Can I Run My Outboard Without The Thermostat?


There are many different opinions out there about running an engine without a thermostat. Some of them are true, some of them are false. Figuring out your specific situation is how we can answer this question, for you!

Can I Run My Outboard Without The Thermostat? Yes, you can temporarily run your outboard without the thermostat and not harm the engine. No, you can not remove the thermostat and continue running the engine for long periods of time without causing harm to the engine.

Getting through the current problem and getting back on the water without harming your engine is the end goal to all boaters. Nobody wants to damage their engine and everybody wants to spend more time on the water. Here is a brief breakdown on why the thermostat is so important.

What Is The Purpose Of The Thermostat?

Running an outboard without thermostatAll manufactures have specific specifications for their engines that give them the longevity and reliability that they hold. One of these specifications is a specific operating temperature.

The engine needs to run in a specific temperature range in order to avoid premature failure. Or extensive wear and tear on the engines components. There was a time where people would take an old 2-stroke and just remove the thermostat and continue running the engine.

The misconception is that this is ok and causes no harm. However, the harm that is done is not something you can see without looking into the engine.

Why Temperature Ranges Are So ImportantCan an outboard run without the thermostat

Temperature ranges are important to ensure the life span of the engine. You don’t want the engine to run too hot and cause internal damage, eventually locking the engine up.

Ensuring that the engine runs in the correct temperature ranges is just as important as making sure that the Engine Gets Broke In Properly when it is brand new!

The hotter the engine gets, the thinner the oil becomes, losing the lubricating factors of the oil!

The same goes for running the engine too cold. If a thermostat is removed completely from the engine and it is ran for a long period of time, the engine will begin to build water into the oil, thus losing lubrication!

Where Does The Water Come From You Ask?

This is the part that most people gloss over. Four stroke engines utilize an oil pan and oil just like your car does. Due to the amazing heat relieving capabilities of being cooled by water, the engine can run extremely cold without the thermostat.

If the engine runs cold, this will create condensation due to the vast differences in the temperatures of the engine and the heat that is being produced by the engine. As time goes on, the condensation builds and water accumulates in the crankcase and in the oil!

If left for too long unchecked, the watery oil mixture eventually can cause internal damage to the engine. Thus, costing lots of money to repair what could have been avoided!

What Else Can Happen From Removing The Thermostat?

Have you ever seen an outboard running at idle on a dock, with lots of water coming out of the exhaust in the back?

Most commonly this is due to a thermostat that is stuck open! At idle the engine with an open or removed thermostat will be dumping water into the exhaust. With the boat at idle and sitting there, it doesn’t have the pressure to push the water down the driveshaft housing and out the prop.

So, it dumps it out the back through the exhaust on the back of the engine above the water!

So, It’s Ok To Run Temporarily Without The Thermostat?

Absolutely! It is ok to run the engine without the thermostat for a little while. Things happen. Troubleshooting, stuck closed thermostat, overheating, the list goes on. So in a pinch, remove the thermostat and get going!

The key is understanding why it is so important to have the thermostat installed and operating properly. Understanding what dangers and damages could be done if left unattended for too long.

Avoiding premature failure and unnecessary damage to your engine is the ultimate goal here, right?

What Happens If The Thermostat Is Stuck Closed?

Outboard engine thermostatsIf the thermostat is stuck closed, it will not be able to get rid of the water as quickly as it needs too. As the water runs through the engine, it absorbs the heat of the engine. It is important that as things heat up, it can disperse the hot water and get fresh cooler water to the engine block.

If the thermostat is not opening properly, then the engine will begin to get hot and quickly raise the temperature of the motor. Thus, causing an overheat problem. Depending on the age and type of outboard,

it can put the engine in Guardian, S.L.O.W. Mode, or Limp Mode. Limiting the power of the engine and only allowing low speed operation of the engine.

Common Symptoms Of A Bad Thermostat

The symptoms are pretty simple to see. If the thermostat is stuck open. The engine will run a lot colder. For 2-Strokes, it can cause a rough or poor running condition. For other non carburated 2-Strokes it can cause a high idle.

It can also cause the water to come out of the exhaust, creating an idle issue. Understand though, that there are a few engines out there that squirt water out of the back, regardless if the thermostat is operating properly or not.

We even made a Youtube Video about this on Our Channel explaining why outboards squirt water out of the exhause above the water line!

If the thermostat is stuck closed. Then you can see an overheat at idle or low speed problems. If the engine overheats at idle, but runs great at mid range or at wide open, then most likely, the thermostat needs replaced. If the engine is overheating no mater what speed, then the impeller is most likely the culprit.

Realize that there are certain manufacturers (Yamaha), that have certain engines that have other problems that cause the overheat at idle issue. For example, the early 2000 (Yamaha), 200, 225, and 250 motors, had a problem with the midsection rotting out and dumping water into the exhaust the same way.

So if you have this issue and have one of those engines, be warned that it might be a lot worse than a thermostat!

Can You Clean The Thermostat?

Yes, and No!Outboard motor thermostat

Yes. If you suck something up and get some grass or something stuck in the thermostat, you can clean it out and put it back in with no issues.

Cleaning the thermostat is actually a good idea to do with your regular preventative maintenance or service. Just getting the old dirt and corrosion off of it can extend the life of it.

No, you can’t just take a green corroded busted 8 year old thermostat, scrape it off and think it’s good to go again. It’s important to remember that the thermostat works on heat and resistance.

Over time from all of the heating and cooling and water abuse that it gets, the resistance can break down and it can open up prematurely and not work the way it is supposed to.

The thermostat isn’t really a serviceable item. Outside of cleaning a LITTLE corrosion off of it and picking a couple pieces of grass out of it.

In most cases, for the price of a new thermostat. It is better to just replace it and forget about it. Keep the peace of mind and stay out on the water without the trouble of worrying about it!

Is There A Difference In Marine And Automotive Thermostats?

Yes, and No.

There are a lot of different thermostats on the market. There are also many companies that produce these thermostats. So, just understand that some manufactures are buying their thermostats from the same companies, regardless of marine or auto use.

Remember that the width, height, and depth of the thermostat is important. Then, also probably even more important is the temperature rating of the thermostat.

They can look identical in size and shape but be many many degrees off in temperature resistance. If you put a thermostat in that opens at 115 degrees opposed to 143, then you have a problem and you aren’t fixing anything!

If you find a good alternative and it works properly, awesome! No worries! Sometimes though it is best to just get the correct thermostat from the manufacturer and not go through the trouble of checking all the differences between the two. But that is entirely up to you.

Hopefully this has been valuable and informational for you about why it is important to keep up with the thermostat maintenance in your outboard!

Something else that you might find interesting is this article that we wrote here about The Different Sections of An Outboard and How They Work! Where we go into the inner workings of the different major parts that make up and outboard motor.

Or we also have a ton of valueable information in this post talking all about Whether Or Not A Boat Is A Good Or Bad Investment!

Check Us Out!

We make all kinds of different DIY boating, How To, and Outboard Service videos on our YouTube Channel! We hope to get the chance to have you subscribe to our channel and allow us to help you keep your outboard in tip top shape and running great!

It’s also a great way to connect with us and ask any questions you might have! We’d also love to have the opportunity to answer any questions you might have in the comment section below!

Thanks for reading. Enjoy your boat and be safe on the water.

Aaron Hilligoss

Aaron has been working in the Marine Industry for over a decade and holds certifications for Yamaha and Mercury Marine. It is not uncommon for him to own and be working on at least three different boats at any given point in time!

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