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Why is there water in the bilge when I run the engine?

2023 January 1

Dear Boat Doctor,

Every time I run my engine, I end up with a little water in the bilge under the engine. The longer it runs, the more water I get. I have looked for leaks and can’t find the problem. What’s causing this and how do I fix it?

Tanya Anderson

Fort Lauderdale, Florida

Dear Tanya,

This is an interesting problem, with a number of potential causes, so let’s try to zero in on the problem. I will assume your engine pan is isolated from the rest of the bilge, so the water must be coming from the engine. 

You have probably done this, but make sure to look for leaks and double check every hose clamps. Check these with the engine both warm and cold. Hose clamps can be tough to tighten with a screwdriver,  so a nut driver or even a quarter-inch ratchet can help make sure they are properly tightened.

With that checked off the list, let’s review the water path on the engine. Water comes in from the raw water strainer, through the raw water pump, to the heat exchanger. Warm water then exits the heat exchanger, goes through a vented loop, and enters the exhaust mixing elbow. Water and exhaust mix in the elbow and go out the exhaust via the muffler and potentially another vented loop.  

A note of caution, if you are leaking exhaust water, you are likely also leaking exhaust, which is very dangerous. The carbon monoxide in diesel exhaust can kill you or at least give you a serious headache.

The condition of the water in the engine pan can give you some clues. If the water is cool and clean, the leak is before the mixing elbow. If the water is warm and dirty, the leak is after the mixing elbow. Once water gets to the mixing elbow, it will be warmed by the engine and gets dirty with exhaust soot. 

My money is on one of the vented loops being the problem. The vented loops stop siphon action that can flood the engine with seawater. They prevent the siphon with a one-way valve, designed to prevent air or water to pass out of the loop, but to allow air into the loop to break the siphon. When the valves get dirty, they can stick open and leak. The leak can range from a fine mist or a drop to a full-scale leak.

All vented loops are serviceable. Simply unscrew the cap and you’ll find a valve inside. Often the valve is a little rubber duckbill, others are a mechanical valve with a spring and plunger. Remove the valve and soak it in vinegar and warm water. The vinegar will dissolve any mineral deposits. If the valve is in bad shape, contact the manufacturer for a replacement part.