Systems+Engines

Ten things diesel mechanics think every boatowner should know Page 2

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3. Don’t run your fuel tank dry.

Know your engine’s burn rate (gallons consumed per hour) and your fuel capacity so you can accurately calculate the boat’s range between fill-ups. Do not delay refueling to the point where you have expended nearly all the fuel in your tank. The last 20 percent should be held in reserve. Sucking the last few gallons of fuel from the bottom of the tank increases the chance of pulling water, sludge, and other contaminants—perhaps even air—into the fuel lines. If you’re not sure you have enough range to cruise comfortably between fuel stops, be conservative: Acquire some jerry jugs and take extra fuel with you. You can never have too much fuel, unless the boat is on fire.

4. Know how to bleed air out of your fuel system

Air locks in diesel fuel systems are a fact of life. A typical fuel system has a lift pump (a vacuum pump) that lifts or sucks fuel out of the tank, draws it through the pump, then sends it to the filters and injectors, where an injector pump sends it to the individual cylinders for combustion. Whenever you open the fuel line between the tank and the engine (for example, to change a filter element), air enters the line. Air may also be sucked into the fuel line through cracked seals and gaskets, poorly fitted connectors and clamps, via the pick-up tube in the fuel tank, and so on. This air must be removed, because even a tiny bubble will block the

flow of fuel, which will prevent the engine from starting or will cause it to stop if it is already running.

To clear your fuel line of air, you must “bleed” it out if your engine doesn’t have a self-bleeding feature or it can’t cope with the air. Consult your engine’s manual to identify the appropriate bleeding points; paint them with nail polish so you can find them easily. Given decent access to the engine, bleeding or venting air is a simple procedure that everyone should be able to perform. Use the engine manual to teach yourself how to do this.

While on the subject of air: When running at any speed, diesel engines require an enormous volume of clean air to support combustion. This air is drawn into the engine through a filter. Periodically, that filter should be cleaned with solvent or with kerosene if it’s a metal-mesh filter, or replaced if it is paper or fabric. Even a small reduction in the free flow of air will dramatically reduce the engine’s performance.

5. Be diligent about checking your lube oil and oil filter.

Diesel engines are rough on oil and usually require more frequent oil and oil filter changes than comparable gasoline engines. Follow the engine manual’s recommendation for service intervals—most suggest an oil and filter change after every 75 to 100 hours of operation; some recommend a change every 50 hours. Unchanged oil is probably the single greatest cause of accelerated engine wear and failure. Changing it is one of the simplest maintenance tasks to perform. When changing the lube oil, change the filter element. Carry spares on board. Between oil changes, use the dipstick to check the oil level. Top it off as necessary from an onboard supply, but do not exceed the “full” mark on the dipstick; more is NOT better.

Note that fresh lube oil is golden or honey brown in color. However, after a few minutes of circulating inside the engine, it turns black from soot, ash, acids, contaminants, and other carbon byproducts of diesel-fuel combustion. This is normal and, by itself, is not enough to warrant an oil change.

6. Minimize risks of fire.

Diesel engines vibrate a lot, and a typical marine diesel has a lot of wiring and hoses attached to it. Over time, fasteners loosen and fail and the wiring and hoses come loose. If a loose hose or wire (such as the primary wiring harness, or the power supply to your fuel pump, or a hose to the hydraulic pump) should come in contact with a hot exhaust manifold, for example, any of these could cause a fire.

From time to time, inspect your engine compartment for these potential risks. Add chafing protection, replace worn insulation, and add extra fasteners if necessary. Consider rerouting wires and hoses where appropriate.

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