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Vintage Yanmars Running Hot

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Question: I have a 29 Phoenix with twin, 1984-vintage, 170-hp Yanmar diesels with 1,000 hours on them. While the boat seems to run fine, I have one issue. During operation, the temperature of each engine rises normally at first but then keeps on going, not to the point of setting off alarms, but close. Pegging the engines (i.e., throttling them up to 3200 rpm) intensifies the problem.

So far, I have replaced all exhaust elbows, turbos, and heat exchangers. Moreover, I have cleaned the oil coolers, flushed the heat exchangers, and replaced the water pumps. All this improved things but the problem remains. The engines run cool (180F or thereabouts) up to 2600 rpm, get warmer (about 190F) at 3000 rpm, and run very hot (approximately 210F) above that.

The seacocks are one inch in diameter with 1.24" I.D. hoses. Do I need larger ones?

—Joe Petner
Lexington, Massachusetts

Professor Diesel: First confirm your gauges are accurate. Borrow or purchase a laser-type heat-detecting tool—they cost about $40 and are available from Sears and other stores. Measure the temperatures at the back of each cylinder head (normally the hottest area of an operating engine) while underway.

If this test confirms gauge accuracy, check the props. Full-load speed for your Yanmars should be 3300 rpm in gear under way. With no load (in neutral) and at normal operating temperatures, momentary full-throttle speed should be about 3425 rpm. However your boat should be propped to run 3400 rpm under load to compensate weight and drag. If your props are too large (in diameter) or over-pitched, your engines will not run up to 3300 rpm, and the props are probably the problem.

Yanmar requires your seacocks, through-hull fittings, sea strainers, and hoses to have a minimum diameter of 1.25". So you’re probably okay. But check your muffler and exhaust hoses—they should have a minimum diameter of 4" and as few bends as possible. And no 90 turns!

Professor Diesel is Larry Berlin, director of Mack Boring’s Training Services division.


This article originally appeared in the June 2011 issue of Power & Motoryacht magazine.