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Outboard Thermostats

Written by Nicholas Kamuda
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Outboard thermostats directly control the amount of cooling water that circulates through the power head of an outboard engine. Though they are just one link in the chain of the cooling system, they play a vital role, and should be periodically inspected and tested. If the thermostat on an outboard is damaged in any way, there is the distinct risk of serious damage to the engine head.

Most boaters can approximate their thermostat's condition by using the boat's temperature gauge. If the gauge begins to dip or spike more than usual, there may be a problem with the thermostat. The thermostat isn't the only source of cooling system problems, however. Broken impeller spines, clogged water pump intakes, and other clogged passageways are also behind many overheated engines.

Though overheating may be a skipper's primary concern (with regards to the cooling system), overcooling can also cause long-term damage. Unlike overheating, overcooling problems only have one source--a faulty thermostat. A simple test of an outboard thermostats condition involves submerging the thermostat in heated water. As the water reaches the thermostat's trigger temperature value (usually written on the side of the thermostat), the thermostat should begin to open.

Purchasing Replacement Outboard Thermostats

Outboard engine supply shops and most boating shops--including commercial Internet sites--should stock a wide variety of outboard thermostats. Many shops accept orders by part number, which can be found in your service manual. The service manual should also provide instructions for accessing the thermostat and other cooling system components in your outboard engine.

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