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Diesel engine reliability

Jan 1, 2003
From Ocean Voyager 2000

From Ocean Voyager 2000

While diesel engine reliability, and longevity, may be legendary, those facts are no accident. Diesel engines tend to be so long lived, even with little maintenance, by virtue of their simple and rugged construction. And when provided with a regular service regimen, they can provide thousands of hours of operation with few, if any, failures.

Rudolph Diesel, the man who gave his name to this engineering marvel, used to modestly refer to himself as "an iceman," in deference to his university professor, Carl von Linde. Linde, inventor of the ammonia refrigeration system, a marvel in and of itself, instilled in the young Diesel a profound understanding of high-compression phenomena. Diesel's first engine, a single-cylinder behemoth (which, incidentally, ran on gasoline vapor), made its debut in 1893, and the rest is, as they say, history. Some of Diesel's earliest engines were used by Adolphus Busch, of Budweiser fame, thereby establishing the commercial viability of the invention.

The diesels found in today's voyaging sailboats, trawlers, and Down Easters are a far cry from the Budweiser models. Advances in casting and manufacturing technology, over the past 20 years in particular, have enabled the production of compact, lightweight, efficient diesels. The horsepower-to-weight ratio continues to increase, while fuel consumption (which is a function of efficiency) continues to fall.

The degree to which any engine, diesel or otherwise, is maintained, will, however, have a profound effect on its reliability and longevity. Meticulously maintained over-the-road fleet diesels regularly achieve 500,000-plus miles before requiring overhaul. Accordingly, it doesn't take much deduction to realize that well maintained diesel engines can be expected to deliver stellar performance.

Maintenance should begin with a clean engine and engine compartment. A clean engineering space (including and especially the area beneath the engine) facilitates regular maintenance and quick, definitive location of leaks, and it reduces the need to take a bath after completing service or repairs. The most desirable type of service to be performed is that which prevents major failures. In other words, discover and prevent problems before they arise. In order to achieve this, it is necessary to develop a "relationship" with one's engine. This can be termed the "look, listen, feel" school of engine care. Make an effort to know what your engine looks, sounds, feels, and smells (never underestimate the importance of the olfactory sense during engine inspection) like when it's running properly. Oil, fuel, and coolant, for instance, all have very distinctive smells when vaporizing on a hot engine. Never ignore the warning signs your engine may be sending out. It never ceases to amaze me how often crew report that an engine alarm sounded, and was ignored, because it was believed the alarm was malfunctioning!

Down to basics

By far, and in my experience, the most common causes for diesel engine malfunctions originate in the fuel system. The incidence of problems related directly to the fuel itself are staggering. Dirt (contaminated with either biological growth or solids), water, and overall quality or lack thereof top the list. It goes without saying that getting the best-quality, cleanest, and highest-cetane (40 is the minimum recommended, but 45 to 47 is preferable) fuel is of paramount importance. Prefiltering through one of the purpose-made filtering funnels is recommended if fuel is suspect in any way, and even if it's not. Dispensing a small quantity of fuel into a clear quart or gallon container (preferably glass) prior to filling the vessel's tanks is also a method for determining quality and absence of contaminants. Good-quality diesel fuel should look translucent, roughly like honey, but certainly no darker than weak tea.

Beyond the fuel, problems encountered with the fuel system itselflines, tanks, filters, pumps (both lift and injection), injectors, and glow plugsare nearly as pervasive. Often, the chain of events is, as one might suspect, contaminated or poor-quality fuel causes one of these components to fail. A cascade effect in the fuel system will easily bring the biggest, strongest diesel engine to its knees for want of clean fuel in the right place at the right time.

Fuel tanks are a breeding ground for accumulated dirt, biomasses, and their deceased carcasses. This soup will happily reside on the bottom of the tank until the vessel enters a seaway, usually when engine power is needed most. The lively motion will stir it up, causing it to be sucked into the pickup. If it doesn't clog this tube immediately, it will travel to the filter (if you're lucky), where it will overwhelm the element(s) in short order. Clogged filters are at best a nuisance and at worst dangerous if maneuvering or crossing in front of another vessel. The best defense against this scenario is the installation of a high-quality primary fuel filter (this is the first filter the fuel reaches on its way from the tank) that possesses some of the following assets: a clear sight bowl, a filter element that can be changed without tools and requires no bleeding (and is readily obtainable), provisions for a vacuum gauge and water sensor, and a Coast Guard-approved drain cock and heat shield. If this sounds like a Racor MA series, that's no coincidence. Most sail auxiliaries will benefit from the installation of an MA500. Overkill? Perhaps. However, these attributes are found in few, if any, smaller filters. Additionally, why complain about extra filter surface area, a larger sediment bowl, or ease of maintenance? The price is well worth the performance. When purchasing, be certain to specify the USCG- and UL-approved MA version (which includes a heat shield and an approved drain cock), not the automotive FG series.

Clogged pickup tubes, on the other hand, can be downright debilitating and more difficult to repair than simply replacing a filter element. Rarely can they be cleared without disassembly and tools. In some cases, they are not even removable. Fabricating a fuel tank that does not incorporate a removable pickup tube borders on irresponsible. In this case, if the tube were to become clogged, the operator would have little recourse other than to install a new pickup (in some cases this could require tank removal or deck "modifications"), and cap off the existing one. If your tank does not have a removable pickup and an inspection port into each baffled chamber, you could be facing unpleasant difficulties down the road. Additionally, clean-out ports enable just thatcleaning out. It's difficult to properly clean out a tank or remove a foreign object through the fill or sender holes, especially if the tank is baffled or when you are voyaging far from the expertise of a professional tank-cleaning service.

Fuel lines can also suffer from neglect and poor installation practices. While the American Boat and Yacht Council only recommends type B1 fuel hose for diesel supply lines, type A1, which is gasoline rated and of a higher overall quality, is better still. Type A1 fuel line is noticeably thicker, offers greater fire resistance, and is less prone to kinking or crushing. As one might expect, it is also more costly, but not prohibitively so. Being thicker walled, it is also more chafe resistant.

Regular maintenance needed

Diesel fuel pumps (both lift and injection), injectors, and glow plugs require regular maintenance. If supplied with clean fuel, injection pumps are quite long lived and reliable (machined to tolerances measured in the 0.0001-inch range, they are sensitive to even the smallest amount of dirt). They may require periodic service, after thousands of hours of operation, to adjust timing and check for wear. Some jerk-type pumps have their own oil reservoir and accompanying dip stick. This oil will eventually become contaminated, requiring replacement. Follow the factory recommendations. Beyond this limited service, injection pumps should only receive internal service from qualified fuel shops equipped with the proper facilities. Injectors, while also quite resilient, lead a much harsher life than injection pumps. Their hellish existence is punctuated by extreme heat (combustion chamber temperatures range from 1,000° F to 5,000° F), pressure (450 to 700 psi), and abrasive carbon deposits. Accordingly, their service interval is much shorter than other fuel system components, at approximately every 1,000 hours. They should be taken or sent to a qualified diesel fuel shop for testing and rebuilding if necessary. Proper reassembly is critical, requiring renewal of all crushable copper washers. While working on the injectors, remove, inspect, and renew, if necessary, the glow plugs, when equipped.

Even with an ample supply of clean fuel, no diesel engine will run for very long without cooling, sometimes referred to as raw, water. Most diesels fall into the "fresh water," or closed, cooling category. Essentially this means that, with the exception of the raw-water pump, injected elbow, and exhaust system downstream of it, the remainder of the engine is cooled only by fresh water, preferably a 50/50 mix of mineral-free water and coolant. Coolant should never be used undiluted, unless specifically formulated to be used that way. Heat generated within the engine is absorbed by the coolant mixture, carried to a heat exchanger, and then transferred to the raw-water circuit. The now hot raw water is then ejected into the exhaust system where it cools and quiets the even hotter exhaust gas. While this system sounds simple, it is fraught with potential failure areas, nearly all of which are a result of poor or improper maintenance.

On its mission to collect heat, the raw water first enters the boat through a through-hull and seacock arrangement. The size of this fixture is usually mandated by the engine manufacturer. It must, at the very least, match the size of the intake barb on the raw-water pump. It is interesting to note how often this detail is overlooked when vessels are repowered. The original seacock may be inadequate for the new engine, and may have been inadequate for the original engine as well. The raw water should then, after passing through a length of wire-reinforced hose (all raw-water hose, especially that used on the suction side of the circuit before the pump, should be wire reinforced and double clamped), enter a strainer. This is usually a bronze and clear plastic affair equipped with an easily removable lid and basket designed to catch weed, marine life, and trash. If anything, err on the side of oversizing this piece of gear. Some strainers present too much resistance even when empty. A larger unit will afford a greater margin of capacity.

Change impellers regularly

The next stop, the raw-water pump itself, suffers from several character flaws that are exacerbated by poor maintenance. The primary serviceable component, the rubber impeller, should be inspected at least seasonally, or every 100 hrs. I usually go a step further and recommend annual replacement. This may sound overly cautious; however, it virtually guarantees trouble-free service. Even the more expensive impellers, for auxiliaries, rarely cost in excess of $25.00. Again, a small price to pay for piece of mind.

In addition to the impeller, there are other components within the raw-water pump that will wear. These include the cam, wear plate (if equipped), cover plate, seals, and bearings. The latter two usually will run for years, and thousands of hours, without so much as a hint of trouble. They should, however, be inspected each time the impeller is replaced. With the seals, look for obvious water leaks (and oil, if gear driven) from the weep hole on the bottom of the casting. Bearings will feel rough when the pump shaft is turned (this can only be done when the pump is removed for gear-driven units or untensioned for belt-driven units). The cam, wear, and cover plates all tend to wear a bit more quickly. Manufacturers' recommendations vary; however, a useful rule of thumb is to replace these three items with ever third impeller. This may vary as well but can easily be determined by simple inspection each time the impeller is replaced. A word of caution: the cam tends to corrode more quickly than the pump body, by design, so inspect it closely. If it appears pink and porous, replace it.

Heat exchangers have some weak links of their own that are, again, usually preventable by following good maintenance procedures. Most are equipped with zincs. While the manufacturer may have set replacement intervals for these, it's best to inspect them regularly, thereby developing a customized schedule. Some corrode more quickly than others, based on a host of factors too numerous to detail here. Needless to say, they can not be replaced too often. Frequently, after some time, the carcasses of depleted zincs will accumulate in the end chamber of a heat exchanger, impeding the flow of cooling water. These can be cleaned out by removing the end cap. Have a spare end cap and gasket set on hand when doing this. These caps frequently develop small cracks around the fixing bolt (usually the result of over tightening). A final word on heat exchangers: many engines have more than one. Crankcase oil and transmission fluids are often cooled by their own heat exchanger, and, while they usually do not have end caps, they sometimes have zincs.

The closed cooling side of the system has several components that bear mentioningprimarily the circulator pump, which pumps coolant around the engine's cooling passages. It is of the centrifugal type and usually requires little maintenance. When it begins to leak, also from a weep hole, or its bearings start to growl, it should be replaced immediately. The thermostat also requires little maintenance other than periodic replacement. The preventive approach is, when changing coolant, which should be done according to manufacturer's recommendations (usually approximately every 500 hours or three years, whichever comes first), replace the thermostat as well. This moves the engine that much closer to 100% reliability. Naturally, belts should be properly tensioned. Over-tensioning will lead to premature bearing failure. The method I prefer to use is: 1/2 inch of belt movement in each direction, for each 12 inches of free belt between pulleys. Some prefer their belts tighter, and this may be necessary if a high-output alternator is in the loop, so to speak. Use common senseif the belts squeal and/or are glazing, increase the tension.

Anticavitation formula

A final note on cooling systems. Many diesel engine manufacturers are now recommending the addition of an "anticavitation" formula to the coolant. Diesels tend to resonate at a frequency that actually causes vacuum bubbles to form around certain components within the closed cooling system, such as cylinder liners and circulator pump impellers. Each time one of these vacuum voids collapses, which could be hundreds of time per minute, they erode a bit of metal. The long-term results could be quite destructive. The additive is formulated to reduce this tendency. Check with specific engine manufacturers for recommendations.

No discussion of diesel engine maintenance would be complete without some mention of lubrication. Diesel engines, by virtue of their operation, tend to be quite demanding of crankcase oil. Most do-it-yourself mechanics have noticed how quickly diesel lubricating oil turns black. The amount of soot, ash, acids, and other contaminants present in the crankcase of a diesel engine necessitate specifically formulated oil, which can contend with this contamination, as well as renewal, which must be religious in its regularity. Most manufacturers recommend oil changes at 100-hour intervals, more often when regularly run under light loads.

Lubricating oil suited for gasoline engines (designated by an S prefix, such as SF or SG, for "spark" ignition), are not suited for diesel applications. As mentioned, diesels are hard on oil, producing more heat, higher loads and suspended solids. Diesel rated oils (designated by a C prefix, such as CF or CG, for "compression" ignition) are specifically formulated to deal with these special needs. The higher the suffix letter, the more sophisticated the additives. Accordingly, a CG rated oil would be more desirable than an older CC or CD lubricant. The detergents in these oils are also designed to neutralize acids caused by combustion byproducts, such as sulfuric acid, which are a result of the mixture of sulfur found in some diesel fuels (to a lesser degree in the U.S. but more so overseas), and water vapor created by the combustion process. This acid will accelerate the demise of internally lubricated components, especially bearing surfaces.

Additionally, diesel-rated lubricants are formulated to hold in suspension a much greater quantity of solids in the form of carbon soot. This scenario is especially pertinent if a diesel engine is run with light loads, such as when charging batteries or running a refrigeration compressor. This tends to cause cold running, which leads to incomplete combustion and additional soot formation, as well as clogging of piston rings and valve stem deposits.

Running a diesel under these conditions will actually accelerate wear. If this regimen sounds familiar, oil change intervals of 100 hours, or less, are a necessity.

Oil, just like any other component in an internal combustion engine, especially a diesel, wears out. Viscosity breakdown and acid contamination cannot be forestalled by filtration. There simply is no substitute for replacement. Follow manufacturer's instructions for grade and weight of oil recommended (based on ambient operating temperatures), and use only OEM filters or a high-quality equivalent (stick with name brands, and cut used ones open to inspect for contaminants and quality of construction). Unchanged oil is probably the single greatest cause for accelerated engine wear and failure. It is also one of the simplest maintenance tasks to carry out.

The marine-diesel engine is here to stay and will continue to be improved upon. Properly maintained, it will render thousands of hours of service.

Steve owns and operates Steve D'Antonio Marine Consulting, Inc. (, providing consulting services to boat buyers, owners and the marine industry.  He's also is an Ocean Navigator contributing editor.