Testing an Outboard Thermostat

Finding water mixed in with your outboard engine's oil is a nightmare. Before you start tearing down the motor looking for cracks in the cylinder head or blown gaskets, you might want to check the thermostat. 

Believe it or not, a faulty thermostat can cause water buildup in an outboard’s engine oil. If you find water mixed in with the oil, test the thermostat first before you panic. Skip the next section of this article and jump to the one that follows for a couple of simple steps on how to diagnose a bad outboard thermostat.

How a Bad Thermostat Causes Water Buildup

The thermostat is a valve that regulates the operating temperature of the outboard by opening to let cool water flow through the engine, and closing to prevent the flow of water from inhibiting the engine’s ability to reach its ideal operating temperature. 

In other words, if a thermostat fails in the open position, water will continuously flow through the engine, and the motor will run too cold. When this happens, condensation forms within the engine due to hot engine oil coming into contact with cold engine surfaces. The condensation forms water, which mixes with the oil and eventually forms a milky residue. You’ll see the residue on the dipstick and on the inside of the oil filler cap (similar to the goop pictured below).

Diagnosing a Failed Outboard Thermostat

Step 1: Remove the thermostat from the motor. 

NOTE: Some Yamaha outboards have two thermostats, in which case you would remove them both.

Step 2: Put the thermostat in a bowl and immerse it in boiling water. As the thermostat heats up, it should open. When you remove it from the boiling water, allow it to cool and it should close again. If the thermostat remains open or closed, it has failed and must be replaced.

Finding Replacement Thermostats

Thermostats are manufactured in various shapes and sizes for different outboard engines. At Boats.net we’ve made it easy to find the right parts for your outboard engine. Simply select your outboard’s brand, year, HP and model, and browse hundreds of OEM replacement parts, including thermostats. 

Many outboard models on our site have Scheduled Service Parts and Cylinder Crankcase parts pages where you can find your outboard model’s correct thermostat. Or you can check out our available thermostats for more options. Please contact us if you need assistance finding a thermostat or any other boat parts and accessories.