When to Replace an Outboard Thermostat

If you can’t remember the last time you replaced your outboard’s thermostat(s), consider yourself lucky if the engine is still healthy and hasn't overheated or failed completely.

Outboard thermostat replacement tips

Outboard motor thermostats are an often overlooked, yet essential little component for controlling the temperature of the engine and ensuring it runs at peak performance. Here are some tips to help you determine when it’s time to change your outboard’s thermostat(s).

Buy a thermostat

Buy outboard cooling system parts

What is a Thermostat?

A thermostat is a valve in the outboard’s cooling system that opens or closes depending on temperature. When the thermostat is closed, it prevents the flow of cooling water passing through the outboard, thus increasing the motor’s operating temperature. As the temperature rises, the thermostat opens and allows coolant to flow through the outboard, thus decreasing its operating temperature.

By constantly opening and closing, the thermostat keeps the motor at its ideal operating temperature. This prevents the engine from overheating, and ensures the fuel combustion is perfectly optimized to provide maximum power while minimizing fuel consumption

Symptoms of a Bad Thermostat

When thermostats go bad, they don’t slowly decline, just simply fail completely. Usually a thermostat remains closed when it fails, cutting off the supply of coolant to the outboard. However, a thermostat will occasionally fail in the open position or get jammed by corrosion or debris, thus allowing coolant to constantly flow though the motor regardless of whether the outboard needs it or not.

Outboard thermostat needs replacing

Symptoms of a failed thermostat (closed position)

  • Engine overheating, particularly at idle and low running speed 
  • Engine going into limited power or slow mode to mitigate against overheating
  • Increased fuel consumption

Symptoms of a failed thermostat (open position)

  • Engine slow to warm up
  • Engine not reaching its correct operating temperature
  • Poor cold weather performance
  • Water flowing from the exhaust pipe at idle

When to Change the Outboard's Thermostat 

Thermostats are fairly robust, and can last for several years depending upon how many hours you put on your outboard, and what types of conditions you run it in. For example, the thermostat of an outboard that runs in salt water will experience far more corrosion than thermostats in outboards primarily used in fresh water, and will need replacing more frequently. 

Outboard engine bad thermostat change

Your outboard’s owner's manual should tell you how often you should replace the thermostat(s). However, those are just guidelines, and if you expose it to a lot of salt water or silty conditions, or don’t always flush your motor after using it, expect to replace the thermostat(s) more often. Each outboard engine thermostat should be replaced at least every two years.

Don’t Put Off Changing the Thermostat

Outboard thermostats can’t be serviced or repaired, so when the time comes to change them out, discard the used thermostat after installing a new one. The good news is that thermostats are inexpensive, and considering how important they are to the health and performance of the motor, buying a new thermostat once every couple of years is not something you want to cheap out on.

Outboard thermostat change tips

If you purchase a secondhand outboard, you should replace the thermostat(s) immediately, because there’s no way of knowing when it was last changed. It's better to do it right away just to be safe. Also, when an outboard has two or more thermostats and you have to replace one because it has failed, replace all the others at the same time. Outboard thermostats wear evenly, so if one fails, the others will soon follow. Lastly, add thermostats to your list of essential spare parts to keep on board just in case the current thermostats in your outboard fail while you’re out on the water.



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