How To Check An Outboard Thermostat

Author: Stuart Buckingham   Date Posted:27 June 2019 

Blog How To Check An Outboard Thermostat

If I were to ask you what the five most important things to check when servicing an outboard were most would rate a thermostat pretty low and some, in fact, wouldn’t even rate it. We are trained to think that the heart of the cooling system is the water pump and it should be checked and changed annually, and this is correct, but if the water pump is the heart of the cooling system then the thermostat should be considered the brain!


A thermostat is designed to hold the cooling water in the block until it reaches the temperature set by the engine manufacturer. Generally, this is between 50 and 60 degrees Celsius. It then opens to release the water before it gets too hot and overheats the outboard.


Because of the saltwater environment that most of us run our outboards in thermostats are susceptible to corrosion, salt build up (from irregular flushing) and sand build up (from beach launching or shallow water usage). These are the instances that can cause thermostat blockages and lead to your engine overheating.


The good news is that checking the thermostat is often a quick and easy task. Here is a quick step-by-step guide to checking your thermostat at home:

  1. 1. Unbolt the thermostat housing located at the top of the cylinder head or exhaust plate (Note: for older V4 Evinrude/Johnson outboards this will be located at the bottom of the exhaust plate between the cylinder heads)
  2. 2. Remove the thermostat and place in a clear jar
  3. 3. Cover thermostat with boiling water


Working thermostat: A working thermostat should open as the wax inside expands from the heat of the water. If your thermostat appears to be working, clean any muck around where the thermostat seats in the block and re-install the thermostat with a new thermostat gasket.


Faulty thermostat: A faulty thermostat can open slightly without opening completely or stay fully closed. If your thermostat appears faulty, you’ll need a new thermostat and thermostat gasket.


Now, I will add to this as I am sure there are the technically experienced readers who have read the above and are shaking their heads. A kettle boils at approximately 100 degrees Celsius so tipping in water of that temperature onto a thermostat that opens at 60 degrees Celsius isn’t telling you if the thermostat is opening within the correct range. So, if you have a temperature probe or thermometer you can wait until the water reaches say 65 degrees Celsius and then pour it on the thermostat to test. But, for the purpose of simply checking whether the thermostat is operational, the above method will work fine.


To shop our range of thermostats, click here.

Comments (3)


By: on 28 June 2019
Will follow up on the thermostat check next time I work on motor, thanks for your advice.thanks Keith


By: on 28 June 2019
That is what I do except for my 2.3 Honda.

thermostat test

By: on 28 June 2019
This is a most important aspect of out board service and should be done every 12 months regardless of amount of use.

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