Marine Engine Maintenance Tips


Antifreeze is used to protect the engine coolant from freezing. In areas of the world where the temperature drops below 32 degrees F (0 degrees C), at least a 50 percent glycol concentration (antifreeze) is needed to avoid cracked blocks. Antifreeze also raises the boiling point of water. As such, a minimum of 30 percent glycol concentration is recommended in warm climates for protection against engine overheating.

All modern diesel engines also require rust inhibitors. Otherwise, the cooling system will rust and lose efficiency and the engine will overheat. In addition, minute holes can form on the cooling water side of the cylinder liner, which will eventually cause water to leak into the combustion chamber and ultimately destroy the engine.

Some brands of antifreeze contain rust inhibitors, although rust inhibitors can also be purchased separately. Be aware that as antifreeze ages, it may still protect coolant from freezing or boiling but the rust inhibitor chemicals may deteriorate. A simple litmus paper test is available to check the proper strength of the rust inhibitors. As a rule of thumb, check your cooling system fluids after every 250 hours of operation.


Batteries are required to do more and more on modern boats, including starting engines and powering radios, radar, lights, pumps and other electronics. Batteries have a finite life, which depends in part on the quality built in by the manufacturer. Regardless of the type of battery in your boat, there are some basic steps you can take to keep it working properly.

  • Make sure the battery is securely mounted. Batteries must be securely held in the battery box. Shock and vibration can seriously decrease battery life and lead to premature failure. Hold-downs should also be tight and free of corrosion.
  • Clean battery surfaces. Keep the top surface of the battery clean, as dirt can conduct electricity and silently rob your battery of stored energy. The electrical connections must be kept clean and corrosion-free. Use grease or a cleaning spray to prevent algae growth.
  • Check battery cable connections weekly. Make sure all connections are tight and free of corrosion. When accessories are added to a boat, additional wires should never be connected directly to the battery. Connections must be made to the power panel, through fuses or circuit breakers properly sized for the type of wire used to carry power.
  • Visually inspect all cables. Battery cables must be kept clean and properly supported along their run to the power panel. Look for cracks, breaks, tears or other damage to the cables. Check end connections for gaps between the connectors and coating. If you notice corrosion on the wires in the cable, replace the cable. Clean the posts regularly and coat with a corrosion retarder.
  • Inspect all starter connections. Regularly check the connections between the battery and starter switch, and between the starter switch and the starter itself. Check the starter mounting bolts for tightness. Lubricate starter bushings and/or bearings when necessary.
  • Check charging circuit connections. Inspect all connections between the battery, regulator and alternator. Make sure connections are tight and corrosion-free. Inspect the wiring for cracks, worn spots or general deterioration.
  • Check electrolyte levels and state of charge. If your battery has cell caps that can be opened, check the level of the electrolyte in each cell monthly. If the level is low, add distilled water. Some batteries (such as “maintenance free” types) are built with sealed cell covers and sufficient fluid to last the lifetime of the battery. The only way to check the condition of these sealed batteries is with a load test, carried out with a specialized piece of test equipment.
  • Track discharging levels. If you are using the battery to run things when the engine or generator is not operating, don’t discharge the battery below 12.2 volts. Never discharge a battery below 11.8 volts. Use a digital voltmeter to check battery voltage. Recharge the battery promptly after using it, as leaving it in a semi-discharged state will shorten its life.

Battery fluid is diluted sulfuric acid, so remember to always use caution when working with batteries. Wear protective goggles and try not to splash acid on clothing, the top of the battery or nearby surfaces. If any acid is spilled, replace the battery cell covers and wash the top of the battery with plenty of fresh water. Periodically clean the top, using a mixture of water and bicarbonate of soda to neutralize any acid residue. Dry the battery top after cleaning it.

With proper care, most moderate quality batteries used on commercial boats can be expected to last at least three years, while premium quality batteries can last for more than seven years. There are no bargain batteries. Finally, make a note of the date on which your battery was installed and post it near the battery where it can be seen each time you check the battery. Time flies and what you may think of as a relatively new battery may be years older than you thought.


The first step for cooling system care is making sure you’re using the proper fluids. Either distilled or deionized water should be used with an approved antifreeze and an approved supplemental coolant additive or rust inhibitor. (Fishermen in warmer climates don’t need to use antifreeze but must still use the coolant conditioners.) Use a low-silicate antifreeze that meets one of the following specifications: GM 6038-M or ASTM #D4985. The fluid in the jacket water cooling system should not consist of plain tap water or water which has been “softened” by a domestic water softener. Tap water is not recommended for engine cooling systems because of additives, contaminants and other chemicals (such as salt, chlorides, sulfates, etc.) found in the water.

Corrosion of the water-cooled side of cylinder liners can occur when supplemental cooling system conditioners are not used or are depleted. Jacket water systems should always be filled with a Caterpillar approved coolant mixture, a 50-50 mixture of distilled water and an approved glycol base antifreeze containing approved corrosion inhibitors or distilled water, plus an approved cooling system conditioner.

Here are some additional tips to incorporate into your regular maintenance schedule to keep your cooling system in good shape:

Replace hoses approximately every three years of engine operation.

  • Replace the fluid in the cooling system or replenish the inhibitor chemicals at intervals listed in your owner’s manual. Although the glycol in the antifreeze mixture does not wear out, the corrosion inhibitors lose their effectiveness over time. You can eliminate the need for multiple additive replacements by using the new Cat® Extended Life Coolant, which requires only one addition of “extender” at 3,000 hours or three years.
  • Consider a coolant analysis program, such as the S•O•℠ coolant analysis offered by Caterpillar Inc., to evaluate the effectiveness of your coolant and check for contaminants.
  • Clean the water pump drain vent and inspect for leakage of the pump seal.
  • Inspect and maintain the sea water coolant system. Clean the sea water strainer, check the pump impeller at least annually (or monthly for rubber impeller pumps) and periodically clean the heat exchangers.
  • Periodically inspect and replace zinc anodes used in the seawater system.
  • Check coolant level before starting the engine.
  • Ensure that the engine room ventilation system is adequate.

In addition, the filler cap on pressurized cooling systems includes a valve that permits the system to operate at above atmospheric pressure. Pressurizing the jacket cooling system increases the boiling point of the coolant, increasing the efficiency of the cooling system. Fluid level in this type of system is normally checked when the engine is cool.

Proper engine cooling system operation is best achieved by always performing all required inspections and checks. It may take some extra work, but it’s better than having to deal with an overheated engine.


Corrosion due to electrical activity can be very destructive and costly. Let’s review the basics so that this silent destroyer doesn’t hurt you!


All hulls which operate in water will be subject to some galvanic corrosion. Place two different metals in sea water, connect them with a wire and current will flow from one to another. The bad news is when the current flows, metal particles from the basic metal will deposit themselves on the noble (more corrosion resistant) metal. Eventually the basic metal will corrode away.

The following items can minimize the problems caused by galvanic corrosion:

  • Use similar metals wherever possible.
  • Make the smaller, more expensive parts (such as propellers, rudders and seacocks) from a more noble metal (graphite, platinum, titanium, stainless steel and copper nickel compounds) than the larger, less expensive items.
  • Insulate dissimilar metals with a gasket or flexible compound to avoid contact (or “electrical conductivity”) between them.
  • Bond similar metals to a “common” ground.
  • Avoid the use of graphite grease. Instead, use a lithium or moly based grease, which are advertised as being “non-conductive.”
  • Provide sacrificial anodes! Since it’s almost impossible to prevent all galvanic action, a sacrificial anode made of zinc is the most common solution. The zinc is placed in strategic locations where it can be monitored. It’s relatively cheap to replace the zinc as it deposits itself on the larger surfaces.

Your engine manufacturer installs zinc rods in the engine’s sea water cooling system. When the new engine goes into service, the zincs should be inspected hourly. As the zinc rods corrode away, a white crust of oxides and salts form on the surface, which will flake off when tapped. Later, as you gain experience, the inspections can be lengthened to daily or weekly until you determine the proper service interval. If the zinc remains clean and like new, it’s not protecting like it should.


Galvanic activity usually progresses slowly, sometimes taking months or years before serious corrosion is apparent. The voltage difference between the two metals may be only millivolts (1/1000ths of a volt). Stray currents, on the other hand, can be thousands of times greater and can destroy expensive components in hours. Here are some things that may cause problems:

  • Poor insulation, especially in damp areas of the boat.
  • Undersized wiring, which causes excessive voltage drops. The electricity then tries to find a better flow path.
  • Cheap appliances which leak electricity.
  • Radio grounds with different voltages than the battery ground.
  • Lack of a common ground point.
  • Tying the AC systems neutral to the boat’s ground system without an isolation transformer.
  • Defective shore power wiring can cause problems between two boats electrically tied together at a marina.

To prevent stray current corrosion, the following practices are recommended:

  • Wire the boat like your house, not your car. Modern homes have three wires to every outlet. One from the electricity source, a return line to the electricity source and a ground wire. (Your car has only one wire that goes from the battery to the appliance. The car’s chassis is used as the return line. There is no path to ground because of the rubber tires.)
  • Use two wire marine appliances, not single wire automotive appliances (e.g., engine alternators and starters, bilge pumps, etc.). Make sure both the electrical supply wire and the return wire are large enough. Devices that work well on land may be unsuitable in a marine environment.
  • Use a common ground for all systems. Use a keel bolt to an external ground plate, not the engine block.
  • Use a Type-B isolation transformer to tie the neutral side of the AC system to the boat’s common ground.
  • Install an isolation switch to disconnect your battery when not in use.
  • Check your boat with a Voltmeter. Look for voltage readings where there should be none.

Finally, if you don’t have a lot of marine experience, call in a pro. Some corrosion problems can be quite subtle and hard to figure out. It pays to hire an expert. It’s much cheaper than replacing expensive components.


Pitting in cylinder liners is a direct result of cavitation erosion. This type of erosion develops from normal mechanical and chemical processes that take place during engine operation.

Cavitation of the cylinder wall begins when air bubbles remove the wall’s oxide film, which protects the metal from coming into contact with oxygen and corroding. Flexing of the cylinder wall (after fuel combustion) causes the cylinder liner to vibrate, and creates vapor bubbles in the coolant. These vapor bubbles form on the outside of the cylinder wall and explode inward, or implode, resulting in tiny pits on the cylinder wall’s protective oxide layer. When vapor bubbles continue to implode, enough energy is released to physically attack the cylinder wall and remove the oxide film. Corrosion and pitting then take place at a high rate.

If a pit breaks through the cylinder wall, coolant can leak into the cylinder and contaminate the lube oil. A sludge forms that can interfere in ring and bearing functions. Wear rates increase significantly and engine seizure may result.

The best way to prevent cavitation from occurring is to follow your engine manufacturer’s recommendations on additive replacement. When using a standard heavy-duty coolant, SCA (Supplemental Coolant Additive) should be added every 250 hours to help replenish the eroding oxide film. Caterpillar has recently introduced an Extended Life Coolant (ELC), which provides a substantial amount of protection and lasts longer than standard heavy duty coolants. ELC eliminates the need for multiple additive replacements – requiring only one addition of extender at 3,000 hours.

If you modify your cooling system, remember to keep the pressure cap furnished with the expansion tank. Removing this cap allows a lower operating pressure inside the engine. That will cause more vapor bubbles to form, resulting in cavitation.


Items in your maintenance schedule fall into one of three categories…preventive maintenance items, revolution-sensitive items and load-sensitive items.

Preventive maintenance items should be performed at the hours indicated on the schedule. If not, engine life and performance will be adversely affected.

The maintenance intervals for revolution-sensitive items are based on hours of operation. The faster and longer the engine runs, the faster the components wear. The load on the engine during operation does not affect these items.

Obviously, load-sensitive items are affected by engine load. The best indicator for determining service intervals for load-sensitive items is total fuel consumed, which will vary as the load on the engine varies. In general, the lower the load, the longer the engine life. If revolution-sensitive components are serviced at the proper interval, the risk of a failure is minimized for most users.

If you decide to exceed the indicated intervals in the schedules, you assume a higher risk of component failure and unnecessary expense.

As mentioned, the wear rate of load-sensitive items is a function of fuel consumed. An engine operating at 100 percent load takes less time to consume the same amount of fuel as an engine operating at 50 percent load. It follows that it takes less time for an engine operating at 100 percent load to wear the same amount as an engine operating at 50 percent load.

Of course, engine load will fluctuate, making it difficult to determine the exact hour interval for servicing. It’s therefore important to keep accurate fuel consumption records to properly maintain load-sensitive items.

Here’s a list of items that require regular preventive maintenance:

Revolution-Sensitive Items

  • Water pumps
  • Alternators
  • Fuel transfer pumps
  • Oil pumps

Load-Sensitive Items

  • Cylinder liners
  • Cylinder heads
  • Connecting rods
  • Pistons
  • Piston rings
  • Main and connecting rod bearings
  • Valve train components

White smoke, which is basically unburned fuel, is often noticed at engine start-up for several reasons. First, fuel is not burned efficiently when the engine runs at idle or at low engine speed without load, which are normal start-up conditions. Cold ambient air temperatures and cold engine coolant temperatures also contribute to inefficient combustion. Another major factor is retarded timing, which means the fuel is injected after the optimum time for complete combustion. (Retarded timing is used to help improve starting capabilities, decrease noise or lower engine emissions.)

Caterpillar has made several improvements to its engines to reduce white smoke at start-up. Various attachments are available to shorten the time it takes for the engine to reach operating temperature by heating the coolant or the inlet air. These include jacket water or block heaters, and inlet air heaters. Attachments vary among different engine models.

Caterpillar has also made iron modifications to help reduce white smoke. Using different pistons to increase the compression ratio or altering the opening and closing of valves can reduce white smoke, too. For example, the 3208 marine engine rated 435 bhp (326 bkW) has a camshaft that closes the inlet valves earlier. This makes start-up more efficient, reducing smoke.

White smoke is typically not as much of a problem with electronically governed engines as it is with mechanically governed engines. This is because fuel injection timing is controlled by software rather than mechanical devices. All Cat electronic engines feature a Cold Start Strategy that is activated when the coolant temperature falls below a certain point. This strategy can advance the fuel injection timing even when the rpm is low, or make the engine run on a portion of the cylinders until the engine is warm. For example, the 3406E marine engine uses only three of the six cylinders during cold mode operation.

Finally, refer to your engine’s operating and maintenance manual for the recommended fuel and lube oil for the ambient conditions in which the vessel operates.


If your engine will be stored less than six months, protect your investment by following these short-term storage procedures:

  • Change the oil. Drain the oil out of the sump, replace it with the appropriate diesel engine oil and change the filters. Start your engine and operate it long enough to pump fresh oil throughout the engine, approximately five to ten minutes. Using the right type of oil and changing the oil and filter at the proper intervals helps prevent sticking piston rings, piston seizure, accelerated wear of the in-cylinder components and bearings. Refer to the owner’s manual for the correct procedure, viscosity and amount.
  • Drain water and sediment from your fuel filter/water separator – and from the fuel tanks, if possible. Replace the filter. Consider using a biocide in the main fuel tanks to prevent the growth of bacteria. Follow the biocide manufacturer’s instructions carefully. Too much biocide can harm the fuel system.
  • Drain water and sediment from the bottom of the fuel injection pump housing, if applicable.
  • Remove and replace the secondary fuel filter on the engine. Replace it with a manufacturer-recommended fuel filter.
  • Top off the fuel tanks with clean, fresh diesel fuel. Use the priming pump to completely fill the primary fuel filter/water separator, secondary fuel filter and fuel injection pump housing with clean, treated diesel fuel.
  • Check the freeze protection of the coolant mixture. Make sure the freezing point of the coolant is lower than the expected low temperature your engine will experience during storage. Verify that the coolant conditioner percentage is between 3 and 6 percent. Drain and flush the cooling system every two years when using a low silicate antifreeze. If you use Cat Extended Life Coolant (ELC) and a mid-life extender, drain and flush the system every six years. Change the water temperature thermostat when flushing the system or if the engine has been operating below 180°F.
  • Drain the engine’s sea water system. Remove and inspect the zinc rods in the cooling system and replace them, if necessary. Flush the sea water system with clean, fresh water and allow the water to drain from all compartments. If your boat uses a water lift muffler, flush the sea water system only when the engine is running, otherwise, water could enter the engine and cause severe damage.
  • Inspect the sea water pump impeller. Replace the pump cover gasket and impeller if it shows signs of damage or excessive wear. At this time, also inspect the sea water pump drive belt for damage or wear. Replace the belt if necessary.
  • Inspect and replace all rubber hoses as necessary. Replace all rubber hoses every two years. This can be performed during the cooling system flushing procedure (i.e. hoses from through-hull fitting to sea strainer, sea strainer to sea water pump inlet, marine gear outlet to the wet exhaust discharge fitting and any other cooling system hoses).
  • Lubricate all fittings on the engine. Refer to your owner’s manual for the proper procedures.
  • Drain the oil from the marine gear and clean the filter/screen according to the manufacturer’s recommendation. Check the gear oil level with the engine running. Fill with the appropriate oil. Do not overfill the gear, but make sure there is sufficient oil for starting.
  • Remove batteries from the boat during the storage period. Fill the batteries with distilled water and fully charge them. Clean the battery and cable connections, and apply a light coating of multipurpose grease to prevent battery connections from corroding. Store the batteries in a cool, dry location during the storage period.
  • Remove the air cleaner filter element. Clean or replace the filter element as necessary. The crankcase breathers should also be cleaned at this time. If your boat is equipped with a closed-crankcase system, check all hoses and fittings and service per the manufacturer’s recommendations.
  • Use an appropriate cleaning solvent to remove all oil and dirt from the engine. Be careful not to allow the solvent or water to enter the intake system through the air cleaner. Allow the engine to dry and touch-up your engine and marine gear with paint as necessary.

An engine’s thermostat (or “temperature regulator”) regulates the jacket water temperature to keep the engine running at a normal operating temperature, usually around 185 degrees F.

If the jacket water becomes too hot, the thermostat “opens” to allow cold water in from either the keel cooler or heat exchanger. When the jacket water returns to a normal operating temperature, the thermostat “closes” to keep the cold water out. An old or deteriorating thermostat, however, may not recognize when to close off the cold water and subsequently overcool the engine. Overcooling can damage an engine as much as overheating because the parts don’t expand properly, and the engine is allowed to run under any load at substandard temperatures.

The biggest problem associated with running too cool is excessive carbon buildup around valve guides and behind piston rings. Other problems include combustion contaminants in the oil and the presence of sulfuric acid, which can attack the rings and valve guides. This can cause excessive wear and shorten the lives of these parts by up to 80 percent.

To get the most out of your engine, change your thermostats annually as part of your routine maintenance schedule. You can also determine thermostat wear by monitoring the coolant temperature under load. If it’s running below 185 degrees F, change your thermostats. (Cat engines usually have two or four thermostats per engine. If one fails, change all of them.) The advantages to a regular thermostat changeout is increased fuel efficiency, longer life between overhauls and increased engine performance.


Engine valve lash should be adjusted at the first oil change on a new engine. On Cat® Marine Engines, the first oil change is typically recommended at 250, 500 or 1000 hours.

A valve lash adjustment is critical because the valves of a diesel engine play a key role in the combustion process. Specifically, the intake valves control the flow of air entering the cylinder, while the exhaust valve controls the flow of exhaust gases exiting the cylinder. Both inlet and exhaust valves must close and seal completely during the combustion process.

Technically, the valve itself is not adjusted, but rather the valve mechanism, i.e., the lifter, push rod and rocker arm assembly. The valve mechanism is adjusted to provide a specific lash (or looseness) necessary to regulate the opening and closing of the valve. If the lash is too loose, the opening and closing of the valve becomes very abrupt which will eventually lead to damage to the valve and/or valve mechanism. If the lash is too tight, the valve cannot close and seal properly in the cylinder head and exhaust gases will leak past the valve. With incorrect lash adjustment, the engine may not operate at full power, fuel consumption may be high and the exhaust smoke and exhaust temperature may be excessive. If left uncorrected for an extended period of time, a catastrophic failure of the valve and engine is likely to occur.

Valve lash adjustment is a relatively uncomplicated procedure.Valve lash should be checked and adjusted as needed as a normal maintenance process every 1,000 to 3,000 hours after the initial adjustment. Consult your Operation and Maintenance Guide for the correct interval and procedure to adjust the valve lash for your engine.


There are two ways to take a Scheduled Oil Sample (S.O.S.) – by using an oil valve probe or by vacuum extraction when the engine is at normal operating temperature. After it’s taken, the sample is placed in a carefully labeled bottle and sent to the dealer’s S.O.S. lab. There it is checked for water, glycol, fuel and trace elements, such as sodium, silicon, chromium, aluminum and iron. If the sample indicates potentially troublesome elements or fluids, your dealer will contact you to discuss corrective measures. Take oil samples regularly, so that a change in magnitude of one or more of the elements can be observed, the correct diagnosis made and remedial measures taken. The goal is to identify potential problems early and avoid major engine failures.


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