Diesel mechanics is not a difficult sub-ject. In fact, all owners of diesel-poweredboats canand shouldlearn the funda-mentals of operating and maintaining theirengines. To run well, a diesel enginerequires clean fuel, clean oil, and a lot ofair. Routine maintenance will virtuallyguarantee years of trouble-free serviceand will keep your busy mechanic at bay.

How a diesel engine works, and why inmany cases it is superior to a gasolineengine as a marine power plant, will not bediscussed here.That has been well coveredelsewhere. What I will describe are 10

things every diesel mechanic wishes everyboatowner knew about diesel engines. Asone mechanic told me, A knowledgeableboatowner makes my job easier. When Ireceive the call, the owner can reportmeaningful symptoms, can explain theengines relevant history, has probably keptup with maintenance routines, and wontgive me grief when I explain that the enginemay need a set of new injectors, a new cir-culating pump,or a purging of the entire fuelsystem.

Here is the list, in no particular orderof priority.

DONT BABY THE ENGINEDiesels dont like to idle in neutral or runin gear at low speeds; they do like to workhard under load. A diesel engine that isproperly matched to its boat can run at itscruising rpm (the sweet spot) for hourafter hour, day after day. Whats the cruis-ing rpm? Generally, the sweet spot isapproximately 75 to 80 percent of themaximum rpm as defined in the ownersmanual. Running at cruising rpm after justa few minutes of initial warmup, the engineshould keep the boat moving at a reason-able speed with minimal noise and littleharmonic vibration. Running in the sweetspot also maximizes fuel efficiencyitruns hot enough to be at its most efficientoperating temperature, but is not beingoverworkedand increases the engineslifespan.

However, the sweet spot will vary fromone engine to another. Volvo-Penta, forexample, recommends that some of itslarger engines be run at 200 rpm belowthe obtainable maximum rpm. My three-cylinder Yanmar diesel has a rated maxi-mum rpm of 3,600. I routinely run it at

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TEN THINGS DIESELMECHANICS THINKEVERY BOATOWNERSHOULD KNOWBy Captain Bernie Weiss

ENGINE

Learn where all the majorcomponents on your engineare. This is the fuel lift pump

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2,800 rpm (75 to 80 percent of the max),which keeps my 12,000-pound sloop mov-ing at 6.5 knots, with the engine consum-ing approximately 1/2 gph.

You should especially avoid idling theengine for long periods. At idle speeds,fuel combustion is incomplete comparedto cruising rpm under load, so excessiveidling only wastes fuel. It also accelerateswear and tear on the engine, causes var-nish to build up on the cylinders, anddeposits soot and carbon on the enginesvalves and in the exhaust system, particu-larly at the manifold injection elbowwhere raw cooling water exiting theengine mates with exhaust gases. Runningat cruising rpm blows this stuff out.However, after running at cruising rpm forseveral hours, a brief cool-down at idlespeed, with no load on the engine, is ben-eficial. A few minutes is enough.

GIVE YOUR ENGINE CLEANNOT CONTAMINATEDFUEL

Diesel engines must have clean fuel. Fuel iscontaminated when it contains water,sediment, or other solids. Fuel may alsocontain biological organisms. To minimize

contamination, dont store your boat forthe winter or let it sit around for weeks ata time with its fuel tanks only partly full.Keeping the tank full minimizes the con-densation of water vapor on the tanksinterior walls, thus reducing the volume ofcondensate entering the fuel system.Diesel fuel left undisturbed in a tank forlong periods should be regarded as a cul-ture medium designed to foster thegrowth of microorganisms.

Since diesel engines have a low toler-ance for contaminated fuel, filtration iscritical. Your engine is equipped with itsown fuel filter, of course, and there maybe a small fine wire-mesh screen at thebottom of the fuel pick-up tube insidethe fuel tank. At least one additional fuelfilter is recommended. This should be ahigh-efficiency unit specifically designedfor diesel service, such as a Racor orDahl filter. This filter should also be awater separator that allows you to drainthe separated water from the bottom ofthe filter bowl before enough accumu-lates for it to continue through the fuellines to the injectors, where it willalmost surely stop, if not damage, the

engine.This water-separat-ing filter should be the pri-mary (first) filter the fuelencounters after it leavesthe tank. Filters mountedon the engine itself shouldbe considered secondaryunits.

The elements in all ofyour fuel filters, includingthose that trap sediment,sludge, and organic mate-rial, should be changed atperiodic intervalsafterevery 75 to 100 hours ofoperation or annually,whichever comes first. Onolder boats and on thosewith possibly contaminat-ed fuel, filters should bechanged more frequently.Always carry spare filterelements on board.

DONT RUN YOUR FUEL TANK DRY Know your engines burn rate (gallonsconsumed per hour) and your fuel capaci-ty so you can accurately calculate theboats range between fill-ups. Do not delayrefueling to the point where you haveexpended nearly all the fuel in your tank.The last 20 percent should be held inreserve. Sucking the last few gallons of fuelfrom the bottom of the tank increases thechance of pulling water, sludge, and othercontaminantsperhaps even airintothe fuel lines. If youre not sure you haveenough range to cruise comfortablybetween fuel stops, be conservative:Acquire some jerry jugs and take extrafuel with you. You can never have toomuch fuel, unless the boat is on fire.

KNOW HOW TO BLEED AIROUT OF YOUR FUEL SYSTEM

Air locks in diesel fuel systems are a factof life. A typical fuel system has a lift pump(a vacuum pump) that lifts or sucks fuelout of the tank, draws it through thepump, then sends it to the filters and injec-tors, where an injector pump sends it tothe individual cylinders for combustion.Whenever you open the fuel line betweenthe tank and the engine (for example, tochange a filter element), air enters theline. Air may also be sucked into the fuelline through cracked seals and gaskets,poorly fitted connectors and clamps, viathe pick-up tube in the fuel tank, and soon. This air must be removed, becauseeven a tiny bubble will block the flow offuel, which will prevent the engine fromstarting or will cause it to stop if it isalready running.

To clear your fuel line of air, you mustbleed it out if your engine doesnt havea self-bleeding feature or it cant copewith the air. Consult your engines manualto identify the appropriate bleedingpoints; paint them with nail polish so youcan find them easily. Given decent accessto the engine, bleeding or venting air is asimple procedure that everyone should beable to perform. Use the engine manual toteach yourself how to do this.

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ENGINE

Every boatownershould know howto bleed a diesel

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S While on the subject of air: When run-ning at any speed, diesel engines require anenormous volume of clean air to supportcombustion. This air is drawn into theengine through a filter. Periodically, that fil-ter should be cleaned with solvent or withkerosene if its a metal-mesh filter, orreplaced if it is paper or fabric. Even a smallreduction in the free flow of air will dra-matically reduce the engines performance.

BE DILIGENT ABOUT CHECKING YOUR LUBE OIL AND OIL FILTER

Diesel engines are rough on oil and usual-ly require more frequent oil and oil filterchanges than comparable gasoline engines.Follow the engine manuals recommenda-tion for service intervalsmost suggestan oil and filter change after every 75 to100 hours of operation; some recommenda change every 50 hours. Unchanged oil isprobably the single greatest cause of accel-erated engine wear and failure. Changing itis one of the simplest maintenance tasks toperform. When changing the lube oil,change the filter element. Carry spares onboard. Between oil changes, use the dip-stick to check the oil level.Top it off as nec-essary from an onboard supply, but do notexceed the full mark on the dipstick;more is NOT better.

Note that fresh lube oil is golden orhoney brown in color. However, after afew minutes of circulating inside theengine, it turns black from soot, ash, acids,contaminants, and other carbon byprod-ucts of diesel-fuel combustion.This is nor-mal and, by itself, is not enough to warrantan oil change.

MINIMIZE RISKS OF FIRE Diesel engines vibrate a lot, and a typicalmarine diesel has a lot of wiring and hosesattached to it. Over time, fasteners loosenand fail and the wiring and hoses comeloose. If a loose hose or wire (such as theprimary wiring harness, or the power sup-ply to your fuel pump, or a hose to thehydraulic pump) should come in contactwith a hot exhaust manifold, for example,any of these could cause a fire.

From time to time, inspect your enginecompartment for these potential risks.Add chafing protection, replace worn insu-lation, and add extra fasteners if necessary.Consider rerouting wires and hoseswhere appropriate.

KNOW HOW TO TROUBLESHOOT THE COOLING SYSTEM

Since overheating is a common problemwith diesel engines, you should familiarizeyourself with the engines cooling systemboth the raw-water and the freshwatersides. Frequently, a simple do-it-yourselfrepair or replacement from a spare-partskit will save you both time and money.

When troubleshooting an overheatingproblem, as Yogi Berra once put it, Youcan observe a lot just by looking. Whenan engine overheats, I first check the raw-water overboard discharge.

If there is no flow or only a dribble, themost likely causes of overheating are: THE RAW-WATER INTAKE SEACOCKis closed. THE RAW-WATER INTAKE is blockedexternally. Check for a plastic bag, a clumpof sea grass, or other material covering orplugging the inlet. THE RAW-WATER FILTER or strainer is

clogged with sediment, sand, goo, grass, orliving critters such as jellyfish and algae.

If there is flow, but it is diminished,consider these likely causes: DAMAGED IMPELLER VANES in theraw-water pump. If the impeller is com-pletely destroyed, there will be no flow.Theimpeller should be replaced every year ortwo, as the vanes become brittle with ageand may snap off. Also, the vane tips maywear unevenly or take on a permanent set,degrading the impellers function. THE HEAT EXCHANGER is dirty orclogged up with sediment and other deposits.Sometimes removing the end cap of a heatexchanger will reveal the cause of overheat-ing. The entire raw-water cooling systemshould be flushed periodically to remove saltand sediment deposits. Sometimes it will benecessary to remove a clogged heatexchanger so that it can be boiled out withan acid bath to purge its water channels andrestore it to like-new condition. THE EXHAUST INJECTION ELBOW isclogged with carbon deposits or othersolids, reducing the flow of cooling waterand exhaust gases. Routinely running theengine at high or moderate rpm underload, as opposed to long periods of idlingat low speeds, will help avert this problem.

ENGINE

Know where the water-pump impeller is andhow to change it. Alwayscarry a spare

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If the flow appears normal, then youshould suspect one of these commoncauses: THE THERMOSTAT is stuck closed andlikely should be removed or replaced.(Note: Some diesel engines will operatejust finetemporarilywithout a thermo-stat.) Its worth remembering that dieselengines are most efficient running at hightemperatures: a coolant temperature of180oF is not uncommon. THE COOLANT LEVEL is low (thisshould be a 50:50 mix of antifreeze andwater). Coolant levels should be checkedroutinely, and more coolant should beadded as needed. Caution: Remove the capto the reservoir only after it has cooled tothe touch.When replacing the cap, be cer-tain it is closed snugly and seals the sys-tem. Frequent replenishment of thecoolant suggests a leak and requires fur-ther investigation. A V-BELT driving the water pump andalternator is broken or slipping. Keep aspare belt on board. Even a new belt needsretensioning sometimes and should beregularly inspected for wear. Expect tofind some slippage or excessive wear ifyou see fine dark belt dust settlingaround the engine mounts or at theengines base near the belt. To check belttension, apply thumb pressure midwayalong the longest belt run; tension is okayif the belt deflects about 1/2 inch. THE ENGINE is overloaded:A rope maybe wrapped around the propeller shaft,the boats bottom may be foul, the pro-peller could be fouled, and so on.

Also, check for air leaks in the raw-water cooling system. Has a hose crackedor collapsed? Are the hose clamps tight?

KNOW YOUR FUEL ADDITIVES After diesel fuel is refined from crude oil,it is modified with additives to reducesmoke, prevent preignition (or knock-ing), improve its cetane rating, and so on.Few aftermarket additives will furtherenhance the fuel, but there are someexceptions. Many diesel mechanics recom-mend 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 contaminat-ing the tank, clogging up fuel filters, andblocking fuel lines. LUBRICANTS, such as Lubricity andStanadyne Performance Formula. Lubricantsprolong the life of seals and rings in theengine. Modern diesel fuels with reducedsulfur contentsulfur acts as a lubricantmay be improved by such an additive. FUEL STABILIZERS, such as Sta-Bil andPri-D. These additives prevent fuel fromundergoing degradation and oxidationduring prolonged storage (as on a winter-ized boat). But if diesel fuel has beenaboard a boat undisturbed in storage formore than a year or so, a mechanic willview it with suspicion; frequently he or shewill recommend that the fuel be replacedbefore starting the engine.

Two precautions when using fuel addi-tives: (1) Follow the instructions on thecontainer. (2) A little bit is better than a lot.

MONITOR FOR EXHAUST LEAKS From time to time, when the engine isoperating, inspect the entire exhaust runfrom the engine to the overboard dis-charge. Look for leaks, both gas and water.Major leaks will be obvious, but early signs

of leaks due to hairline cracks in hosesand water-pot muffler systems may not bereadily apparent. Diesel exhaust containsacidic sulfur and other gases that may poi-son the air within the boat and, over time,may cause nearby metals to corrode. Todetect air leaks, look for telltale traces ofblack soot. Water leaks should also beimmediately repaired. Leaks never get bet-ter on their own; they must be addressedas quickly as possible.

READ AND UNDERSTAND THE OWNERS MANUAL THAT CAME WITH THE ENGINE

In the case of a new boat, this will be partof the package of literature that is passedto you by the dealer. If you have an olderboat, request a manual from the enginemanufacturer. If the engine is old and nolonger in production, a search on theInternet will often prove fruitful. At thevery least, a manual will tell you the rec-ommended intervals between oil changes,the type of lubricants to use, and anyother salient information. Better yet, gethold of the engines workshop manual.Consider going to a diesel-engine course.Many manufacturers offer one- and two-day hands-on courses that can be invalu-able in helping you maintain your engine intop condition.

CONCLUSIONLearning how to operate and maintainyour marine diesel engine properly is notdifficult. With just a little knowledge andskill and a simple tool kit, you can saveyourself both time and money.Frequently, evaluating a problem is nomore difficult than studying the symp-toms, which will either lead straight tothe problems cause or suggest the mostlogical possibilities.

The key to a satisfying experience withyour diesel engine is not necessarilyknowing how to repair it, but rather howto maintain it so that you do NOT have torepair it. Treat yourself and your enginekindly by following the recommendedmaintenance procedures and schedules.

ENGINE

The key to a satisfying experience with yourdiesel engine is not necessarily knowing how to repair it, but rather how to maintain it sothat you do NOT have to repair it

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