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Preventing Problems in Outboard Motors

Installation is important. So too is the condition of the propeller as well as the gasoline and motor oil you use. You’ll also need to guard against carbon buildup ...

The BoatUS policy holder was cruising at planing speed off the Florida coast in a 27-footer rigged with twin, 200-h.p., oil-injected outboards. Suddenly, the starboard motor began to lose power and had to be shut down. Back at the dock, a tear-down and inspection of the failed powerhead revealed massive damage due to overheating and subsequent water intrusion. The cause? The oil feed line had broken at its hose clamp and the engine had been running without lubrication.

The boat's owner swore that no low-oil alarm had sounded, and, in fact, the surveyor who examined the boat and outboard couldn't find one! How could that be? According to Lincoln Davis, a Mercury-certified master mechanic with 30 years of experience, "More than half the troubles we see at our dealership can be traced to faulty installation — the way the engine was initially set up on the boat." He explains the problem (Claim #921703) this way: "In most outboards, the low-oil sensor is on the engine itself, but the horn is part of the control box or the key-choke harness. If the installer used non-standard controls or an incorrect but still compatible harness, nobody would hear an alarm if the engine overheated."

Davis recites a litany of installation errors he has encountered, starting with a 25"-shaft motor bolted onto a 20" transom. That seemingly unthinkable misstep resulted in lots of spray and very sluggish performance due to the extra drag on the lower unit. He also describes an outboard that was set up with a wiring harness designed for a smaller engine, a blunder that caused an electrical short and subsequent fire behind the dash.

Less dramatic, but equally problematic, are installation mistakes like setting the outboard too high on the transom, which results in severe propeller cavitation; positioning the engine off-center, which makes the boat pull to port or starboard; using steering and/or control cables of the wrong length, which causes binding and premature failure; mounting the key switch so that water can drain into the ignition, which leads to short circuits and starting failures; and neglecting to treat both ends of every wiring connection with dielectric (non-conductive) grease, which also results in shorts and, sometimes, fires.

Boat owners don't have a lot of control over these aspects of the installation process, but — whether your outboard is brand new or several years old — you and your mechanic can go over each of the areas mentioned to confirm that they were handled properly. And, since your engine's warning systems are the best line of defense against $5,000 repair bills, ask your dealer — each year — to short out the alarm horn so that it sounds off in your presence. According to Davis, built-in sensors rarely fail, but alarm-system wiring and horns develop problems readily — especially in salt water.

The Proper Prop

There's another step you can take to minimize wear-and-tear on your engine. Ask your mechanic to identify the size and type of propeller on your outboad, make sure there is a working tachometer on board and ask what the tach should read at wide-open throttle.

This process is critical because next to installing the motor properly and using the highest-quality two-cycle oil (more on this later), correctly propping an outboard is the most important factor in engine life. Here, "pitch" is the crucial element. Simply put, pitch — expressed in inches — is the distance a propeller would travel forward in one complete revolution under ideal conditions, i.e., no cavitation, slippage, or friction.

"Underpropping", which means installing a propeller with too little pitch, can lead to over-revving the outboard, causing subsequent failure of the reed valves (one-way "gates" between the carburetors and crankcase) and even the crankshaft. Both are very expensive to replace. Conversely, "overpropping", or installing a wheel with too much pitch, can result in coking of the rings, predetonation (usually undetectable), and, so, piston failure, necessitating a costly rebuild. With the high alcohol content and generally poor quality of today's gasoline, says Lincoln Davis, the likelihood of this latter problem occurring is very high.

As a rule of thumb, after break-in, an outboard should be able to run at or near its maximum rated r.p.m. when lightly loaded. This benchmark provides a margin for those times when extra weight (like water skiers or additional passengers) causes a drop in r.p.m. If, despite your dealer's assurances, you find that your engine isn't operating at the right r.p.m., ask that the prop be changed to one of more appropriate pitch. (The dealer may also recommend a wheel of different diameter and/or blade design, as these factors also affect an outboard's performance and, therefore, its longevity.)

A final note: At some point, you're going to hit a log or rock, thereby bending or breaking one or more of the blades on your propeller. This may, or may not, cause noticeable vibration and/or cavitation, but it will almost certainly alter the effective dimensions of your wheel. The undamaged propeller might have been the right one for your boat/motor combination, but impact may have changed both pitch and diameter, leaving you either underpropped or overpropped. Have your wheel rebuilt (usually about $50-$75) or — if the damage is too severe — buy a new one.

A Healthy Diet

Even if your outboard was installed and propped correctly, other factors that are completely beyond the control of your dealer or mechanic can lead to serious problems.

As everybody knows by now, gasoline refiners have taken out the lead and introduced all sorts of additives to boost performance. Under EPA mandate, some areas of the country get only "reformulated," or "oxygenated," fuel, whose makeup may vary from place to place and truckload to truckload. Nor is the quality of gasoline the same from one region to the next. All of this is bad news for outboard owners.

Low octane, now a fact of life, can lead to piston meltdown if the outboard's timing is advanced too far or if the engine is running too lean. (Unfortunately, both conditions usually go undetected until the engine breaks down or is examined by a mechanic.) Today's gas is also much more likely to create carbon buildup on the piston rings, whether or not the engine is propped correctly. BoatUS Marine Insurance Claim #928471 proves both the above points. When the seized outboard in question was disassembled, the number-five piston and the adjacent cylinder wall were all but destroyed.

Here's what the surveyor concluded: "The powerhead damage is a result of a lean fuel mixture and poor quality fuel — despite reports of only 'high test' use — resulting in high carbon buildup and stuck rings. This condition led to overheating and piston disintegration due to the higher temperatures — not an entirely uncommon finding in recent powerhead failures."

Do such cases mean that all outboards will develop serious fuel-related problems? Certainly not, says Lincoln Davis: "In general, engines built after 1988 can handle today's gas if they are properly tuned and otherwise maintained on a regular basis."

Unfortunately, however, the gas of the 90s harbors yet another demon. The alcohols — particularly ethanol — that are now routinely added to gasoline in many refineries attract water from the air. Collecting in your fuel tank, H2O is then fed to the outboard, where pistons can't compress it, spark plugs can't ignite it, and internal metal surfaces succumb to its corrosive properties. To make matters worse, water can work its way into a fuel system even if there's no alcohol in the gasoline. Condensation inside metal tanks is one source, particularly in boats that see intermittent use. Loose or cracked fuel lines, as well as untightened fuel-fill caps, can also allow water intrusion. Regardless of the means, the results can be disasterous.

Several claims in the BoatUS files illustrate this point dramatically. In one case (Claim #9404959), a 200-h.p., V-6 outboard suffered catastrophic failure halfway across the Gulf Stream, resulting in a long tow. A tear-down of the engine revealed that the powerhead, carburetors, and fuel-system components were all severely damaged by the water that had accumulated in the fuel tank. Similar water-related havoc was discovered in Claim #921934MP, where the surveyor reported "The seal for the inlet servicing the main tank is not watertight. In addition, the clamp that secures the hose from the inlet to the tank itself was not adequately tightened and slid down the length of the hose."

Obviously, the average boat owner has no control over the amount of alcohol in the gasoline at the local pump, but it is possible to prevent the damage water-contaminated fuel can cause inside a two-stroke outboard. First, you and/your mechanic should carefully check all the hoses, connections, and fittings involved in delivering gas to your engine. Look for leaks, cracks, and looseness that might invite water. Doubling up hose clamps throughout the system is an excellent preventative measure.

Second, if your boat has an internal fuel tank (or even a large, above-decks reservoir), your outboard should be set up with a remotely mounted water-separating filter. There are several types, but most of the mechanics we talked to recommend a Racor unit that has a clear plastic "bowl" at the bottom of the screw-on metal canister. Fitted with a drain, this design allows you to detect water virtually at a glance and allows you to remove it without disassembling the whole filter system. Needless to say, however, the Racor, and every other type of separator, can be overwhelmed by large quantities of water. Always carry spare filters.

Battling the Black Stuff

As for the carbon deposits — the piston-ring "coking" — mentioned earlier, veteran outboard mechanics advise a two-pronged strategy: First, use aerosol "de-carboning" spray at the intervals recommended by your engine's manufacturer (usually every 50 hours). Some Johnson/Evinrude outboards have a fitting onto which you can screw the end of the short length of tubing that's attached to the can of OMC's formulation. In other cases, you'll have to spray the stuff right into the carburetor(s). Adhering slavishly to this de-carboning routine can as much as double the working life of an outboard that's particularly prone to coking (some models are far worse than others; talk to your dealer).

The second and even more important damage-avoidance weapon in the outboard owner's arsenal is TCW-3 oil. This latest generation of lubricants for water-cooled, two-stroke engines is specifically designed to counteract the carbon buildup that results from today's gasoline. These oils are also extraordinarily "emissable", meaning that they blend thoroughly with gasoline — and stay blended. Note, however, that TCW-3 is a standard, not a brand. On the one hand are a host of relatively inexpensive, "no-name" oils carrying this designation; on the other are the TCW-3 lubricants made to the specifications of the major outboard manufacturers. "I don't care if you use Mercury, Yamaha, or OMC oil," says Mercury mechanic Davis, "but don't buy the cheap off-brand stuff! When I pull an outboard apart, I can tell right away how much money the owner has been spending on oil."

Beating the Heat

As indicated by the examples already cited, the vast majority of outboard-related claims in the BoatUS files involve outboards damaged by overheating. However, we haven't yet discussed the two most common causes of this usually fatal condition: blockage of the water-intake louvers in the lower unit and failure of the water-pump impeller.

Our records show that blockage usually occurs at speed, when the outboard snags a plastic bag or a piece of broad-leaf seaweed like kelp. In no time — a matter of seconds at cruising r.p.m. — the flow of cooling water to the powerhead is interrupted and one or more of its component parts overheat to the point of failure. Is such damage preventable? Yes, but only if your rig has a working temperature alarm and only if you respond to the alarm instantly, by shutting down the outboard and clearing the blockage.

Unfortunately, many boat operators hear the horn and think, "I wonder what that's telling me? Maybe its just a false alarm." By then, it's too late. And, as one Florida Member discovered (Claim #9307100), a dash-mounted temperature gauge is little help. Here, as in most cases, by the time the needle registered a problem — and the helmsman noticed it — the extensive powerhead damage was already done.

The water-pump impeller, located in your outboard's lower unit, is the critical element in the engine's cooling system. Essentially a small rubber paddlewheel that rotates on a metal hub within a plastic or metal housing, it's rather vulnerable. Sand or mud in the water can lead to abrasion and premature failure; gasoline by-products can erode the bond between the hub and the rubber around it, causing slippage and rendering the impeller ineffective; momentary overheating can damage the impeller blades; and simple age and normal wear-and-tear can either impair the impeller's performance or cause it to fail completely when the blades shear.

To prevent the engine damage that quickly results from any of these scenarios, you must replace the impeller "regularly". The recommended interval varies from outboard to outboard and from manufacturer to manufacturer (you may have to make a phone call here; most owner's manuals don't list the interval). And, as noted, the number of hours you put on your engine, and the waters in which you operate also affect impeller life.

Suffice it to say that at least once a year, the lower unit should be "dropped" and the impeller inspected very carefully — especially for minute cracks and wear along the curl lines on the blades. At the same time, you or your mechanic should grease the splines at the top of the drive shaft and inspect, if not replace, the lubricant in the lower unit (check for fine metallic grit, a burned odor, and a milky color that would indicate water). Remove the propeller, too, and grease the splines on the shaft.

All the components dealt within this process — the impeller, drive shaft, and lower-unit gear set — are subject to exceptional stress, and failure in any of them will usually cause serious damage and huge repair bills.