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The cooling system of a boat motor is one of the most vital necessities it can have. Worn or damaged water pump/impeller, clogged passages, cracked/kinked hoses, among other things can easily create an overheat condition, which is one of the deadliest conditions a motor can face. Overheating can and does cause premature failure due to swelling of pistons and rings in the cylinder bore, warping and/or cracking of cylinder heads, blown cylinder head gaskets, degradation of oil lubricating properties, etc. The cooling system of a marine engine is armed with an electronic audio alert device simply called a "warning horn". If you hear a shrill high pitch sound coming from under the dash, or from the throttle box while under power, heed the warning, and do not continue operation of motor. You should also test the function of the warning horn periodically by disconnecting the brown wire attached to the engine temperature sensor, and with key switch in the "on" position, directly touch the brown wire to a good known ground source. Warning horn should sound off, if not, horn and/or wire needs to be checked or replaced.
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There are two types of cooling systems commonly found on inboard/outboard boat motors.
Figure 1 shows a typical fresh water dependent cooling system. With this system, the motor is totally dependent of fresh seawater being present to provide proper cooling characteristics. Fresh seawater is normally drawn by a belt driven pump from a thru hull fitting in hull of boat on inboards, or by water pump in stern drive through the lower unit water passages on an inboard/outboard, and then exits the boat through the exhaust.
 
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Figure 2 shows the typical "enclosed" cooling system. This configuration utilizes contained antifreeze in the main part of the motor and also uses a heat exchanger much like the purpose of a radiator in an automotive application. The difference being instead of relying on air flow to cool the antifreeze, fresh seawater is pumped through the heat exchanger and then exits the boat through the exhaust.
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Now that you hopefully understand the basics of the cooling system, lets look at the source of the process, and the things that can go wrong, causing an overheat situation.
Figure 3 shows a typical belt driven seawater pump. This pump is the heart of the cooling system, and if anything goes wrong here, an overheat problem is inevitable. Failures can include worn or damaged impeller, worn or damaged housing, and impeller shaft lock up. It is recommended that the impeller be changed every two years regardless of amount of usage. An impeller will usually go bad due to dry rot from non-usage, rather than being worn out from excessive usage, and can also be damaged from running motor with insufficient water supply. When replacing the impeller, make sure to apply a thin layer of light weight grease to the inner surface of the impeller housing. Damage to the pump housing can be caused from lack of winterization procedures, and/or running motor with insufficient water supply. If the shaft locks up, there is no real way to service, so the pump must be replaced as a whole.
  
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Figure 4 & 5 shows location of water pump located in stern drive. Failures here, are typically the same as in figure 3, except that shaft lock up does not usually occur. If it does, you've got much bigger problems, and you need to refer to the drive section for further information.
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Moving on to the thermostat, the next possible culprit of an overheating problem in the cooling system. Figure 6 shows the typical thermostat arrangement in the thermostat housing. Most thermostats are designed to open at 160 degrees water temperature. As long as the thermostat opens as designed, everything is fine, if it doesn't then overheating will surely result. To test the thermostat, remove and place in pan of water on cook stove. With a reliable thermometer, heat water until thermostat opens. It should open at approx. 160 degrees. If it does not, discard and replace with new thermostat.
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Finally, the third most common reason of an overheat problem with the cooling system can be attributed to clogged water passages, usually in the exhaust manifolds. Since most exhaust manifolds are made of cast iron, rust and corrosion are always present. If you have an overheating problem, and the impeller and thermostat are in good shape, remove the riser from the exhaust manifold Figure 7, and inspect water passages for blockage. if the passages are blocked, its basically the same thing as the thermostat not opening, and water circulation is not allowed to escape through exhaust as designed.
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In summary, change water pump without fail EVERY two years regardless of usage. Also, if you operate your boat in salt water, make sure to flush your engine and cooling system after every use. Following these simple guidelines will protect your marine engine from serious damage, and the mechanical skills required are fairly minimal. If you should have any questions, feel free to email us.
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