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27 ways to keep your engine running smoothly

Spark plugs, fuel, belts, cooling systems and more



Proper maintenance, accurate troubleshooting and a keen eye (and ear) will go a long way toward increasing engine reliability and the life of your power plant. It’s the old story: Be good to your engine and it’ll be good to you. Here are 27 tips to keep your engine running smoothly, covering everything from spark plugs and belts to your fuel and cooling systems.


If your outboard runs poorly, as if it is almost out of fuel, check the remote fuel primer bulb, which is part of the supply hose to the engine. The bulb should be firm while the engine is running. These bulbs have been known to fail at inopportune times, so it would be prudent to carry a spare on board. For many brands, their orientation on the boat is critical due to the internal valve configuration. You will find that keeping the ball vertical will help maintain pressure in the system.

When your diesel engine suddenly loses rpm or is slow to respond to increased throttle, suspect a fuel filter blockage. To avoid the costly breakdown, you can install a vacuum gauge on your primary fuel filter. By monitoring the gauge readings, you will be able to determine when your filters begin to clog, and service them before they shut down the fuel supply.

Water in the fuel system is an all-too-common maintenance issue. Over time, condensation in the fuel tank collects in the fuel filter and can starve the fuel system, shutting the engine down or at least affecting its performance. I recommend an aftermarket fuel filter/water separator that has a separate bowl attached to the filter element. A quality filter, such as those made by Racor, separates water from the fuel, and the bowl allows you to monitor the volume of accumulated water and drain it from the system without changing the filter element.


If you drain your fuel filter/water separator but it quickly indicates water is present again, look for water entering your fuel system. Aside from condensation within the tank, many fuel/water problems begin with poor tank design or an improper seal on the filler neck, which allows rain or seawater to enter the tank. Check the seal on the fuel filler cap, and if the filler is mounted through the deck, be certain that water cannot pool around the filler. The fuel tank vents often are overlooked as points of water entry. The vents should be installed so that the openings are facing down and aft, preventing waves and spray from being forced into the tank.

Proper filtration of diesel fuel is critical for reliable engine operation, but your engine manufacturer’s filtering is only the beginning of the process. Aftermarket fuel filter systems should be sized (i.e. flow rate and capacity) based on your engine’s fuel transfer pump maximum flow rating, not on the engine requirements alone. Micron rating (size of particles that will be trapped within the filter) should be selected in accordance with the engine manufacturer’s recommendation to avoid restricting fuel flow.


Cylinder crossfire (rough running, backfiring) can occur following a spark plug change if the plug wires are reinstalled in the wrong firing order sequence. This commonly occurs when plug wires are located below the exhaust manifold and out of sight. Prior to removing plug wires, I mark each wire with the corresponding cylinder number, either using small wire ties (for example, four ties for cylinder No. 4) or white correction fluid from a stationery store.

Many spark plug holes are difficult to see, as they are below manifolds or plumbing hoses. To make plug installation easier, push an old spark plug boot onto the end of the new plug to provide additional length and grip surface when starting the plug in its threads.

When replacing spark plugs, refer to the specification sheet for the proper heat range; don’t assume that the plugs being replaced are correct. By using a plug that is too hot, you run the risk of burning a hole in the piston, while using a plug that is too cold for the application can build up excessive carbon and easily foul.

Difficulty starting a gasoline inboard engine is often caused by wet ignition wires. Any moisture that reaches the spark plug wires can collect and run along the wire, compromising the connection to the spark plug. When installing the wires, be certain that each has a drip loop — a small loop in the wire, running below the plug boot — creating a path for any moisture to drip off before it gets to the plug. In addition, I wrap self-fusing silicone tape around the plug boot where it seals on the wire, eliminating the path for moisture to travel.

Moisture that finds its way inside the distributor cap can corrode the contacts of the cap and rotor, making starting difficult and promoting cylinder crossfire. Always use the gasket that comes with marine distributor caps, and never install caps intended for automotive use, as they are not ignition-protected.

Determine your engine’s health by reading the spark plugs. Properly reading spark plug color is an art, but a quick color check will give you a feel for what is occurring inside your engine. A light tan/gray color on the firing nose indicates that the plug is operating at optimum temperature and the engine is in good condition. Soft, black, sooty deposits that appear dry indicate an excessively rich fuel mixture. An oily, wet coating on the nose can be caused by engine oil leaking past worn valve guides or piston rings. A chalky white insulator, rapid electrode wear and the absence of deposits are sure signs of overheated plugs. Water droplets or rust indicate water intrusion into the cylinder, either from water in the fuel or a cooling system breach as with a bad head gasket.


At least once each season, remove and clean the flame arrestor on inboard gasoline engines, or replace the air filter element on diesels. One of the requirements for combustion is air, and plenty of it. As the engine draws air through the flame arrestor or air filter, it brings with it the salt air and an atomized, oily mix that often exists in the machinery space, which then clings to the surface of the flame arrestor or air filter, trapping any additional airborne particles and closing off the tiny openings. The flame arrestor is made up of metallic grating and clogs easily but can be cleaned, while many diesel filter elements require replacement. These simple steps can increase the performance and fuel efficiency of your engine.

Be certain there is adequate ventilation in the machinery space; engines that can’t breathe can’t perform efficiently. Engines require a large volume of air for optimum performance, a fact that is often overlooked by boatbuilders, especially when various engine performance options are available. During a sea trial, I like to run the boat wide open while attempting to open the engine hatch. If the hatch offers resistance, it is an indication that your engine is starving for fresh air, creating suction in the machinery space and holding the hatch shut. The compartment vents likely will need to be enlarged to accommodate the engine’s induction requirements.

Difficulty starting a gasoline engine often can be attributed to a faulty or incorrectly adjusted choke. Although mechanical chokes on gasoline engines are slowly disappearing, there are still many power plants so equipped, and I have rarely seen them function properly, mostly due to neglect. A faulty choke will prevent easy starting when cold, often allowing the seawater pump to backfill the exhaust system and hydrolock the engine. To check choke operation, open the throttle (with the engine cold and flame arrestor removed) while looking down at the choke butterfly. It should quickly snap shut but open slightly as the engine is cranked over. Once the engine starts, the idle speed should elevate and the choke butterfly should slowly open. As the engine warms, the butterfly should open completely as the idle speed drops to normal. There will be slight tension holding the butterfly open, as it shouldn’t fluctuate when the engine is being operated normally. A choke that stays partially closed will allow the engine to run excessively rich, resulting in poor fuel economy and performance.


A quick check of the cooling system on outboard engines is to place your hand into the water stream from the outflow tube. The stream should be strong, and should warm as the engine reaches its operating temperature. However, it should never be hot. High water temperature, low pressure or intermittent flow from the outflow tube indicates potential water pump issues.

Maintaining the pressurized cooling system is critical, as it serves to increase the boiling point of the coolant, which increases its cooling efficiency. The heat exchanger pressure cap plays a vital role in cooling system integrity and function. It is designed to maintain a predetermined system pressure, typically around 16 pounds. The cap’s pressure release point should be checked at least annually with a test instrument. As part of a preventative maintenance program, I routinely replace the pressure cap at first signs of seal degradation or corrosion.

High or low coolant levels in the expansion tank indicate separate issues. Check the coolant level in the expansion tank prior to starting the engine. Low coolant can indicate a system leak and may allow the water pump to cavitate, trapping air bubbles within the cooling jacket of the engine. Trapped air creates hot spots and steam, which leads to component failure within the engine. If the level rises beyond the “hot” line after the engine has been run, seawater may be leaking into the closed cooling system.

Progressive overheating can be caused by the slowly degrading zincs within an engine’s heat exchanger. The seawater portion of your cooling system typically contains zinc anodes, which corrode over time and can break up into small pieces, potentially blocking the heat exchanger cooling tubes. Remove and inspect the zinc anodes monthly, and remove the heat exchanger end caps to inspect the tubes annually or every 300 hours, whichever comes first.

Check that the engine accessory drive belts are properly tensioned. Belt tension and condition are critical for proper engine cooling, as the belts typically run both the engine water circulating pump and the seawater intake pump, in addition to the alternator. Twist the belt over by hand and inspect the bottom and sides for signs of deterioration, such as cracks, grease or oil saturation, glazing or peeling. Cracks are considered normal wear, while glazing or shiny spots are the result of belt slippage. Fraying may be caused by bent or damaged pulleys. Replace belts as indicated by condition. When retensioning or replacing V belts, I begin by adjusting the tension to a point where the pulleys cannot be turned by hand. Run the engine to allow the belts to seat under load, then readjust the tension.

Coolant does more than transfer heat, and should be changed regularly since it degrades over time and with use. The additive packages contain corrosion inhibitors that lose their effectiveness with age. A general rule is to change the antifreeze solution every two years if using the green, ethylene glycol type, and every five years if using the extended-life pink type. In addition, the coolant’s alkalinity should be checked annually using litmus paper. If the litmus paper turns blue, it indicates the proper alkaline level. If it turns pink, the coolant has become acidic and needs to be replaced immediately.

Use the correct ratio when mixing antifreeze with water to top off your system (as specified by the engine and/or antifreeze manufacturer). Do not add pure antifreeze, as excessive amounts of ethylene glycol can cause the chemical to gel on the sides of the cooling passages in high-temperature areas like the water jackets around cylinders and exhaust manifolds. This buildup will retard heat transfer and create uneven cooling, leading to troublesome hot spots. A mixture that is too weak will allow corrosion to form, again retarding the heat transfer process.

Preventive engine maintenance should include replacing the seawater pump impeller. Opinions differ regarding replacement intervals, but my advice is to replace the impeller annually if you run in an area where you may draw sand through the system, or if you venture offshore where water pump failure could be a serious problem. Impellers should be replaced during recommissioning, not winterizing, as the rubber impeller takes a set over the winter, which can shorten its useful life.


Properly tuned engines must be able to run within their recommended rpm range when fully loaded. Not reaching the upper portion of the rpm range at wide-open throttle can overload the engine and reduce its service life. The engine’s rpm can be adjusted by changing the pitch on a prop. Reducing pitch increases the rpm, while increasing pitch decreases rpm. (An inch of pitch change is roughly equal to 200 rpm.)

Black, white, blue and steam exhaust from a diesel reveals various engine issues. Black exhaust contains hydrocarbons that aren’t fully oxidized (partially burned fuel). This commonly is caused by lugging the engine, which often is a result of overpropping (excessive pitch) or a fouled bottom and running gear. It can also indicate defective fuel injectors, restricted air intake or low-grade fuel. Blue exhaust is the result of lubricating oil being burned in the combustion chamber. Valve guides, seals and worn piston rings are common culprits. Pure white smoke is actually a mist of unburned fuel, possibly indicating poor compression or resulting from poor fuel atomization in the combustion chamber. The white smoke will settle to the surface and leave a fuel residue behind. Steam (quickly dissipating) in colder climates is fairly normal, while visible steam in warmer climates can be the result of cooling water leaking into the exhaust system, a bad raw-water pump, or a restriction in the raw-water system, leading to overheating of the water entering the exhaust and producing steam.

Routine oil changes help prolong engine life but typically don’t indicate if anything is going wrong. One of the most valuable tools to help discover problems before they result in major damage is an oil analysis. Oil samples should be analyzed routinely to establish an engine’s baseline. A comprehensive report will be generated listing the various wear metals found and in what quantity (typically in parts per million, or ppm), which is compared to generally accepted industry standards, as well as previous samplings if available. In addition, conditions such as rich fuel mixture, depleted additive packages, and formation of corrosive acids can be determined and monitored.

If an engine doesn’t crank over rapidly, the most common cause is poor battery connections. Clean, secure terminals are critical not only for starting but to be certain that the charging system has an opportunity to recharge the battery while under way. The battery case must be clean and dry, as accumulated contaminants or moisture act as an electrical drain and can discharge the battery while the boat is inactive. Engines that crank over rapidly but won’t start are missing either fuel or spark. The electronic control modules on many of today’s engines require clean, dry, corrosion-free electrical connections and ground wires. Inspecting, cleaning and applying corrosion inhibitors to the connections should be part of your annual maintenance procedures.

Outboards should be flushed after each outing, even when the engine is run in fresh water, unless the lake or river is as clear as the water from a garden hose. When using a freshwater flush adaptor, never turn the faucet more than a quarter turn, as the high pressure from city water supplies can damage internal seals in the cooling system.