Inspecting Marine Gasoline Engines

The condition of your boat’s engine is one of the most important safety and performance considerations while out on the water. In this article, originally published by BoatUS, provides a run-down of how to inspect your boat’s engine. Featuring a step-by-step guide on how to check your oil, how to address cooling system issues, compression, and more, this article will provide the basics for good engine maintenance. 

Inspecting Marine Gasoline EnginesReading through the claim files, you get the uneasy sense that most skippers didn’t have a clue their engines were developing a serious problem until the minute that they sputtered and died. Unlike an automobile’s engine, which tends to run and run until the car is traded in, a boat’s engine is more likely to develop problems, even when it is relatively new. These problems are considered normal wear and tear, and are not likely to be covered by a boat’s insurance policy.

There are three reasons for this disparity between automobile and marine engines. First, a boat’s engine works much harder than an automobile engine — up to twice as hard. While an automobile engine “strolls” through most of its life, a boat’s engine has to spend many hours swimming upstream. Second, the boat’s engine works in a much more hostile environment, water, often saltwater, which can corrode vital parts quickly. And, finally, marine engines develop problems because they are sometimes idle for weeks or even months. If they aren’t laid up properly, and many aren’t, this idleness encourages rust on internal parts, which is a major source of damage in marine engines (Seaworthy, October 1995).

How healthy is your engine? One obvious clue to an engine’s condition is its age. While operating hours are certainly a factor, the number of hours on an engine doesn’t bother mechanics as much as the number of years it has been in service. Marine mechanics report that a commercial boat engine that is used every day will often be in better shape mechanically after several thousand hours of use than a recreational boat’s engine with only 500 hours.

In this issue of Seaworthy, we’ll give you some simple tricks for diagnosing the condition of your boat’s engine. These tricks won’t tell you everything about your engine, but you’ll have a better idea of what’s going on internally and when to contact a mechanic to make adjustments that might save on more costly repairs later.

Examining the Oil

The number one rule of any 4-cycle engine is to change the oil and oil filter routinely, according to manufacturer’s recommendations. Oil’s role in keeping the engine running is critical — it lubricates all bearing surfaces as well as the crankshaft and camshaft bearings, timing gears, pistons, and valves. Marine engines must work very hard, and an oil cooler must be used in most engines to reduce the high internal temperatures.

Some things to look for in the oil:

  • Pull the dipstick and smell the oil. A strong burnt smell indicates the engine has been overheating — not a good sign. Next, wipe the dipstick on a clean white napkin. Oil that is thick initially and then slowly spreads over the napkin indicates the presence of fuel, which usually means that an overrich fuel condition (often caused by a sticky choke) is allowing fuel to leak past the rings into the oil. This condition can cause premature wear on bearings, rings, and pistons. If either condition exists — burnt oil or fuel in the oil — contact a mechanic.
  • Check under the oil filler cap, which is at the highest point in the system, as well as inside the valve cover (use your finger) for indications of condensation and/or rust. The latter is the #1 enemy of marine engines. When oil contains a lot of water it looks creamy or “frothy”, although this might not be apparent until the engine has been running for awhile. Even if the oil looks clean, you can put a drop or two on something hot, like a coffee maker burner. The drop should have a smooth edge — a jagged edge indicates water. Depending on the amount of water you suspect is in the oil — this is a judgment call and you may want to consult a mechanic — the engine could have a serious problem. Some condensation is to be expected, especially if the boat has been idle for a long time. Too much water, however, can indicate a blown head gasket, a cracked cylinder head, or a corroded cylinder liner.
  • Oil pressure should be noted at start-up and then again after the engine has warmed, to make sure it is up to manufacturer’s specs. Low oil pressure at start-up — below about 25-30 (this varies by manufacturer, so check your manual) indicates a potential problem with the cam bearings, oil pump, or a faulty pressure sending unit.
  • Oil tests are relatively inexpensive, can be done by a boat owner, and tell you what’s in the oil — metals, water, or fuel. A test’s value, however, depends on how recently the oil has been changed. To be effective, the oil being tested should have been in use for several weeks. Samples should be taken from the mid-sump area after the engine is warmed. Taking several oil samples over a period of months or years tells you a lot more than taking a single oil sample. Contact local auto supply stores like NAPA, if they don’t have the kits, the store manager should be able to recommend someone who does.
  • Pull the plugs. Tips that are a brown or beige color indicate normal combustion, while a gray color indicates detonation (Seaworthy, January 1996) in the cylinder.

Complete and original article originally published on

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