MJM Yachts 40z Express - Manual (Page 12)

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To run well, a diesel engine requires only clean fuel, clean lubricating oil, coolant, and lots of air.
Below are ten important maintenance issues that diesel mechanics wish their customers knew:
1. Don’t baby the engine. Diesels don’t like to idle in neutral, or even in gear at low speeds; they
do like to work hard under load. What’s cruising RPM? Generally, 75-80% of the maximum RPM.
Excessive idling leads to gradual build-up of detrimental varnish on the cylinders, and deposits
soot and carbon on the engine’s valves and in the exhaust system, particularly at the manifold
injection elbow where raw cooling water exiting the engine mates with the exhaust gases. Run it
hard. However...after running at cruising RPM for several hours, a brief cool-down at idle speed,
with no load, is beneficial. A few minutes is enough.
2. Give your engine clean fuel. Fuel is “contaminated” when it contains water, sediment, other
solids, or biological organisms, some of which thrive in diesel fuel. To minimize contamination,
don’t store your boat for the winter or let it sit around for weeks at a time with fuel tanks only
partly full. A full tank minimizes condensation of water vapor on the tank’s interior and the growth
of micro-organisms. Fuel filters trap sediment, sludge, water and organic material and should be
changed at periodic intervals.
3. Be conservative in your estimate of fuel consumption. When under way, do not delay re-
fueling to the point where you have expended nearly all the fuel in the tank. The last 20% should
be held in reserve. To suck up the last few gallons is to chance sucking up water (tank
condensate), sludge, and other contaminants - perhaps even air - into your fuel lines.
4. Know how to vent (“bleed”) the air out of your fuel system. Air locks in diesel fuel systems
are a fact of life. The typical diesel fuel system operates with 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 the injector pump sends fuel 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, etc. This air must be
removed, because even a tiny air bubble in the fuel line will block the flow of fuel, and without
fuel, the engine will not start; if running, air in the fuel line will cause it to stop.
To clear your fuel line of air, you must vent or “bleed” it out. Consult your engine’s manual to
identify the bleeding nuts; paint them with white nail polish so you can easily find them again, in
the dark, at sea. Given decent access to the engine, bleeding or venting air is a simple procedure
that everyone should be able to perform. Using the engine manual, teach yourself how to do this.
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. Carry spares on board. Between oil
changes, use the dipstick to check the oil level. Top it off as necessary from your on-board lube
oil inventory, but do not exceed the “full” mark on the dipstick; more is NOT better.
6. Minimize risk of fire. Diesel engines vibrate a lot, and the typical marine diesel has a lot of
wiring and hoses attached to it, crossing it, behind it and near it. Over time, as the engine
vibrates, the fasteners may loosen and fail, the wiring and hoses droop or fall. One hates to see 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) cross and contact a hot exhaust manifold, for example; this could
be a prescription for fire due to abrasion of insulation around wire, or chafing through the wall of a
hose. From time to time, inspect your engine compartment for these potential risks. Add chafing
protection, replace worn insulation, and supplement the fasteners if necessary. Consider re-
routing wires and hoses where appropriate.