We have been cruising on our 1991 35.5 Legend for the past 5 years. This summer we took our boat from Lake Champlain NY, our usual cruising ground, down to NY City, Cape Cod across the Long Island sound, north to Candem Maine and back to our home base. In 8 weeks we covered around 1500 nautical milles, adding 225 hours to our Yanmar 3GM30F. We had very few mechanical problem. One Water pump belt to replace as preventive maintenance, one oil change and a newly discovered overheating problem above 2800 rpm. In the past we never had to motor at 2800 rpm or above as our boat speed, with our fixed 3 blade props at 2600 rpm is 6.4 to 6.5 knots. In this trip however, in few occasions, we had to push the motor a bit more. This weekend, I decided to tackle the problem. After reading the archives on this site, Torresen Marine Diesel Forums and visiting Yanmarhelp.com I started examinng all possible causes and checking both, the raw water system and the coolant circuit. All the usual was in great shape, hoses, seacock, strainer. I then changed the Impeller, drained the coolant, opened both ends of the heat exchanger to make sure all the cooling tubes were perfectly clear with no deposit . I was ready to take the tube assembly out for cleaning but did not have to as the tubes were very clean. I had never drained the coolant in 5 years! To my surprise (maybe I should not have been surprised after all) there was a lot of brown sludge in the top portion of the aluminiun tube in the heat exchanger now visible under the fill cap. I knew I had to clean this stuff. I used a bottle of Prestone super flush (safe for all metals) and followed the instructions on the label. So much brown debris came out when I drained the stuff after a heat / cool cycle that I decide to redo the treatment a second time. It is easy to do but takes a long time as you have to make sure the motor comes to working temperature and then cools down before flusing the stuff out. After 2 Prestone flush and a clear water flush, I refilled the coolant with a mixture of Dexcool antifreeze and left the dock for the last time for a test run. I am glad to report that the problem is totally resolved. I can now run the motor for as long as I want at 3400 rpm with no problem. I wanted to share this with all as overheating is a problem often discussed in forums. I am the living proof that systematic checking done by a non mechanic guy can resolve the problem. I plan to change the mixing elbow later this fall when the boat will be out of the water. Changing the coolant and checking for brown deposits will now be part of my regular maintenance. Hope this might help other owners with the same problem.
