Question: I have a 1994 Mainship 31 with a pair of 315-hp Yanmar diesels with 2,000 hours on them. While running at full throttle (about 33 knots) for five minutes, the starboard engine temperature creeps up enough to set off an alarm. If I ease ‘er back to half-throttle, the temperature returns to normal after a couple of minutes. The starboard engine has always run a bit warmer than the port. What causes this, and can it have long-term effects? Frank Tew, Godwin, North Carolina
Professor Diesel: It’s not uncommon for one engine to run warmer than the other. However, the disparity should not be so significant as to set off an alarm. You’ve got a problem.
Most overheating issues occur on the seawater side of the cooling system, so start there. Begin with the seacock, making sure its flow is not restricted by barnacles, salt, or dirt. Then, make sure the seacock is fully open, and while the engine’s running, check the hose between it and the seawater pump, as well as your sea-strainer seals, for leaks. If everything’s fine, change the impeller in the seawater pump after you’ve examined the pump for wear—you may have to repair or rebuild it. Next examine the exhaust-mixing elbow for restrictions. Carbon and salt can reduce water flow and boost temperatures. Because the engine has 2,000 hours on it, have the heat exchanger, oil cooler, and intercooler cleaned.
Now for the freshwater side. Make sure the drive belt on the front of the engine is tight and healthy. After changing the coolant (if it is more than two years old), test the pressure cap on the header tank and thermostat at a radiator shop. Finally, replace your water-temperature sensor—it may be faulty—and make sure your drivetrain isn’t causing some kind of resistance due to poor prop-shaft alignment, a stuffing-box issue, a bad strut bearing, or a prop problem.
Professor Diesel is Larry Berlin, director of Mack Boring’s Training Services division.
This article originally appeared in the October 2010 issue of Power & Motoryacht magazine.