With four stroke outboards, inboards and stern drives, it's imperative to change crankcase oil. That’s because a marine engine is a mini-refinery that ever so slowly evaporates lubricant's lighter molecules. Heavier molecules stay in the sump and become thicker and stickier, just like varnish or sludge. Then the sludge circulates throughout the oil passages and ultimately causes the oil control rings to stick tight in the piston lands. No longer expanding and contracting they no longer scrape excess oil from the sides of the cylinder walls. So with every stroke of the crankshaft excess oil pumps into the combustion chamber. What once was a healthy engine is an oil burner.

Now, because you were too lazy to schedule an oil change, you're stuck pulling the dipstick so often that it becomes a real hassle. The alternative is to inadvertently run the engine out of oil. Do this, and it will die.

Even worse than the monotony of pre-flights is when errant oil burns in the combustion chamber and is transformed into carbon, an abrasive that circulates throughout the engine, scoring cylinder walls and bearings. Do not believe the well-meaning slip mate who tells you that oil burning is good for an engine because it means there's more lubrication in the combustion chamber. Once blue smoke clouds around the transom, an engine's days are numbered.

On stern drives and outboards, regularly check the gearcase oil for water. If there's only a drop or two, check at more frequent intervals. As little as an ounce of water means there's a serious leak and immediate professional attention is required. Know that water can migrate into the gearcase because spinning gears can act like an oil pump and force lubricant out through a leaky seal. When the propeller stops, the gears stop; but now there's a vacuum created by the missing oil. Water is drawn in. A marine mechanic finds exactly which seal is leaking by pumping air into the gearcase, then submerging it in water.

A trail of bubbles leads to the failed part. Usually, it's the propshaft seal that's been torn up by monofilament fishing line picked up by the propeller. But on older motors, sometimes the shift rod or driveshaft seals wear out.

Make sure the engine is properly cooled. Overheating is the single most common cause of a ruined marine engine. On closed cooling systems, this means making sure there's plenty of 50/50 blend antifreeze and water in the tank. On freshwater-cooled inboards, stern drives and outboards make sure the inlet and outlet hoses have not collapsed. Older engines can overheat if their hoses break. It's for this reason we squeeze hoses to make sure they're still supple, and not stiff and cracked.

Should an engine overheat, shut it down as soon as possible. Otherwise, heat and pressure will destroy crankshaft journals, rod and main bearings. Cylinder walls score. Compression plummets. The net result is that the engine will never run right no matter how many times you change the spark plugs and distributor cap. Even if you don't run the engine right up to the eve of destruction, other bad things can happen. The cylinder head could warp. If it does, slapping on a new gasket isn't enough. Instead, you must remove intake and exhaust manifolds and the cylinder head, which then has to be milled flat. Figure on spending several hundred dollars and losing the boat for a week to a month.

Keep the engine in perfect tune. Tune-ups not only keep performance razor sharp, they also conserve fuel and extend engine life by making sure all the fuel that flows into the combustion chamber burns. Otherwise raw fuel washes oil off the cylinder walls. Lack of oil equals friction, which causes wear. Another related malady, the raw gasoline mixes with the lubricant in the sump, diluting the mixture and diminishing the oil's ability to cool and lubricate.

Tuning a marine engine involves more than just changing spark plugs. On inboards and stern drives the flame arrestor must also be cleaned. Otherwise a dirty arrestor restricts air flow and the mixture will be too fuel-rich. Because marine engine compartments run so hot, the spark plug wires are another weak link. Heat cracks their insulation, and voltage that' s supposed to fire the spark plug leaks into the atmosphere via wet, ambient air. Test for cracked wires by studying the engine at night. Stray voltage shows up as blue arcs in the night air. The light show can be helped along by spraying a mist of fresh water on the wires.

Finally, easiest way to insure long engine life is to make sure it's spinning the correct pitch propeller for the kind of running you do. The wrong pitch can lug an engine down, or over-rev it. Both conditions waste fuel and cause wear. But with the right propeller, acceleration is perfectly matched - Home Page -

© Copyright 2007 by Tim Banse