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Johnson Thermostats

Written by Nicholas Kamuda
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Johnson engines all over the country use Johnson thermostats as part of their all-important cooling systems. The cooling system is responsible for maintaining an even running temperature in the power head of the engine. Johnson thermostats are the regulators of cooling systems, directly providing the engine with the resources that it needs to run smoothly.

The thermostat on many outboard engines is located along the cooling system's intake route, above the water pump. Raw water is channeled in via the water pump, and directed towards the engine via the impeller. On its way to the engine, the water has to pass through heat exchangers, oil coolers, and the thermostat.

The most common warning sign that something may be damaged in your cooling system may be erratic temperature gauge behavior. Before any other troubleshooting, you may find that it is a good idea to check the accuracy of the temperature gauge against a trustworthy thermometer. If the gauge is in fact accurate, and still displaying extreme temperatures, there may be some damage in the cooling system. Checking the pipes for blockage, debris, or other signs of clogging, and checking the impeller pump for damage may help you pinpoint the problem.

Quick Tests for Johnson Thermostats

Johnson thermostats, like other marine thermostats, open when hot. A quick and easy functionality test for a thermostat is to partially submerge it in a pan of hot water. The valve should open at the temperature indicated by the thermostat (usually written on the side), and should be fully open approximately 20 degrees F higher. This quick test should not be the basis for a comprehensive diagnosis, but it may give you an idea of whether or not the thermostat is functioning properly.

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