Replacing the Transmission Oil Cooler

Engine Check Surprise

Have you ever been doing an engine check just an hour before you were expecting guests to join you for a day on the boat, only to find water in the transmission oil? I was recently caught in this predicament.

Prior to leaving the slip I always perform a full engine room check. This includes checking all the fluids on our Yanmar 6LY3 engines. Recently, when I pulled the dipstick on the port transmission, I noticed it looked milky. This is an indication that there is water in the oil. The transmission holds 1 gallon of oil and has its own oil cooler in a closed-loop system.

Performing pre-departure engine check

Water in the transmission meant that the oil cooler had failed and needed replacing. I chatted with a couple of experienced boaters who assured me that replacing the oil cooler was as easy as it looked. There are only four connections, water in and water out, oil in and oil out. Simple and straightforward.

I ordered a new one from While waiting for the new one to arrive I drained the watery oil from the transmission. Then I filled the transmission with some fresh oil and a full can of Sea Foam. This works to liquefy harmful residue and deposits, as well as, help to separate water from the oil.

Taking my time, I removed the damaged cooler and had the new cooler installed in a couple of hours. I then drained the oil and seafoam mixture from the transmission and replaced it with a gallon of fresh oil.

Time to test my work! With the boat tied up in the slip, a little more securely than usual, it was time to run the engines and put the transmission into gear to circulate the oil within the transmission. 

After warming the engines and circulating the fresh oil, I then drained it again and put in one more gallon of fresh oil. I also changed the oil in the starboard engine so as to keep them both be on the same timetable.

All in all, this was a fairly straightforward repair. This was yet another new experience as the mechanic of our Floating Home. I’d call it a complete success!

This video shows my repair work:

3 thoughts on “Replacing the Transmission Oil Cooler”

    1. HAHA! It was a little untamed with the heat and my headlamp! Longer hair, but always a trimmed beard! Miss you brother.

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