MJM Yachts 40z Express - Manual (Page 13)

7. Know how to trouble-shoot the cooling system. Since overheating is a common problem
familiarize yourself with engine’s cooling systems: the raw water (sea-water) system, as well as
the fresh water (internal circulating coolant) system. The most likely causes of overheating
are:
* Raw-water valve closed.
* Raw-water through-hull blocked externally. Check for a plastic bag, or a clump of sea grass or
other material, covering or plugging the inlet.
* Raw-water filter / strainer clogged with sediment, sand, goo, grass, or living critters such as
barnacles, jelly fish, and algae.
* Defective or destroyed impeller in raw water pump. The impeller should be replaced every year
or two, as the rubber vanes become brittle with age and may snap off.
* Heat exchanger dirty or clogged up with sediment and other deposits. The entire raw water
cooling system should be flushed periodically to remove salt and sediment deposits.
* Exhaust elbow restricted by carbon deposits or other solids, reducing discharge of cooling water
and exhaust gases. Routinely run the boat at high RPM for several minutes to clear.
* Thermostat stuck closed; likely will require removal or replacement. (Note: Some diesel engines
operate OK - temporarily - without a thermostat.) Coolant temperature of 170-180o F is normal.
* Low level of coolant (50-50 mix of antifreeze-water). Coolant levels should be checked routinely
at the expansion tank, adding more as needed.
* Broken or slipping V-belt, which drives the water pumps and the alternator. Even a new belt
deserves re-tensioning and inspection. Suspect slippage or wear if you see dark “belt dust”
settling at the engine’s base. Belt tension is OK if pressure deflects the belt by about 1/2 inch.
* Overloading of the engine: Rope wrapped around propeller shaft, dirty bottom, fouled propeller,
or air leaks in the raw water cooling system. Cracked or collapsed hose? Hose clamps tight?
8. Know your fuel additives. When crude oil is refined as diesel fuel, it acquires additives to
reduce smoke, prevent pre-ignition (“knocking”), improve its cetane rating, etc. Few additives
further enhance fuel. Some diesel mechanics actually recommend: Biocides, such as Killem and
Biobor, lubricants, such as Lubricity and Stanadyne Performance Formula, and fuel stabilizers,
such as Sta-Bil and Pri-D. BUT 1) Follow the instructions on the container. 2) Routinely, a little bit
is better than a lot.and 3) Be guided by your mechanic’s advice.
9. Monitor for exhaust leaks. From time to time, when the engine is operating, inspect the
complete exhaust system from the engine to the through-hull and overboard discharge. Look for
leaks, both exhaust (air) and water. Major leaks will be obvious, but early signs of leaks due to
hairline cracks in hoses and water pot muffler systems may not be. Diesel exhaust contains acidic
sulfur and other gasses that may poison the air within the boat. To detect air leaks, look for tell-
tale traces of black soot. Water leaks should also be immediately repaired. Leaks never resolve
spontaneously; they must be addressed as quickly as possible.
10. Properly dispose of hazardous waste. Be sure to properly dispose of used coolant, used
engine oil and transmission fluid, contaminated fuel, old filters soaked with fuel and oil, etc. These
fluids are generally poisonous to people, pets, wildlife, and the environment, and some of them
can be recycled.
Edited from an article by Captain Bernie Weiss at www.AtlanticYachtDelivery.com.