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HSH Yacht Articles

Diesel engines: repair and maintenance


Jeremy R. Hood

After waiting out a tropical depression anchored on the Tarpon River, Florida, I set out with a crew of two to make my first Gulf Crossing. It was a bumpy start as we hit a sand bar where none was shown on the chart but we were soon over it and with no damage we headed out to sea leaving the flat Florida skyline behind us. Winds were light and variable and by late in the evening we were wallowing in the left over swells making little or no way and so I decided to start the engine. It started, ran for a few seconds and died. And that was that. After a number of tests both then and next morning I realized the cylinder head gasket had blown and we would be without an engine until we could put in to port for repairs. And so we made the decision to carry on without our engine. Though I had spares aboard including new gaskets, I chose not to begin the repair at sea even though I may have been able to get the engine to run. Later when I did undertake the repair I found that the cylinder head was severely pitted and the repair was not possible without some machining. But engine failures such as this are relatively uncommon and on most occasions having the right tools, a few spares and some basic understanding of diesel engines you will be able to cure many problems. Crossing the gulf last year I spent quite a time in the engine rooms of the various boats and only on one occasion did we come in to port with no engine at all.

Problems with diesel engines are usually relatively simple ones. And with a little regular preventative maintenance you can avoid the need for repairs at sea. Diesel engines require clean fuel, clean oil, fresh air and a satisfactory supply of cooling water. Merely ensuring you take care of these needs will go along way to eliminating potential problems.

Regular Maintenance

Talking with Jim Glandt, service manager at Blue Water Ships Store as part of the preparation for this article, he gave first priority on a maintenance schedule to a boats fuel system. To make sure that your engine receives a clean supply of fuel there are several things that you can do. Diesel fuel is not always clean to begin with and in many areas of the world it is sensible to filter the fuel as you fill your tank using a funnel with a filter in it, though here in the U.S. it should not be necessary. Fuel tanks should be kept as full as practical especially if the boat is to be left for long periods, as having a large quantity of air in the tank can lead to condensation and hence a build up of water in the fuel. When water is present in your fuel then growth of algae may begin and to prevent this, regular use of an algicide is recommended. Here in Texas with the heat of summer this problem seems to be particularly acute and there are many boats each year who have fuel problems as a result. All is often well when sailing in the bay but when they venture offshore the more turbulent conditions stir up the algae and other debris from the bottom of the tank and this soon gets sucked in to the fuel system blocking the engine fuel filters with the result that the engine cannot receive sufficient fuel. Fuel filters themselves should be changed at least once each season, and because they may become clogged when out using the boat, having spare filters aboard along with the tools to fit them may help you get an engine started that has stopped. Though you may usually pay to have your boats engine serviced it is a good idea to change out your fuel filters at least once yourself so that if you need to do this on the water you know how to do it. I recall a couple of years ago helping an owner prepare his boat for an offshore passage and as part of this we went out and bought filters for the engine. As often happens the exact filter was not available but a standard equivalent was. We purchased several of these and took them back to the boat and began fitting one of them. Though it was sold as an equivalent and indeed it had the same thread and top fitting as the old filter, the new one was about 1" longer than the old and physically would just not fit in the space allowed for it on the engine. Had we not tried fitting one then, the boat could have set off with a box of unusable filters.

Expensive problems result when your engine is not properly lubricated and old engine oil can be the culprit. Dirt and sludge in your engine oil can act as an abrasive on bearings rather than a lubricant and after a period of time you will cause wear in your engine that will be costly to repair. When I went to see Dennis Osborne at Lakewood Yacht Service (who specialize in diesel engine servicing and repair), I was surprised to find that this was his number one point regarding maintenance. Different manufacturers recommend changing the engine oil at different intervals so check to see how often you should do this on your engine. The differing figures can partly be explained by the size of the engine sump. If your engine only takes a small quantity of lubricating oil then it will likely need changing more frequently. The most commonly suggested interval is every 100 hours but this may be as little as 75 hours or as long as 200. And changing the oil means changing the oil filter too. Practically the job is often difficult and dirty. Using a hand pump to suck the old oil out of the dipstick hole is tedious and whenever I do it, no matter how careful I try to be, some oil manages to get thrown around in the process. Those owners who take their engine maintenance seriously will often arrange to fit an electric pump permanently to the sump drain so that the whole process may be accomplished quickly and easily. Though this installation may be a little time consuming, the time and energy saved later will more than compensate for this. But I have to own up. I have not fitted such a pump on my boat and though I do change the oil at least fairly regularly, I hate doing it!

A diesel engine needs an adequate supply of air. Because larger engines require so much air, special air intakes will be installed to force air in to the engine room though on smaller boats with smaller engines air is often allowed to enter the engine through cockpit lockers or via the bilge. So if you have air intakes, check to ensure they are not blocked, and if you don't, then check to see how your engine receives its air supply. Neatly sealed engine rooms may be quieter than you anticipated when the engine dies through lack of air! Air entering the engine is usually filtered to remove particles that may cause damage. Usually these filters are the replaceable type used in cars though sometimes they are a type requiring cleaning at regular intervals. A good time to change or clean the air filter is when you change the oil.

Next on our list is cooling water. To ensure that your engine runs at the correct temperature you will need an adequate supply of cooling water. On raw-water cooled engines the surrounding sea water is used directly, though most modern diesel engines use raw water to cool the fresh water which in turn cools the engine. In both cases raw water passes from the sea-cock below the water-line to a filter/ strainer. Most of these may be cleaned by turning off the sea-cock and then opening the strainer and removing the filter. If it is in good condition it may merely be washed and replaced. Once refitted don't forget to open the sea-cock again before attempting to start your engine! On fresh-water cooled engines the heat exchanger is used to transfer heat from the fresh water to the raw-water. Over time this may become blocked and so good preventative maintenance will include inspecting this and perhaps removing it every season to thoroughly rinse it out. Often heat exchangers will have there own zinc anodes to minimize electrolysis and these need to be checked. If yours has decayed more than about 50% it's time to replace it.

In performing routine maintenance on your engine Dennis at Lakewood Yacht Service reminded me of a few other things to check. Though he didn't feel the need for regular replacement of the raw water pump impeller he felt that it should be removed and inspected for cracked or broken blades. Belts on the engine are often used to connect pumps, alternators and compressors and these need to be inspected for wear (which may result from misalignment) or cracking due to old age. Belts should be adjusted so they are the correct tension. Motor mounts are often ignored but if they wear or the engine vibration allows them to come loose then damage to the stuffing box and cutlass bearing may result. Check them to make sure they are tight and have not broken.


Though regular routine maintenance will eliminate many potential problems, sometimes you will find that your engine which ran just fine before you worked on it will not start or, if it starts, will die after a short while. Most often this is the result of air entering the fuel system and becoming trapped somewhere. In my experience over 90% of the problems encountered on diesel engines are caused by air in the fuel. Last year I recall arriving at a boat ready to assist the new owners in taking her to Corpus Christi. The boat had recently been taken out on a sea trial, surveyed and subsequently all major recommendations had been carried out so I felt pretty good about setting off. We stowed our provisions and set off under engine, heading out of the marina into an easterly wind which meant motoring towards the Houston Ship Channel where hopefully we could sail on a reach down to Galveston and out through the jetties. But we had barely got to the end of the Clear Lake Entrance Channel when the engine died. We set a sail and returned to the fuel dock under sail where it was a simple job to bleed the air from the fuel line and get it started again. The next time we made it nearly to the Ship Channel before again it died and we returned, this time back to our slip in the marina. Here I began a check of the fuel system replacing filters, tightening unions and inspecting everything I could. We ran the engine at the dock for several hours and all seemed at last OK. It wasn't, and again it failed when we next set forth. By this time I was becoming tired and the owner annoyed. We postponed our departure until the next day when he could get a trained mechanic to solve the problem. Next morning the mechanic arrived. He proceeded to check everything as I had, and failed to find a problem. We ran the engine at the dock under load and sure enough it failed. Eventually at some time after lunch on this second day he pronounced it cured and we set off. We made it to the end of the jetties before it died! Eventually we reached Corpus though many hours were spent in the engine room each time bleeding the system and getting the engine to run for a while. In the end we ran our starting battery down with these numerous attempts and chose to make our arrival under sail saving the house battery for the navigation lights and radio rather than drain it also in a last attempt to restart the engine. Months later I saw the owner and he told me how a mechanic there had eventually, after many hours, tracked down the problem. We had all known it was air getting in but until then no one had been able to find out where. The problem was the fitting on the fuel tank itself.

Air in the fuel

Air in the fuel will often cause your engine to fail when out on the water and so knowing how to bleed air from the system will be essential if you want to get it started again. If you changed the fuel filters yourself you probably learned then how to reconnect the system and bleed air from the secondary fuel filter or injection pump intake. Doing this at the dock is good practice for when it becomes necessary on the water. In most cases bleeding the system to remove air will allow the engine to restart, but unless you find the cause of the problem it will almost inevitably reoccur as it did with me last year. Understanding the fuel system helps. From the tank, air is sucked towards the lift pump on the engine passing as it does through the primary fuel filter. After it has passed the lift pump it is under pressure being pushed on towards the secondary filter and injection pump. Leaking seals on the suction side can cause air to be sucked in; on the pressure side leaks cause fuel to be sprayed out. And so if you are getting air in your fuel it must be entering between the fuel tank and the lift pump on the engine. Knowing this narrows down the sources of potential leaks and a methodical check should reveal where the problem lies. I guess last year on that boat I was just not methodical enough.

Poor starting

Maybe the last time you took your boat out it was fine but now you have arrived at the boat with the plan of a long weekend away from the pressures of work, a time to relax and unwind. And instead of this your engine won't start! Understanding a little about how the engine works helps in finding the problem. Air enters the engine, is compressed and hence heated. At this point fuel is injected into the heated air where it ignites and the engine starts. Poor starting can result from not enough air (blocked filters) or too cold air which may be because of failed glow plugs (if fitted) or weak compression. If your engine has gradually been getting harder to start each time, then weak compression could be the cause. Older, worn engines often begin to suffer from this and are harder to start than normal but, once started, they appear to run just fine. But the biggest cause of poor starting is batteries. A check can be made on all connections to ensure good clean contacts but this will not cure the problems of an old, weak battery. Unless your engine is being turned over quickly enough the air above the piston has time to seep past the piston rings and poor compression will result. Anyone who has tried to hand-crank a diesel knows that getting the flywheel up to speed is the most important prerequisite of successful starting. And so if your battery is weak and the engine is sluggish it will almost never start. With a weak battery you can try opening the decompression levers if fitted and turning the engine over. With these levers open no compression results so the engine turns easily and the engine oil is given a chance to circulate and perhaps warm up a little, become less viscous, and help seal the pistons. Once the engine is turning over fast, closing the levers may induce it to start. On multi-cylinder engines with separate compression levers for each cylinder it may be possible to get one cylinder to start first before closing the lever on the other.

Black smoke, White smoke

When in Rome the color of the smoke may indicate the election of a new Pope, but when its from your diesel engine the cause is a little more mundane. On a normally running engine the exhaust should be clear and no smoke seen at all and so any smoke indicates a potential problem. Black smoke is the most common, and it indicates that unburned fuel is being ejected with the exhaust gases. This can result from clogged injectors causing fuel to not atomize correctly and hence not burn. If this is the cause then the injectors will have to be removed and serviced by a company specializing in this work as, once disassembled, they have to be rebuilt to precise standards and then set to a correct opening pressure. But most often black smoke is a result of a badly fouled prop or boat bottom. If either of these put excessive load on the engine the injector pump will deliver maximum fuel to the engine but because of the load the engine will be turning slowly and will not be able to use all the fuel. If the prop or bottom is badly fouled you may have to reduce the throttle setting and accept a slower speed though clearly something needs to be done as continuing to overload the engine could cause major damage. Knowing this will also lead you to realize that having a too large propeller will result in the same symptoms and the same potential damage. If you have recently changed props or just bought a boat and this happens you should seriously consider whether the prop should be changed or at least modified. One other cause of black smoke can be a blocked exhaust elbow where cooling water is injected in to the exhaust. This is most likely on older engines where a build up of soot has occurred over the years. To check it will be necessary to remove and inspect the elbow when the engine is cold.

When I started my old MD2B Volvo engine the other day for the first time in a while (shame on me), clouds of blue smoke issued out of the exhaust! Blue smoke is an indication that your engine lubricating oil is finding its way into the combustion chamber and is being burned. If this occurs it is an indication that the engine needs at least decarbonizing and perhaps a more major overhaul. I have yet to diagnose the cause on my engine.

White smoke results from water vapor and will often be seen when an engine is first started. Continuing white smoke may result from water in the fuel or a cracked or leaking cylinder head gasket allowing cooling water to enter the cylinder. It is often indicative of one cylinder failing to fire and thus the smoke may be accompanied by a loss of power from the engine.


If your engine overheats and you continue to run it severe damage may result. Overheating occurs when the normal heat produced by the burning fuel is not being removed from the engine quickly enough. It can result from having insufficient fresh water in the engine, a stuck thermostat or a failed fresh water pump but the single most common problem is with the raw water. If the water strainer is partially blocked it may not allow sufficient water to enter, if the pump impeller fails the water will no longer be forced through the heat exchanger or if the heat exchanger is blocked the water will stall and insufficient flow will occur. If the strainer is clear and the pump is OK then the heat exchanger is the next most likely cause of overheating. Removing the end caps (if fitted) may allow you to flush and clean it but often the whole assembly will need to be removed. Occasionally a chronic overheating problem may occur during the summer when the temperature of the raw sea water is already high. Some engines seem more prone to this problem than others and one apparent solution in some instances is to fit a larger size heat exchanger. Dennis at Lakewood Yacht Service recommends this occasionally but in discussing this with Jim at Blue Water Ships Store he was of the opinion that a properly installed and maintained system should work OK. He did point out though that with the shallow water and the Texas heat of summer, if an engine is prone to overheat this is one area of the country where its going to happen.

Though you may not feel as familiar with your diesel engine as you do with the gasoline one in your car, they are really much more simple and in my experience 95% of problems can be avoided or solved by regular routine maintenance and taking the time to learn how to undertake basic repairs such as replacing filters and bleeding air from your engine or how to change out your pump impeller. Not only is it sensible to undertake these tasks at least once, on your own, at the dock, it will ensure not only that you do have aboard the correct spares but also the correct tools to undertake the work. Heading back up the Ship Channel late in the afternoon on a still Sunday after a weekend trip to Freeport I was at the helm when I heard the engine note change and as a result noticed that cooling water was no longer being ejected from the exhaust. We pulled off to the side of the channel and dropped an anchor. First thing to check was the pump impeller. But to get to it was almost impossible. The cover pate with its six machine screws was clearly visible but getting a screwdriver to the heads was impossible. Reluctantly we realized that we would have to free the alternator, remove the belt and then take off the whole pump. Then we found the bolts holding the pump were different sizes, corroded and themselves almost inaccessible. Eventually we did manage to remove the pump with the limited tools available and then it was relatively easy. The impeller had failed, we did have a spare and once fitted the we were able to replace everything. The job really should take only 20 minutes or so on most boats but not having the correct tools caused it to take over two hours!

One friend that I met when cruising told me that whenever he bought a spare for his engine he fitted it at once, keeping the one he removed as a spare. Not only did he ensure that he had the correct tools and that the spare was the correct one, the engine got a treat too!

Over the years I have been sailing, problems with diesel engines have inevitably occurred and in solving them my knowledge and experience has grown. But like many new to sailing I had to start somewhere. If you are looking for a good book on the subject then the one by Nigel Calder (Marine Diesel Engines, pub by International Marine) is a good place to start.

In the preparation of this article I had the benefit of speaking with Dennis Osborne at Lakewood Yacht Service and with Jim Glandt at Bluewater Ships Store. Not only were they extremely helpful and informative, I learned a few things in the process! Thanks guys.

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