mjm yachts 50z Downeast, Owner'S Manual

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CHAPTER 3
INTRO - THE TOP 10 CAUSES OF ENGINE FAILURE
It doesn’t happen often and if you’re familiar with the most common causes of engine failure you can cut
down on the chances of a breakdown. As an introductory to this chapter, we want to familiarize you with
this list of causes, compiled by
Motorboating Magazine Here are the Top Ten to be aware of:
1. NO FUEL : This is probably less of a problem on a fuel-efficient MJM than on other boats, but lack of
owner attention to fuel consumption is the primary culprit for engine failure. A boat’s fuel tank can be
nearly dry as a bone – even when the gauge claims there’s a 1/4 of a tank left. This makes sense when
you realize that at cruising speed, with fuel pushed aft, the gauge shows the tanks reading higher than
when the boat is at rest. A good rule of thumb is to never pass a fuel dock (no matter what the price) if your
gauge is showing less than 1/3 full.
1b. AIR IN FUEL LINE: If air gets drawn into the fuel lines because of either a small leak in a fuel line
connection or the Racor Filter lid gasket/filter basket tabs have interfered with the lid being secured fully,
you may find the engine will turn over, but won’t start. Check the Racor to insure the fuel level is within
an inch of the top. Check the engine owner manual for the location of a manual primer pump.
1c. COMPUTER SETTING : On the Yanmar electronic engines, we’ve encountered several instances
where after shutting down the engine for several hours (on a picnic), it was only possible to start the
engine after many tries or not at all. The problem was that the setting that determines the amount of fuel
to be injected into the engine upon starting was not set
2. DIRTY FUEL: Engine problems are caused by dirt and water in the fuel. Your engine RPMs may
become erratic. Debris, stirred up from the bottom of the tank by wave action, is drawn into the fuel line
and clogs the fuel filter element. Starved for fuel, the engine begins to run poorly, and then not at all.
Water in the fuel can drive you mad. Moisture condenses out of the highly humid air on the inside walls
of a fuel tank, then runs down into the fuel. Water can also be introduced at the fuel dock from a
contaminated fuel supply. Fuel floats on top of water and the fuel pick ups are near the bottom of the
tank. A fuel/water separator protects against this by handily extracting the water. Check the bowl daily
and drain off the accumulated water. For severe contamination, or arrange to have the fuel “polished” by a
competent marine diesel service.
3. FUEL BUGS: Diesel engines suffer from microbial bugs growing in the fuel. If left unchecked, these
critters clog filters. If you leave the same diesel fuel in the tank for any length of time, a fuel conditioner
similar to that supplied with your boat by the builder will kill the bugs and break up any hydrocarbon
residue into particles that will burn completely in the combustion process.
4. TIRED/DAMAGED WATER PUMP IMPELLER: As boats age or if
an engine isn’t operated for a long period of time, a worn-out circulating
water pump is another engine killer. Impeller blades are commonly
made of a rubberized material that stiffens or distorts over time and can
break off entirely, reducing coolant flow and clogging the heat
exchanger. Periodic engine maintenance procedures can prevent this
problem. Spares are provided. Shown at right is an impeller that would
have soon failed. It was discovered, then replaced during the 50 hour
inspection on a 29z that had not been run for 11 months. Another cause
for impeller disintegration is running the engine with the raw water intake
shut off. By the time that the overheating is discovered and you shut down the engine, the impeller may
already have been destroyed or damaged. This happened on a 34z when the operator forgot to be sure
that the raw water intake valve was in the proper position.
5. HARD HOSE: Another issue to be concerned about with older boats. As water intake hoses age,
they lose their resiliency and collapse under suction, causing a restriction in the flow of engine coolant.
This results in over-heating. Prevention is easy: Visually inspect cooling hoses and squeeze them to be
sure they retain shape and set.
PROPULSION SYSTEM