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How do I diagnose my engine problems?

2019 November 1

Dear Boat Doctor,

I am having some problems with my Yanmar 4JH2DTE, and I’m embarrassed to say I may have created one of the problems myself. I installed a new Balmar alternator and an AltMount serpentine belt kit. The whole kit seemed to fit well, but I have a high-pitched squeal that I can’t quite identify.

I also seem to get roughly a quart of water in my engine drip pan for every hour of run time. I have looked for the leak, but I just can’t seem to find it. How do I isolate and solve these problems?

John Abramson

San Diego, California

Dear John,

You’ve got two potentially serious problems on your hands. The squeak could be a few different things: a failing bearing in the water pump or alternator, an overtightened belt that is putting a lot of side load on the pulleys or a belt alignment problem.

The belt should be snug but not tight. Aim for about ¾-inch of slack in the longest belt run. Check the tension, and then check the engine for squeaks if you make an adjustment.

If this doesn’t help, look at belt alignment. Serpentine belts are sensitive to belt alignment, and all of your pulleys must be on the same plane or at least within a millimeter of each other. Lay a 24-inch straight edge against the pulley faces and see how things line up. The AltMount kit should have the crank pulley and water pump aligned, but your alternator is aligned with washers on the saddle bolt.

If you still have a problem, it is probably a bearing in the alternator or water pump. A great way to tell which device is squeaking is  to carefully hold a large screwdriver handle to your ear and touch the blade on the alternator and then the water pump. If either is squeaking, the sound will be transmitted through the screwdriver.

The water in the engine pan is more straightforward to find. There are two possible sources for the water: the raw water cooling system and the exhaust system, or some combination of the two.

If the water in the pan seems clean, this would imply the cooling system is leaking. The water comes into the boat through the strainer, into the raw water pump, through the heat exchanger and then into the exhaust mixing elbow. I would check all the hose connections, and watch the hoses and pump very carefully when the engine is running, just a small drip could collect the amount of water you are seeing.

If the water is dirtier, it is likely an exhaust leak. Your wet exhaust starts at the mixing elbow, continues in the exhaust hose to the muffler and then out of the boat. Again, check all the hoses, the mixing elbow and the muffler itself. 

When you are looking for leaks, run the engine at cruising speed to get the same pressure in the water systems as when you are operating, the leak may not happen at idle speed.