Marine Diesel Engine Maintenance Tips

When it comes to marine diesel engines, proper maintenance and repair is not something to take lightly. Even something as simple as forgetting to install an o-ring when replacing the oil filter can be a costly error. The nature of boating is such that the water you are moving through is constantly trying to slow or stop you. Because of this, even traveling at slow speeds can exert your engine more than you might think. Problems may be arising without any signs, so preventative maintenance is a must. The diesel marine engine is by no means a simple part, but there are four areas that, when cared for properly, will go a long way in ensuring the proper performance of your engine. These are the lubrication, cooling, electricity, and fuel systems. This blog will discuss how to care for each.


The first and most important engine component to maintain is its lubrication system: the oil. Not only does oil reduce friction in moving parts, it is also responsible (even more so than the cooling system) for keeping the pistons and cylinders cool. Furthermore, oil also acts as a seal for parts such as the cylinder walls, valve stems, and turbochargers, where they keep contaminants and corrosion under control. Fortunately, the lubrication system is very easy to maintain - simply change the oil seasonally or based on engine hours and change the oil filters whenever you change the oil. Additionally, changing the oil every fall before winterization (the process of preparing your boat to be stored for the winter) is imperative because old oil can become acidic and corrode the interior of the engine.

Furthermore, the oil should be checked prior to every trip. During this, you are only checking whether the engine block is full enough, not the color of the oil. The color is not an indicator of the oil life, only the engine hours. When servicing the oil filter, change it according to the boat’s manual and remember to replace all the seals and gaskets as well. A common problem when doing this is buying the wrong oil filter for the engine, which can restrict flow and cause harm. Apart from the oil and filter, be sure to take a look at the oil line’s route from the oil reservoir to the engine and check for signs of external damage or rust. Over time, either of these can lead to leaks causing your engine to seize up.


Though it is technically not part of the engine, the cooling system is one of the biggest sources of problems. A marine engine’s cooling system comprises two components: the raw-water and fresh water systems. The raw-water system draws in seawater through an in-line pump and subsequently leads it through a lube oil cooler and into a heat exchanger where it cools the engine’s freshwater. From here, it runs through the mixing elbow to cool the hot exhaust gas before it exits the exhaust. The freshwater system is a closed loop that pumps water from the reserve tank throughout the engine to the cylinder jacket, turbocharger, and cycle head. Following start-up, a thermostat in the system regulates a valve that cycles the heated water through a heat exchanger, where the seawater cools it.

A common problem within the raw-water system is impeller failure. A healthy pump impeller looks similar to a gear, featuring rubber veins that spin and flex as the pump operates. Over time, impellers absorb seawater and harden causing veins to warp, bend, or break. Once this happens, it will not be able to pump a sufficient amount of water. Best practice is to remove the impeller before winter and replace it each spring. Within the freshwater system, among the most important things to maintain is the pressure cap. Check regularly that it is sealed and able to increase the pressure enough to raise the boiling point of water from 212 degrees Fahrenheit to 250. Otherwise, the water in your engine will boil and your system will overheat.


The DC electrical system of an engine is another common source of trouble. Fortunately, it is less threatening than other issues, as the electrical system shutting down will only prevent the engine from starting, rather than overheating. Nevertheless, you never want to be stuck on the water without a running engine. The most common problem in DC systems is loose or dirty connections. Connections to check include the cable from the battery to the starter motor, the oil pressure switch, the cooling water temperature switch, and any other wire involved in the engine.

Problems can also arise from the electrical system’s fuses, solenoid, or alternator. The alternator in particular is a common problem, as these can work loose from the engine over time due to vibration. To prevent this, regularly inspect the bolts holding the alternator in place and be sure they are properly torqued. Just as important is checking that the belts are the correct size and type for your engine and are properly tightened. If a belt is too loose, it can cause the engine to overheat.


Finally, let’s look at how to keep your engine fuel clean. Frankly, apart from keeping your tank topped up to prevent microbial growth, there isn't much you can do about the fuel itself. However, the fuel filter can serve as a helpful defense. Be sure to check the inspection bowl for water prior to every trip. If you see water, drain the filter and start up the engine again. If you see water again, that means the fuel is bad. Diesel engine owners should also know how to bleed the fuel system should air get into it and shut down the engine. To do this, loosen the filter’s screw to bring up the hand pump and pump until only diesel fuel comes out. If this doesn’t work, try venting the fuel line fuel-injection piping with a 17mm fuel line wrench.

No matter the type of marine diesel engine components you need, Purchasing 3sixty, a trusted supplier of parts for a wide range of industries, can help. Owned and operated by ASAP Semiconductor, we are an online distributor of marine parts as well as parts pertaining to the aerospace, civil aviation, defense, electronics, and IT hardware markets. We’re always available and ready to help you find all the parts and equipment you need, 24/7-365. For a quick and competitive quote, call us at +1-714-705-4780 or email us at


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