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Are you familiar with an outboard engine’s thermostat? We weren’t, until the other day that is. Of course, I had heard that the engine had such a part but until our last bit of engine servicing where we became intimately familiar with its cooling system, I had no idea where the thermostat resided nor its purpose.

From what I have read, the thermostat’s job is to control the amount of cooling water which reaches the engine. When the engine is first started and is not yet up to its recommended operating temperature, the thermostat only lets a small amount of sea water flow through the engine block. As the engine increases in temperature, so does the amount of water that the thermostat allows to flow through it.

Problems can arise in this system when the thermostat gets all clogged up, either with minerals from the sea water or by bits of rubber broken off from the impeller (a different problem altogether!). Sometimes they get stuck in the open position while at other times, they get stuck closed. As you might imagine, neither scenario is good. You can see in the image below what an old Yamaha thermostat looks like when compared to a shiny new one.

We returned from California with shiny new thermostats for our Yamaha engines.

Remember where you are reading this (Zero To Cruising) and thus, don’t take it as gospel but I have read that if you’re in a pickle and you feel that your thermostat is causing you problems, the engine will still run with it removed. It may not run well, and it may burn more fuel than it should, but you should hopefully still be able to make it back to harbor so that you can find a replacement.

Today, we’ll be taking a crack at servicing our port side engine. We plan to change the impeller, lower unit oil, engine oil, spark plugs and give it an overall cleaning and lube job. Hopefully we don’t run into any unexpected surprises that turn this one day job into a multi-day exercise like the last time!


  1. Mike,
    Is your engine service based on hours used, calendar time, when something breaks or just when you think about it? I would guess hours used would be primary, but since the engines sit in salt water without running a great deal of time, I would think there is a minimum time (calendar) that you need to service the engines? Just curious.

    • All of the above. We change engine oil and spark plugs based on engine hours (ex. oil every 100 hours). Jobs such as today’s are done by calendar (lower unit oil and impeller once per year). Of course, when something breaks, it jumps to the top of the to-do list!

  2. Hey you two,

    Actually, what will happen is the engine won’t come to temperature as soon as it should. This causes it to run rich (it still thinks it’s cold) so fuel mileage is reduced.

    You’re right though, it really won’t hurt anything to remove it for the short haul.


    • What Jim said… In a motor with a radiator it is a little different though. The thermostat also keeps coolant from flowing too fast through the radiator, that way it has more of a chance to cool..

      • Do diesel engines in boats have radiators?

        • Not in the normal sense no they don’t Mike.

          Usually, either the engine is cooled directly with sea water, like your
          Yamahas are.

          OR with an exchanger system that brings in sea water and ‘extracts’
          the engine heat through the exchanger.

          Still in all, they all work in basically the same way.

          Joe hit it right on the head: The longer the coolant is in the radiator
          the better it radiates the heat out of the system. The thermostat, even
          when wide open, still induces a bit of a restriction in the cooling

          WATER is the best coolant. It’s all about the surface tension of the water.
          Less surface tension, like water, lets the water molecules sneak
          into all the little nooks and crannies of the engine and yank that
          heat out.

          In a radiator or closed loop system you need to run at least
          25% anti-freeze for it’s corrosion inhibitors.
          Unfortunately the anti-freeze doesn’t help with the cooling.
          Again, water is the best.

          SO, you asked what time it is and I told you how to build a watch.


  3. Good luck with that! From the looks of the above, it’s in the nick of time.

    • Hard to say. The one in the starboard engine looked even worse and we tested it, it still functioned (perhaps not within specified parameters, I don’t know because we actually didn’t check that).

      • Well, maybe in the nick of time as to peace of mind. I know I would be worrying about a failure if I had replaced one looking like that, knowing that the other probably looked as bad. Plus, you now have an good, practical data on when to change them next. But that’s just me, I’m a worrier.

  4. Something else Mike, when you get a new thermostat, you need to put it in a pot of boiling water and confirm that it will in fact open and then close as it cools down when you take it out, before you install it in your engine. I have seen new ones that did not work and that kind of sucks, when your trying to trouble shoot a problem.

  5. Incidentally, I should think that the ‘used’ one in your pic is still perfectly good. Just no longer “pretty”. Almost certainly worth keeping it as a spare.


    • Definitely. We always keep stuff like that as spares. We keep all our old spark plugs too. We have made a few islanders happy by offering them plugs for their engines.

  6. Hi Mike –

    All the same comments apply to an automotive gasoline engine. (Well except the one about the thermostat keeping the water from going thru the radiator too fast.) The thermostat’s job is to allow the engine to heat up quickly, and to prevent a water pump that can supply enough cooling water for extended WOT operation from over-cooling the engine at partial throttle or idle.

    Yes, in an emergency, it would be far better to run with the thermostat removed than to run with one that is stuck closed – that would surely ruin your engine. In a cast iron car engine, lack of cooling will result in a warped head (read: engine tear down and machine time). In an aluminum outboard engine, it would probably result in a piston stuck in the bore or worse, ie. junk.


    (and no, diesel boats don’t have radiators – they either use a sea water cooled heat exchanger or a keel cooler)

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