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The Boat Doctor gets schooled

2017 October 1

Dear Boat Doctor,

At the tail end of a recent cruise, I went to start the engine and heard the distinct thud of a dead battery. But when I checked the batteries, all was well. I tried the starter again and, looking into the engine compartment, I saw the engine crank about a half-inch and stop hard. What’s going on and how do I fix it? 

The Boat Doctor

Dear readers,

It’s true: The Boat Doctor himself gets schooled from time to time and this is exactly the situation I sheepishly found myself in. Rather than brush it under the rug I thought I’d fess up and share my experience with you.

This felt like a classic hydrolocked engine, where water comes up through the exhaust or raw water intake, fills the muffler and flows water into the engine. The water finds its way through an open exhaust valve into a cylinder. Water, or any liquid is not compressible, so when the engine tries to crank it is stopped hard.

This most typically happens when an engine won’t start but the operator keeps cranking. The cranking pumps cooling water into the muffler but there is not exhaust to blow it out. It fills up and fills a cylinder. It can also happen if particularly violent seas are driven into the exhaust. But my boat was in a slip for more than a day and ran just fine when it went in.

I checked the oil with the dipstick and the level looked good, so if any water got in there, it wasn’t much. The water would be laying in the bottom of the pan, since it is denser than the oil. I could easier pull it out with my oil change pump. To prevent any more water from entering I closed the strainer through-hull.

On my engine raw water is pumped through the heat exchanger, through a vented loop and is then injected into the mixing elbow on the engine. If the vent on the vented loop was clogged, it could setup a siphon and fill the muffler. I pulled it off and it felt a little sticky, I found the culprit. I rinsed it well with warm soapy water and reinstalled it.

We had been experiencing a nasty surge in the harbor, and apparently that was enough for the water to slowly make its way over the loop and fill up the muffler.

Getting the water out was another matter. A local Yanmar mechanic was onboard early the next morning and we pulled the injectors and got the water out. One cylinder had maybe a quarter cup of water in it. From there, we drained the muffler, it was full and there was water up to the mixing elbow. Next, we pulled out a little oil through the dipstick, we got maybe a quarter cup of water out of the pan before we got a flow of clean oil. A quick spin with the starter, after covering the injector ports with a rag, got any residual water out. Finally, we reinstalled the injectors and bled them.

Thankfully the engine ran fine. A quick freshwater hydrolock doesn’t do any real damage itself, but trying to crank the engine when it is locked can.

Just to be careful I ran the engine up to temperature and changed the oil, I wanted to be sure I got all the moisture out. Since oil is relatively cheap, I changed it again after I got back home.

Here are my takeaways from this snafu:

1. Siphon break valves need service, you should inspect and clean them at least once a year, more often in salt water.

2. I will be changing the siphon break on my engine every three years, worthwhile $20 insurance.

3. It’s a good idea to close the engine through-hull when you turn off your engine.

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