Small Diesel Engine Maintenance

Your engine is likely the single most expensive piece of equipment aboard. However, it seldom gets the attention that its replacement cost should warrant.

With the summer behind us, and boating schedules less demanding, the off-season is a great time to catch up on preventive maintenance. If you own a sailboat in the 28-60’ range, chances are you have a Yanmar, Volvo Penta, Westerbeke, Universal, or Perkins engine. While you might have something different, the discussion points to follow will probably apply just the same.

Below is a brief summary of the major maintenance items that should be on your radar.

Engine oil and Filter

The importance of changing your engine’s oil and filter as recommended by the manufacturer cannot be overstated. Failure to do so can cause accelerated wear on internal engine parts that cannot be reversed without a major overhaul. Over time, even with little use, the oil can absorb different contaminants that affect its ability to protect critical internal parts. Most manufacturers recommend changing oil and filters every 100 to 250 hours, or at least once a year. Unless you’re actively traveling, most boat owners will find the once a year interval will apply. Make sure the oil you’re using meets the manufacturer’s specifications. Most people are aware of this maintenance point, as the dangers of neglecting it are the same as your car.

Fuel Filters

Clogged fuel filters are a common cause of engine shut-downs. Most every small diesel will be equipped with a primary fuel filter located off the engine, and a secondary on-engine filter. Primary fuel filters will be from a different manufacturer than your engine (like Racor, for example). Secondary filters can be purchased from a dealer for your engine, or sometimes can be purchased through aftermarket sources as well. Always carry spares of both of these filters, and be educated on the process to replace them. Since any air in most diesel’s fuel delivery system will cause it to stall and not re-start, understand the bleeding process as well. If you are doing more than day sailing the bay (Intracoastal, offshore cruising), you may be interested in upgrading to dual primary filters, vacuum monitoring systems, and fuel polishing set-ups. Similar to engine oil, your engine manufacturer will recommend an engine hour, along with a time interval for replacement. Most manufacturers recommend yearly replacement, at minimum.

Sea Water Pump Impeller

If you’ve been in boating for any length of time, chances are you have experienced the dreaded engine alarm, sounding the alert that your engine is overheating. At that point, you have a short period of time to shut it down to avoid major damage. The most common cause of an engine overheat underway is a failed impeller. The sea water pump impeller is what moves cooling water through your engine and exhaust. It’s made of a flexible rubber composition and has blades or fins attached to a hub. With age and use, the impeller can deteriorate and loose fins as they break away from the hub. This diminishes its ability to pump sea water, until enough blades are lost and the engine overheats. To complicate the issue, broken off fins tend to travel downstream and lodge in the next cooler or heat exchanger. Even after installing a new impeller, these old fins, if left in place, can restrict flow and continue to cause overheating issues. It is extremely important that any and all missing fins from an old impeller be retrieved before returning the engine to service. When changing the impeller, it is also a good opportunity to inspect other pump wear items that can affect the volume of water it moves (wear plates, cams, and cover plates). Typically, manufacturers recommend a two year service interval, along with engine hours (250 on average). However, we have found this interval can vary, especially when operating in areas of the lake with higher silt content. If you are performing yearly maintenance anyway, we recommend replacing the impeller at the same time. It’s cheap insurance against a tow in and a frustrating day on the water.

Transmission Oil

Transmission oil is all too often overlooked. However, most marine gears have clutches in them that wear with use. As they wear, metal particles from clutch plates and other parts become suspended in the oil. As many small transmissions do not use an oil filter, fluid changes are the intended method to remove these contaminants from the oil. Like engine oil, there are also other contaminants the oil absorbs over time that can affect its ability to protect internal parts. While the engine hour interval is commonly twice that of the transmission oil, most manufacturers recommend a yearly service, at minimum. As each transmission can be different, make sure you understand where and how to check the transmission fluid and keep it filled to the proper level.


While it’s true that antifreeze requires less frequent replacement, especially with some of the extended life products on the market today, it can do just as much catastrophic damage to your engine if neglected. Along with cooling your engine, there are also additives in it that protect various engine metals and parts from corrosion. The elements eventually break down, and internal parts can begin to rust and corrode. If you notice a change in the color of your antifreeze, this is a strong indicator that it is well past its useful life expectancy and bad things are happening inside your engine. It should be replaced as soon as possible. Generally, most manufacturers recommend a service interval of 500-600 hours, or every 3-4 years.

Heat Exchanger & Cooler Anodes

Know your engine. Some manufacturers use pencil-type anodes in antifreeze heat exchangers and engine & transmission oil coolers to protect the materials they are made from. If your engine requires them, they will need regular service to stay effective. Two bad things happen when they are neglected. First, the part is no longer protected from Galvanic corrosion. Over time, the cooler or heat exchanger will deteriorate and eventually require replacement. Second, the anode material will separate from its holder from physical erosion caused by water flowing through the system. The pieces break free and lodge in the small internal water passages and cause water flow restrictions. This in turn leads to high operating temperatures and possible engine overheating. The cooler or heat exchanger will then need to be removed, serviced, and possibly replaced. Heat exchanger and cooler anodes are very inexpensive (usually less than $10), but can cause thousands of dollars in headaches when neglected. If you’re engine doesn’t require them, you’re off the hook.

Engine Alignment

Engine alignment actually refers to aligning the centerline of your transmission output shaft with the center line of your propeller shaft. This is typically done by adjusting nuts at each motor mount while measuring the largest gap at the prop shaft coupler/transmission flange joint. The engine is positioned to typically achieve a gap of less than .004” to .006”. Not something most boat owners choose to attempt themselves. However, what you need to know is this is something that needs to be done regularly, usually every 1-2 years. Many newer engines use very soft engine mounts to absorb vibrations (Yanmar especially). As the rubber in these mounts ages, the engine will settle and the alignment will be disrupted. Other changes in the vessel over time can affect alignment as well. A miss-aligned engine will cause abnormal vibrations underway, as well as accelerated wear on everything from cutlass bearings, transmission parts, and engine mounts (not to mention the rattling pans in the galley…). These changes happen gradually and usually go unnoticed by the boat owner. Many engines are aligned at installation, and never touched again. This would be similar to never doing a front end alignment on your car, and the resulting expensive repairs it can cause. If you’ve never had your alignment checked, add it to your punch list.

Inspection of Belts, Hoses, etc.

There are likely many regular inspection points around your engine. These vary from engine to engine, but are usually visual, and include checking belt tension, hose condition, the presence of any abnormal leaks of fluid accumulations, etc. This is where it helps to have a trained eye look over your equipment. Impending problems generally provide some sort of advanced warning, if you know what signs to look for. Keeping a clean engine and compartment will greatly aid in identifying abnormal conditions.

The purpose of proper maintenance is to keep you out on the water, and minimize expensive repairs. The items listed above are a generic list that applies to most engines. However, the best information comes directly from your engine’s manufacturer, and can be found in your owner’s manual. Never hurts to read the instructions…

If you have any technical questions, would like to schedule preventive maintenance, or would like to be set up on our Chief Engineer Maintenance Program, please contact our office today!