How to Tell If a Outboard Motor Is Air or Water Cooled?


Outboard engines have two means to cool their combustion temperatures. One type uses a raw water pickup that circulates water through the engine and exhaust passages, which cools the engine block, head and main components by direct contact. The other cooling means, typically found in a two-cycle outboard motor, uses the flow of outside air over heat-dissipating fins to cool the block and head. Other subtle differences exist in the construction and design of both engine types, and a pilot or boat owner can recognize these differences with some close observation.

Things You'll Need

  • Owner's service manual
  • Remove your boat from the water and place it on a trailer, if you have it moored or docked. If you possess the original owner's service manual for the engine, open it up to the model, year and make page, which should be near the front of the manual under "Engine Specifications" or a similar term. Find the motor identification plate on the outboard motor, usually located on the front or back of the engine case. Make sure the manual motor identification matches in the information on the identification plate.

  • Read the manual specifications and look for the words "Cooling System" and "Engine Type." If you see the cooling system specifies "air cooled," then you can be positive that the motor uses air instead of water to cool the motor. If you see "two-cycle" under engine type, it will confirm the diagnosis. Water-cooled outboard motors will be listed as "water-cooled," which have a raw water pump, and contain the words "four-cycle" under engine type.

  • Examine the top of the engine where the cylinder head fastens to the engine block. If you have an upper engine cowl with a large intake, unclasp the snaps and pull the cowl back on its hinges or remove it completely. Look for large fins on the cylinder head that protrude upward, and any type of metal deflection plate that might be used to direct an air current. Cooling fins usually have magnesium or aluminum construction -- very light weight and thin in design. These fins absorb cool air and dissipate heat, typical of an air-cooled engine. Water-cooled engines will not have these large fins.

  • Examine the bottom of the outboard motor shaft, known as the "lower unit." Water-cooled outboard motors draw lake or seawater up through a water intake passage on the part of the motor that sits below the waterline. These can be identified as small holes or ports in both sides of the engine, or contain a port with a screen over it. There will also be an exhaust port higher up on the lower unit used for expelling heated exhaust water from the motor. An air-cooled engine will have no intake or exhaust ports on the lower unit.

  • Look up the fuel requirements for the motor in the service manual. If the directions indicate a mix of oil with the gas, generally a 2-percent ratio, it indicates a two-cycle engine type that corresponds to an air-cooled outboard motor. If you have no current service manual, look for a metal placard on the gas tank or engine case that denotes a two-cycle oil-fuel mix, or mix ratio number. Water-cooled outboard engines will run on straight gasoline, and will not be required to have a premix of gas to oil.

  • Unscrew the engine clamps from the transom and disconnect the quick-release fuel line and any ignition wire from the engine. Heft or feel the weight of the engine. Air-cooled engines, in addition to their smaller size, will typically weigh 20 to 30 percent less than a water-cooled engine rated at the same horsepower. You can make this comparison by testing the weight of both types of engines if you have both types available for your inspection. Water-cooled outboards have more major engine components than air-cooled types, giving them greater weight.

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