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Does an increase in horsepower explain a rise in exhaust temperature?

Question: I have a 1989 38-foot Bayliner with twin 175-hp Hino diesels that have been retrofitted with turbochargers, thus raising each engine to 210 horsepower. As I go from cruise to wide-open throttle (approximately 23 mph at 2800 rpm), turbo exhaust temperatures increase from 700F to 950F on the port engine and 850F on the starboard. When I cut back to 2500 rpm, the temperatures return to 700F. When I purchased my boat in 2002, the exhaust temperatures didn’t change like this. I’ve checked the cooling system, making sure the thermostats open properly and the heat exchangers are clean. I’m now running at 2500 rpm, which reduces my top speed to 15 mph.

— Roger Blanchette Torrance, CA

Professor Diesel: The rise in exhaust temperature could be normal because of the increase in horsepower. Horsepower is basically controlled heat. The more fuel and air you put into a diesel the higher the exhaust gas temperature is going to be. But the difference in the exhaust-gas temperatures between the engines may be a concern. First, I’d suggest you determine your actual engine temperatures using an infrared thermometer. This will assure that your gauges are not bad. Next, I’d closely examine the entire seawater system on the port engine. Replace the impeller on the seawater pump and make sure the pump itself, all related belts, the seacock, and sea strainer are in good working order. Also, look for excessive bends in the engine’s exhaust hose or restrictions in its exhaust-mixing elbow. And finally, bear in mind that full-load speed for most pleasureboat engines is a one-hour rating. That means under perfect conditions the engine will be able to safely run at wide-open throttle for one hour out of ten. After that hour is up, you should cut back to recommended continuous engine speed.

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

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

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