Prolong Marine Engine Life

How to extend the lifespan of outboard motors and stern drives.

The secret to prolonging the life of a four-stroke marine engine is common-sense simple: Change the crankcase oil regularly, keep the ignition system in tune, and monitor the cooling system. If you follow these three rules, your outboard motor, stern drive or inboard gas or diesel will grow old gracefully. On the other hand, should you neglect to maintain these systems the odds are good that your engine's health will suffer, and its lifespan will consequently be shortened.

The health of an inboard's lubricant is critical, because marine power plants run hotter than their automotive counterparts. They also work much harder. They're either idling dockside or under constant load. Offshore, there are no hills to coast down. Even when trolling or transiting a no-wake zone, a marine engine has its nose to the veritable grindstone.

Ambient air temperature in marine engine compartments is higher than the temperature of the air surrounding an automotive engine. All this heat and hard work combine to push marine engine oil to the edge of the envelope. In fact, marine engines are like mini-oil refineries, cooking off a lubricant's lighter molecules and leaving behind the heavier molecules. As the engine logs hours, its oil transforms into a sticky varnish--which is why oil changes are so vital.

Oil changes exchange varnish-laden oil with fresh oil. When oil changes are neglected, varnish begins to build up on the piston rings and valve stems, causing them to stick. When valves stick, the affected cylinder loses power. When piston rings stick, they no longer press tightly against the cylinder walls, but remain lodged motionless in their lands, no longer able to scrape excess oil off the cylinder walls and dump it back into the sump. Instead, they pump an excessive amount of oil into the combustion chamber, where it burns and creates carbon that adheres to valve stems, piston tops and rings.

Visible only as smoke, this abrasive oil-based carbon accelerates wear on whatever critical surfaces it touches. The bearings wear and splash more oil onto the cylinder wall than the oil-control rings can scrape away. Excess oil is sucked up into the combustion chamber and makes for a smoky engine.

Oil fouls the spark plugs and reduces combustion efficiency. Because the engine isn't running anywhere near peak efficiency, fuel economy is horrible. And there isn't a set of points and plugs in the world that will remedy the situation. Sure, a new set of plugs will pep up the engine, but only for awhile--until carbon deposits squeeze the life out the engine once more.

Another lubrication problem relates to fall lay-up. As gasoline burns, it deposits acid residue in crankcase oil. Acid content isn't really a problem when oil is changed on schedule. But if old oil is left in the engine over the winter the acid etches bearings, piston skirts, crankshaft journals, and other critical wear surfaces. Simply changing the oil during decommissioning prevents this premature wear.

Similarly, dirty oil wreaks havoc on internal parts. Oil attracts grit like a magnet. Even with the spark arrester bolted tightly to the top of the carburetor, some microscopic particles will find their way into the oil. As hours are logged, this abrasive material gets between the bearings and crank journal.

In addition to changing oil regularly and during lay-up, tune-ups are another vital element in extending engine life. A weak ignition spark does a poor job "lighting the fire." As a result, not all of the fuel/air mixture burns. Not only do performance and fuel economy suffer, but unburned, abrasive hydrocarbons also deposit themselves on pistons and combustion chambers.

In extreme cases, unburned gasoline washes lubricant off cylinder walls, causing excess heat and friction to score them. And when gasoline gets into the crankcase sump, it dilutes the oil, reducing lubrication even further. Rings begin to stick, which pulls the plug on compression. If the oil rings are damaged, the engine becomes an oil-burner.

So exactly how often should you change oil and tune the engine? At a minimum, always change the oil during fall lay-up. My rule-of-thumb is: Never log more than 100 hours without an oil change. Engine manufacturers all recommend specific tune-up intervals and oil-change intervals (as well as what type of oil to use). Make sure not to exceed these intervals.

Most boats don't come equipped with odometers or hour meters, so without a log book, you can lose track of how long it's been since the last oil change. You should maintain a record of the number of hours you put on your engine. Unfortunately, most boat owners are somewhat lax when it comes to oil changes. If the family car were neglected the way the typical family boat usually is, major repair bills would begin piling up around 50,000 to 75,000 miles. But, if pampered with frequent oil changes and periodic tune-ups, that same car engine could log up to 300,000 miles without an overhaul.

The third rule for engine longevity is to maintain the cooling system. Most sudden engine failures are due to overheating. When an engine loses its cool, rings score cylinder walls and heads warp.

The best approach is to do a pre-flight check before every outing. Begin with the lower unit. Make sure the water pick-up is clear. Plastic baggies, marine growth, and other debris can all plug the opening, restricting coolant flow. Next, check each cooling system hose. Look for cracks or breaks. Squeeze each hose. Brittle or mushy hoses should be replaced. Also eyeball the side of the block. If the area around the freeze plugs looks suspiciously free of oil and other grit, a cooling leak may have steam-cleaned the area.

If the instrument panel isn't already equipped with a temperature gauge, install one. Then, for the record, note the baseline temperature. Any variation up or down will not only tip you off that something has gone haywire, but point you in the right direction to correct it.

Pitch Man

Even a perfectly intact propeller can seriously shorten engine life if it has an incorrect pitch. Incorrect propeller pitch causes an engine to over-rev. Over-revving strains connecting rods, crankshaft bearings and other reciprocating parts. Eventually the engine will fail--much sooner than it would have with the correct propeller installed.

Unfortunately, there's no way to tell whether an engine is over-revving without a tachometer. So if you're not absolutely sure that you have the correct propeller installed, then either install a dashboard tachometer, or consult a marine mechanic and have him check out your rig.

Stern Drives

Essentially, a stern drive power plant demands the same care as an inboard. Change the oil and filter on schedule, keep the engine in tune, and make sure the cooling system is up to par. But a stern drive requires special attention because a stern drive has a drive leg. Here, too, the answer to longer life is simple: Make sure the propeller is in good shape, and the gearcase is supplied with untainted oil.

Regularly check the propeller blades. (Propeller maintenance is similarly important on inboard boats.) Make sure no dings or gouges have rearranged the leading edge, which must be sharp and cleanly defined. Torn-up or bent blades rotate out of balance and transmit damaging vibration throughout the entire lower unit. A helmsman can often feel an out-of-balance propeller right through the cockpit sole. Vibration will eventually crack the drive leg's propeller shaft, driveshaft and gears, requiring an expensive repair.

When I was working regularly as a marine mechanic, by far the most common failure on stern drives was gear damage. In most cases, the damage could have been prevented if the owner had regularly checked the gearcase lube. To do so, crack the bottom drain plug and let the liquid drain out. A few drops are all you need: If you see nothing but lubricant, tighten the screws and get on with your life. But if water seeps out, keep the plug loosened and continue to let the liquid drain out until pure gear oil begins to flow.

If less than an ounce of water came out of the gearcase, a seal is leaking, but not seriously. Monitor the situation carefully, checking the gear oil before every outing. If more than an ounce (a shot glass full) came out, stay at the dock and find out why.

This lubricant test is best performed after the stern drive has been idle for at least a couple of hours. That way any water will have settled to the bottom of the gearcase. If you don't wait, the water froths up into a milky-white liquid. You can tell water is present, but not how much.


Outboard motors are like stern drives--to insure long life, you should crack the drain plug on a regular basis and examine the gear oil. You should also make sure the propeller is in good shape and the cooling intakes are clear.

Outboards differ from both inboards and stern drives because they are two-stroke, not four-stroke, engines. That means they burn their lubricant in the combustion chamber. While oil changes aren't necessary on outboards, fuel quality and choice of oil are crucial.

There's little doubt that what's killing outboard power heads in the mid-'90s is 87-octane gasoline (see box). Stern drives and inboards are not affected by low-grade fuel the way outboards are, because outboard motor combustion chambers attain a much higher temperature. The extremely high combustion temperature of outboards breaks down 87-octane gasoline into carbon molecules. These molecules form deposits that build up on piston tops and compression rings. (Outboard motors do not have oil-control rings.

Junk Gas Crisis

Why is 87-octane gasoline suddenly causing problems? According to industry sources, the answer is one of economics. Refiners wanted to squeeze every last drop of gasoline out of a barrel of crude oil. The new formulations accomplish that at the expense of fuel quality. In fact, the chemical structure of some of these 87-octane blends is considered so poor that fuel engineers don't even think it's worthwhile to blend in a quality additive package.

If you want to run a quick and dirty test to see exactly what we're talking about, run two or three tanks of 87-octane through your tow vehicle, then switch to 89 or 90. You'll probably notice dieseling and pinging with the 87-octane, but not with the 89. That's why to also use 89 in your boat.

On piston tops, the carbon creates a heat dam, preventing BTUs from transferring to the cylinder walls and into the coolant. Once the heat dam builds up sufficiently, the pistons melt in their bores. When the carbon builds up on piston rings, the rings stick and score the cylinder walls. The first symptom of this malady is a loss of power, followed eventually by a blown engine.

The secrets for longer life for outboards are never to fuel up with anything less than 89-octane, and to use TC-W3 lube oil. This new two-stroke lubricant includes an additive package that breaks up carbon deposits, preventing premature wear.

Some manufacturers market fuel supplements that do the same thing as TC-W3. If you've been running 87-octane, it's not too late to repent, because carbon build-up can be reversed. You can even use up your remaining old (TC-W2) oil. Simply begin using 89- or higher-octane gas and treat it with a carbon-busting fuel supplement like OMC's Carbon Guard.

The price difference between 87- and 89-octane gasoline is a few cents per gallon. If you burn as many as 500 gallons in a season, you've only paid another $50 for fuel. To my way of thinking, that's a pretty cheap way to extend the life of an engine.

The bottom line is that if you want your outboard motor, stern drive or inboard to live beyond the date the last payment is due, don't feed it junk fuel. But if you must, at least supplement the fuel with an additive package that provides the necessary protection