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Trouble-shooting an overheating boat engine

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Question: I have a 1990 Carver Yacht with two 375-hp Caterpillar 3208 diesels. The starboard engine works fine, but at 2000 rpm the port engine overheats. Once the temperature hits 200F, I slow down to prevent damage and everything returns to normal.

So far my mechanic has changed the engine’s raw-water pump (after changing the impeller twice); added new belts; vetted water pressure within the cooling system; checked the heat exchangers; and pressure-tested the cylinders.

Since none of this has helped, he’s now proposing I flush the cooling system with de-scaler; install scoop-type exterior strainers to improve on-plane water flow; and thoroughly scrutinize the port prop and transmission.

The issue may be an old one, incidentally. I bought my boat “as is” some time ago, and during the pre-purchase sea trial, my mechanic noted that the temperature alarm on the port engine was disconnected. I missed the importance of this at the time.
—Carlos Lozano
Sunny Isles Beach, Florida

Professor Diesel: Overheating can arise from ancillaries as well as an engine. Start by examining your port engine’s mixing elbow, exhaust hose, and muffler. If any of these are restricted, exhaust buildup will affect performance. Hoses should be changed every ten to 12 years, depending on engine hours.

Examine the drivetrain next. The tolerance between the prop-shaft coupling and the coupling on the transmission should be between .004 and .006 inches. Moreover, make sure the shaft is not bent, examine the strut bearing and stuffing box for excessive wear, and make sure the transmission has the correct gear ratio and a shift lever that moves fully forward when engaged.

And finally, consider these questions. Does the port prop have the right pitch and diameter? Are there barnacles in the port engine’s seacock? Is air entering somehow? Are there loose connections or blockages? And is there a non-standard alternator or power-steering pump? Non-standards can affect performance.

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


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