for use in diesel engines. Yanmar markets its own coolant, and I would also recommend Texaco Extended Life Coolant. You'll notice both of these coolants are deep pink in color. When changing out your existing coolant it is important to remove the old coolant and then flush the system thoroughly with fresh water.