outboard motor parts

DIY inboard outboard boat motor parts & accessories

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The cooling system on an outboard is a very simple and compact application. Water is inducted through the lower unit by a water pump impeller, and then forced upward to circulate throughout the powerhead, and eventually exits through the exhaust system. There are a couple of things pertaining to the outboard cooling system that need regular maintenance, or require attention from time to time.
First, lets look at the typical water intake opening, and water pump assembly Figure 1. Water is drawn into the water pump through the intake opening of the lower unit, where it passes through a cavity that ends up entering the pump itself. At that point, the impeller pressures the water into the long inlet tube leading to the bottom side of the powerhead. Regularly check water inlet grate to make sure nothing is obstructing water flow. The water pump impeller should be replaced every two years, regardless of usage. An impeller will normally go bad from dry rot attributed to non-usage more so than wearing out from excessive usage. One thing to remember on an outboard is the warning horn. You will hear a loud constant shrill whistle if your outboard is in an overheat condition. DO NOT ignore this warning as an outboard can totally seize up within a matter of seconds if the overheat is severe enough. This is the reason to change impeller as recommended WITHOUT FAIL!

boat parts

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Upon reaching the powerhead, water is directed towards the thermostat and bypass valve Figure3. The bypass valve acts as a pressure relief until such time the thermostat opens and allows circulation through the powerhead to begin. Most outboard thermostats are factory designed to open at approx. 160 degrees.


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The two things that need regular maintenance or may cause problems are the impeller and thermostat. The impeller will be the cause of 95% of outboard overheat situations. It is rare that a thermostat goes bad, but it does occasionally happen. If you find that a thermostat is bad, do not simply decide that you can discard it and operate regardless, like some would with an automotive application. The thermostat of an inboard plays a very crucial role in warming the engine to a satisfactory operating temperature. If an outboard is operated without a thermostat installed, it will surely suffer in some way from poor performance all the way to causing internal damage. An outboard is designed to operate at specific engine temperatures, and if it does not obtain that temperature, it will run very sluggishly, and also cause fuel/oil mixture to not totally burn, which causes excessive carbon buildup. Carbon buildup in an outboard is one of its worst enemies. This condition usually most often causes problems with the piston rings becoming stuck in the piston ring grooves. If this happens, you can pretty well kiss your powerhead goodbye, because it most certainly will allow the piston to touch the cylinder wall, and immediate scoring will result. Once a piston/s is scored, it is only a matter of time before the engine will self destruct unless it is caught in time to perform an overhaul.
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.