7. Know how to troubleshoot the cooling system.
Since overheating is a common problem with diesel engines, you should familiarize yourself with the engine’s cooling system—both the raw-water and the freshwater sides. Frequently, a simple do-it-yourself repair or replacement from a spare-parts kit will save you both time and money.
When troubleshooting an overheating problem, as Yogi Berra once put it, “You can observe a lot just by looking.” When an engine overheats, I first check the raw-water overboard discharge.
If there is no flow or only a dribble, the most likely causes of overheating are:
• The raw-water intake seacock is closed.
• The raw-water intake is blocked externally. Check for a plastic bag, a clump of sea grass, or other material covering or plugging the inlet.
• The raw-water filter or strainer is clogged with sediment, sand, goo, grass, or living critters such as jellyfish and algae.
If there is flow, but it is diminished, consider these likely causes:
• Damaged impeller vanes in the raw-water pump. If the impeller is completely destroyed, there will be no flow. The impeller should be replaced every year or two, as the vanes become brittle with age and may snap off. Also, the vane tips may wear unevenly or take on a permanent set, degrading the impeller’s function.
• The heat exchanger is dirty or clogged up with sediment and other deposits. Sometimes removing the end cap of a heat exchanger will reveal the cause of overheating. The entire raw-water cooling system should be flushed periodically to remove salt and sediment deposits. Sometimes it will be necessary to remove a clogged heat exchanger so that it can be “boiled out” with an acid bath to purge its water channels and restore it to like-new condition.
• The exhaust injection elbow is clogged with carbon deposits or other solids, reducing the flow of cooling water and exhaust gases. Routinely running the engine at high or moderate rpm under load, as opposed to long periods of idling at low speeds, will help avert this problem.
If the flow appears normal, then you should suspect one of these common causes:
• The thermostat is stuck closed and likely should be removed or replaced. (Note: Some diesel engines will operate just fine—temporarily—without a thermostat.) It’s worth remembering that diesel engines are most efficient running at high temperatures: a coolant temperature of 180oF is not uncommon.
• The coolant level is low (this should be a 50:50 mix of antifreeze and water). Coolant levels should be checked routinely, and more coolant should be added as needed. Caution: Remove the cap to the reservoir only after it has cooled to the touch. When replacing the cap, be certain it is closed snugly and seals the system. Frequent replenishment of the coolant suggests a leak and requires further investigation.
• A V-belt driving the water pump and alternator is broken or slipping. Keep a spare belt on board. Even a new belt needs retensioning sometimes and should be regularly inspected for wear. Expect to find some slippage or excessive wear if you see fine dark “belt dust” settling around the engine mounts or at the engine’s base near the belt. To check belt tension, apply thumb pressure midway along the longest belt run; tension is okay if the belt deflects about 1/2 inch.
• The engine is overloaded: A rope may be wrapped around the propeller shaft, the boat’s bottom may be foul, the propeller could be fouled, and so on.
Also, check for air leaks in the raw-water cooling system. Has a hose cracked or collapsed? Are the hose clamps tight?
8. Know your fuel additives.
After diesel fuel is refined from crude oil, it is modified with additives to reduce smoke, prevent preignition (or “knocking”), improve its cetane rating, and so on. Few aftermarket additives will further enhance the fuel, but there are some exceptions. Many diesel mechanics recommend the following:
• Biocides, such as Killem and Biobor. These products kill bacteria, fungi, algae, and other microbial life in your fuel tank. They will prevent sludge from contaminating the tank, clogging up fuel filters, and blocking fuel lines.
• Lubricants, such as Lubricity and Stanadyne Performance Formula. Lubricants prolong the life of seals and rings in the engine. Modern diesel fuels with reduced sulfur content—sulfur acts as a lubricant—may be improved by such an additive.
• fuel stabilizers, such as Sta-Bil and Pri-D. These additives prevent fuel from undergoing degradation and oxidation during prolonged storage (as on a winterized boat). But if diesel fuel has been aboard a boat undisturbed in storage for more than a year or so, a mechanic will view it with suspicion; frequently he or she will recommend that the fuel be replaced before starting the engine.
Two precautions when using fuel additives: (1) Follow the instructions on the container. (2) A little bit is better than a lot.
9. Monitor for exhaust leaks.
From time to time, when the engine is operating, inspect the entire exhaust run from the engine to the overboard discharge. Look for leaks, both gas 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 readily apparent. Diesel exhaust contains acidic sulfur and other gases that may poison the air within the boat and, over time, may cause nearby metals to corrode. To detect air leaks, look for telltale traces of black soot. Water leaks should also be immediately repaired. Leaks never get better on their own; they must be addressed as quickly as possible.
10. Read and understand the owner’s manual that came with the engine.
In the case of a new boat, this will be part of the package of literature that is passed to you by the dealer. If you have an older boat, request a manual from the engine manufacturer. If the engine is old and no longer in production, a search on the Internet will often prove fruitful. At the very least, a manual will tell you the recommended intervals between oil changes, the type of lubricants to use, and any other salient information. Better yet, get hold of the engine’s workshop manual. Consider going to a diesel-engine course. Many manufacturers offer one- and two-day hands-on courses that can be invaluable in helping you maintain your engine in top condition.
Learning how to operate and maintain your marine diesel engine properly is not difficult. With just a little knowledge and skill and a simple tool kit, you can save yourself both time and money. Frequently, evaluating a problem is no