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Troubleshooter: How to Doctor Your Outboard, Inboard, or Stern Drive Marine Engine

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Learn how to diagnosis what your marine engine and/or its drive may be trying to tell you.

Nothing man-made is totally trouble-free. As dependable as today's marine engines are, it's inevitable that someday some component will begin to cause problems. When that happens, it helps to know what some of the symptoms signify, so that you can either make a repair yourself, or have a reputable marine mechanic do it for you. However you finally handle the problem, here's how to make a diagnosis based on what your engine and/or drive may be telling you.

By far the most vulnerable component on an outboard motor is its lower unit. Within the confines of the aluminum housing are the water pump and the gear case assembly. In my days as a marine mechanic, and later as a service manager, the most commonly repaired item was the water pump, with propeller shaft seals a close second.

Overheating is the first clue that a water pump is no longer pushing coolant through the power head. Although overheating can also be caused by something as simple as a discarded plastic baggy plugging up the pump intake. On newer models, your first clue that an outboard is overheating is the sound of an alarm horn or buzzer, and engine rpm being forced to a lower speed. On older outboards without such electronic warning systems, loss of power and rough running are signals that an outboard may be overheating.

Repairing or replacing a faulty water pump isn't difficult, but you absolutely must have the proper tools--which include a shop manual as well as the right set of sockets and box wrenches. Accessing the pump is simply a matter of removing the lower unit's bolts, and sliding the housing down and away from the motor. There can, however, be some complications. Because, depending on whether the outboard is a mechanical or electrical-shift model, you must also disconnect either the linkage or the wires located inside the housing. If the outboard has been run in salt water, the housing may have corroded to the point where you must heat it with an acetylene torch in order to separate the cases. And then there's the O-ring located right below the splines of the drive shaft. Sometimes it sticks in the recess inside the crankshaft, and unless you know it's there, you could lose it or inadvertently install two rings during reassembly. These are just some of the reasons you need a shop manual or a videotape that shows how to go about the procedure.

As for propeller shaft seals, when they fail, they leak rather obviously. When the outboard is running, its spinning gears create high pressure that forces gear lube out through the leaking seal. When the outboard is shut down, the vacuum created by the lost lube sucks in water. The presence of water in the gear lube tells you the prop shaft seal is leaking.

The water shows up when you crack the lower screw in the gear case. If the outboard hasn't been run for some hours, the oil and water will separate. The oil floats on top of the water, and the water drips out of the drain screw when you crack it. You can also test for water immediately after running--when the oil and water are still mixed together. The presence of water produces a milky-white froth.

The quickest way to spot a leaky seal is with a pressure-tester. This tool is much like a bicycle pump, only instead of inflating a tire, it fills the drained gear case with air pressure. A gauge on the tool records how much pressure you pump in, and then notes whether any leaks out. If it does, remove the gear case and dunk it, still pressurized, into a tank of water. Just as a tire leak will produce bubbles, the bad seal's leak will leave an underwater trail that points toward the source. Although the drive shaft seal or mechanical shift-linkage seal could also be leaking, this is not very common. Replacing a propeller shaft seal requires two special tools that are probably not in your personal inventory: a seal remover and a seal installer. You may, however, be able to borrow them from your dealer.

The importance of instructional tools cannot be overemphasized. An exploded diagram that shows all of the individual parts in relation to each other, plus a text or video that explains how to do the job right the first time, are invaluable. In many cases, the cost of a shop manual or tape are less than what you'd pay for a single hour of shop time. Also, the knowledge they provide may well be the difference between completing the job and dragging a basketful of parts and a red face into a repair shop.

Besides the gear case proper, propellers can also cause problems. But this usually happens only if you inadvertently run over an underwater obstacle and the blades make contact. Obviously, bent or otherwise-damaged blades detract from an outboard's optimal efficiency--the motor's acceleration and fuel economy both suffer. But, beyond that, when damage is so severe that it knocks the prop totally out of balance, vibration can cause stress fractures inside the lower unit and power head. Most of the time, however, you'll be able to feel this rough running right through the soles of your deck shoes and have fair warning that a repair is in order.

Diagnosing Stern Drive Marine Engine Maladies

There are obvious similarities between outboard and stern drive lower units, so it's not surprising that they develop similar problems. Like outboards, stern drive lower units are prone to failed water pumps and prop shaft-seal leaks. Stern drive engines are another matter entirely. One of the things I really like about these engines (and inboards, for that matter) is the way they like to tell you what's wrong with them. Most of the time they do it with either smoke signals or the color of their fluid leaks.

Black smoke simply means that the engine is running rich. In other words, it's not burning all of the gasoline it's consuming. On a marine engine, that doesn't necessarily mean it's getting too much gas. Instead, it could mean that it's not getting enough air. So, the first thing to do is to remove the flame arrestor. Clean it in a suitable solvent. But before replacing it, make sure the choke isn't stuck in the closed position (carburetors only). If it is, figure out whether the linkage is broken or stuck, then repair it accordingly. If, when the engine is running cold, the choke does not remain closed, and does not open gradually as the engine warms, a repair is in order. On a marine engine with an electric choke, the wire that runs from the ignition switch to the choke can become corroded and fail to carry the proper current. If and when you check it, also check the fuse to make sure it hasn't blown.

Black smoke could also be caused by something as simple as a weak ignition spark that's insufficient to ignite all the fuel. This, too, is fairly easy to diagnose and repair. Simply remove the spark plugs and replace them with new, properly gapped ones. Given the cost of new plugs, it's not worth cleaning and re-gapping the old ones. Since most modern stern drive engines don't have points, about the only requirements for a tune-up these days are to clean the spark arrestor and change the plugs.

Blue smoke is another matter, but many boat owners routinely confuse blue smoke with black smoke, and consequently miss the significance of the clue. Blue smoke means oil is somehow migrating into the combustion chamber, where it burns and comes out the exhaust. The burning question is: Where did the oil come from? The answer could be as simple as having had too much oil in the sump, or having worn rings and/or valve guides. If the latter is the case, you're in for an expensive overhaul. There is, however, another possibility: The crankcase oil may not have been changed regularly. If it hasn't, engine wear may not have any bearing at all on the problem.' The oil-control rings may be stuck tightly into their piston grooves because of all the varnish that's been circulating through the engine.

Sometimes changing the oil and adding a quart of diesel fuel to the crankcase will loosen stuck rings. But I don't recommend this approach. I've successfully freed a number of engines with Teflon lubricants like Tufoil. Its micro powder can work rings free in about five to 10 hours.

Inboard Marine Engine Ills

Inboard engines are relatively trouble-free. Reduction gears and water pumps are located high and dry inside the boat. Underwater obstructions can damage the propeller and its shaft. But when they do, the resulting vibration is usually felt immediately through the cockpit sole.

Like stern drive engines, inboard power plants also tip you off with smoke signals. They also make sounds that can point you in the right troubleshooting direction. If the engine turns over at its normal speed, but refuses to start, that tells you the battery is charged and that either the ignition or fuel system is at fault. Conversely, if you turn the key and all you hear is a click-click, then the battery may have a low charge, or the solenoid and/or starter may be bad. The best way for a lay mechanic to check a battery is to spend about a dollar and buy a battery tester that measures each cell's specific gravity. These are the medicine dropper type that draw in battery electrolyte and float from one to four tiny balls. The more balls that float, the higher the battery's charge. Knowing a battery's charge is important when troubleshooting because it allows you to proceed without wondering if the battery may be at fault.

If you find any liquid other than water in the bilge, its color and consistency can lead you to its origin. Lime-green liquid is antifreeze, while cherry-red liquid is from the transmission or from the power-steering unit. Engine oil is most typically carbon-black. Any liquid present indicates a leak.

Evaluating EFI Marine Engines

Anyone who tells you that weekend mechanics can't work on marine EFI is wrong. Because marine and automotive EFI are so similar, you can take an adult education class that deals with auto EFI. In certain respects, troubleshooting EFI is much simpler than dealing with the old battery and CD ignitions of yesteryear. You'll need to invest in an electronic scanner that can read and erase error codes. These cost between $100 and $1,000, depending on how sophisticated a unit you need. But keep in mind that, at a minimum, old-style ignitions required a timing light and dwell meter whose total cost was about equal to a bare-bones scanner that not only diagnoses EFI ailments, but also allows you to set ignition timing electronically and without having to wrestle with distributor rotation and hold-down bolts.

Prevention Tips

The best time to spot a malady is before it has a chance to complicate your life. That's why preventive maintenance makes so much sense. This means keeping a sharp lookout for puddles of liquid in the bilge and purple haze drifting off the transom and keeping an ear tuned to strange thumps and whines emanating from the engine compartment.

Write down what you notice. If you do so, this record can be invaluable at a future date when faced with a problem you have to troubleshoot. In troubleshooting marine engines, knowledge isn't simply power, it's the most valuable tool in your kit. Always take the time to properly lay-up and commission engines and drives. And periodically preflight your boat--i.e., check all fluid levels and make a thorough inspection of the engine compartment, as well as of drive legs and running gear below the waterline.