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The Truth Be Told

Five common myths about your marine diesel.

Some years ago Power & Motoryacht conducted a series of focus groups with our readers in order to get a better idea of who they are and what they were looking for in a boating magazine. We got a number of interesting revelations, including one in which half of the respondents thought we should show more pretty women on the cover and the other half thought we should not.

Mangusta 94 engine room

When it comes to your boat’s engines don’t believe everything you hear.

Fortunately, we were able to glean other, more useful insights, particularly on the subject of how you keep up with changes in systems, technology, and skills. When we asked you to name your main source of information concerning these subjects the most frequent answer was, as we suspected, boating publications. But a close second was talking to fellow boaters. This came as no particular surprise—and I’m sure it doesn’t to you—but what we didn’t expect to learn was that when there’s a conflict between what you read in a publication and what you hear from fellow boaters, a sizable minority of you will side with what you hear on the docks. 

Most of what I’ve learned from my fellow boaters has been valuable, but every once in a while someone tries to pass off something that’s totally bogus. My particular area of expertise is engines, so I’m pretty sensitive about pontifications on that subject. And year upon year I keep hearing five particularly egregious fallacies making their way through the boating community. 

1. It’s okay to let your diesel idle for extended periods because it burns so little fuel. 

This one is not restricted to mariners. I’ve seen lots of diesel pickup truck owners leave their engines running while they’re absent. The genesis, I think, is from seeing unoccupied long-haul trucks with their engines running, and the practice does have some basis in fact. Diesels do burn very little fuel at idle because unlike gasoline engines, their throttles do not restrict the amount of air they ingest. As for truckers, the practice seems to have stemmed from the need to keep diesel fuel from getting too cold in winter and gelling. In any case, letting your diesel idle for anything more than a short duration is a bad idea because while the engine will use little fuel, what fuel it does burn will not combust completely because the operating temperature is too low. Unlike gasoline engines, diesels need to be under load to reach optimum operating temperature—if they’re not, unburned fuel can cause needless pollution and even can dilute the lubricating oil, increasing wear. Plus, every time a piston travels up and down a cylinder, the rings and cylinder walls wear just a little bit. The best rule is, if you’re not underway, turn off the engines.

2. You should let your engine idle for a few minutes to warm up before getting underway. 

As noted above, a diesel will not warm to operating temperature until it is under load. Thus you need only let the engine idle long enough to fully circulate the oil—30 seconds is plenty. But don’t immediately put the pedal to the metal. A few minutes of idle speed will warm the oil so it flows better. 

3. When shutting down, it’s a good idea to goose the throttle once or twice before turning off the engine.

I have no idea where this one comes from, but I suspect it has something to do with watching Harley owners goose their engines whenever they get a chance. I’ve heard some boaters aver that doing this makes sure there’s a good supply of oil throughout the engine before it shuts down. Poppycock. An idling diesel circulates more than enough lube to keep everything well oiled, and revving the engine then shutting if off can leave unburned fuel in the combustion chambers and even starve the turbocharger bearing, because the turbo will keep spinning after the oil supply dies. And again, this practice wastes fuel.

4. Coolant temperature is the best way to measure whether a diesel engine is fully warmed up. 

This one’s obviously derived from our experience with automobiles, and it’s not really a critical error unless you’re one who can’t wait to mash the throttles. Because a diesel needs a load to warm to full operating temperature, coolant can reach 180 degrees Fahrenheit while the lube oil is not at optimum viscosity. Not all boats have oil-temperature gauges; if yours does not, play it safe and bring your engines up to speed slowly, even if your coolant temperature is where it’s supposed to be.

5. Most of the wear incurred by a marine engine takes place at high rpm. 

This certainly seems logical. Running an engine at WOT has to be harder on it than running it at idle, no? But in truth, the highest wear rates take place at startup, especially if you’re imprudent. Lube oil is designed to cling to interior components, but the film is micron-thin. And once an engine is running, it takes a few moments for the cold oil—even modern multi-viscosity oil—to flow freely. That’s why commercial diesels have block heaters and pumps that circulate lube oil before the engine starts. If your engines have neither, your best option is to give them 30 seconds to a minute before you advance the throttles.

The modern marine diesel is a pretty durable piece of machinery, and it can no doubt withstand any or all of these bad practices. But eliminating them will help yours live longer and run stronger—no matter what your dockside pals say.

Want to learn even more? The diesel engine is the beating heart of your boat. Arguably, a good running engine is your most important single piece of safety gear. Diesel engines are economical, long-lived and reliable, yet there are a number of ways in which they can fail. This seeming contradiction brings us to the distinction that has shaped the Online Marine Diesel Maintenance & Troubleshooting course.
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This article originally appeared in the September 2014 issue of Power & Motoryacht magazine.