# Removed thermostat from outboard. Thoughts?



## sealover (Jun 27, 2009)

My 4 HP Yamaha had been running hot. I've never been happy with the little trickle of cooling water dribbling out of it, but was told because it's such a small motor that's what to expect. I replaced the impeller and didn't see any noticeable difference. I started thinking about what else could be causing a restriction in the coolant system so I gave the assembly drawings (from outboard parts websites) a second look. There's a thermostat??? Yup. I pulled the exhaust cover off and saw a very gunked up thermostat. Most of the gunk was clear/white and gooey. I thought maybe it was salt and touched it to my tongue to verify. No real taste at all. Maybe a baby sea nettle got sucked in through the water intake? Either way I don't have a spare $56 (!!!) thermostat laying around and I wasn't about to wait a week to get one so I put it back together without it. Put it back on it's engine stand (a hand truck) and put the bottom end in the test tank (5 gallon bucket) and ran it. God love it, the little bugger started up with the second pull (having sat since Columbus day when it's running-hot problem caused me to drift uncomfortably close to the Carnival Pride) 

Healthy stream of cooling water which was coming out warmer than it was going in. 

I use the motor for 2 minutes to get off the dock, and 2 minutes to motor back to the dock when sailing. Once the Chesapeake gets warm I throw it on a little aluminum rowboat and use it for crabbing. 8 hours of half throttle for a minute, idle for a minute. Little bugger uses a gallon and a half doing that from before sunrise to early afternoon. 

Do I even need a thermostat? Even today in 40 degree weather it ran fine in the test tank. (and on some year-old gas from my stash of pre-ethanol stuff)

I can't see how sucking 70 degree water on a 70 degree day will do any harm, and I don't think 2 minutes of sucking 50 degree water in April will make a difference. I've torn enough scorched pistons out of weedwackers to know that heat is what does these in -- taking longer to warm up (or not warming up at all) doesn't seem like nearly the problem. At the recommended 100:1 mix I doubt it's going to foul out the plug real quick, but I carry a spare and a wrench just in case. (I've never needed them, even with the first two seasons I had the thing not ever running more than half throttle or more than five minutes.) 

Why even put one in there? Maybe duck hunters running around breaking ice with their little Jon boats would benefit from a little restriction in the cooling system? I suspect the EPA was happy to see it there to allow it to warm up and properly burn off some more hydrocarbons. The lifetime total gallons I've ran through the little bugger is less than one tank of gas in the tow car so I'm not really concerned about that. 

Thoughts?


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## mdbee (May 2, 2007)

Maybe try cleaning the old one & the testing it for proper operation?

Although I doubt it would happen on your small engine, there is a thing called "cold seizure" on high performance engines. The tolerances are not correct until brought up to operational temp.


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## sealover (Jun 27, 2009)

I'll throw it in my morning coffee to see if it responds, but I couldn't get it to budge with physical pressure. I should have described it a bit better. Not only is it gunked up with sea nettle poop, I think there's rock formations growing on it. It's a 93 and I've had it since 08 I guess. 

It's sort-of froze open I guess. I've replaced a good number of car thermostats but never on an outboard. It looks to me like it might have seized up open. Or maybe they don't shut fully to allow some flow? 

The coolant passages that I could access when doing the impeller and now with the exhaust cover off were open, but the walls had blobs of that clear goo on them, plus other chunks of funk. I'd be tempted to throw a fistful of Barkeeper's Friend in the 5 gallon bucket and let it clean out the cooling system real good, but I bet that'd chew up my new ($44) impeller.


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## pschoonveld (Sep 9, 2005)

The only reason for a thermostat is to limit cooling while the engine warms up. The motor needs to be warm to heat the fuel/air mixture and achieve reliable ignition. 

Removing the thermostat only removes a hindrance to maximum cooling. If you only run the motor in warm weather, you should see a minimal difference in performance. In cold weather, it may misfire or stall out when the choke is removed (choke serves to increase the fuel in the fuel/air mix, which is easier to burn, but not as effective).


If you want to clean it out, use white vinegar. It's acidic enough to pull out the gunk, but not harsh enough to cause issues. Run pure white vinegar either in a bucket (removing the prop can help this be less messy) or use flushing bellows (aka earmuffs) through the motor without the thermostat in or after warmed up. Leave it sit overnight and run through freshwater the next day. 

Do not use Barkeepers. The grit which makes it great for cleaning makes it hell on the motor.


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## sealover (Jun 27, 2009)

White Vinegar -- Thanks! Good call on removing the prop too. I have the "earmuffs" but they've never worked well with this motor -- the water intake is on a spot where that muff doesn't fit against it well at all. 5 gallon bucket does the trick, and works great with the hand truck/stand. If the rest of the passages were cleared out and not restrictive I wouldn't feel bad about putting another $40 thermostat in it. Especially knowing that from now on I need to run it in the bucket for 5-10 minutes after a salt water run, and I can run it in vinegar every year or so. I'll try soaking the old thermostat in CLR to see if that cleans it up, but I think it's gone.


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## LakeSuperiorGeezer (Oct 8, 2010)

If it's stuck open, it's gone. My little 1.2 HP outboard made in 1984 does not have a thermostat. Maybe you have electrolysis going on between the metal in the thermostat and the aluminum in the outboard. Also, I would run 50:1 gas oil mixture as I bet the 100:1 is an EPA requirement to cut down on pollution. However, these new 2 cycle oils burn very clean so lets not worry about that. Nothing on my sparkplug and my outboard also has the 100:1 recommendation although I go 50:1. Nothing like the piston friction being high enough from inadequate lubrication to scuff the piston skirt and causing a sudden stoppage to causing naughty words and one does have to keep the grandkids in mind as they have a way of pointing out these sorts of language errors.


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## svHyLyte (Nov 13, 2008)

Let me first say I am not an engine guy, But...

I had a problem with overheating in an outboard some years age. Plenty of water discharge but still over heating. It turned out that the thermostat had been removed at some point and not replaced. The addition of a proper thermostat resulted in the cooling system functioning normally. The mechanic explained that the thermostat helped regulate the speed of the water flow which was necessary to allow for heat transfer to cool the motor. Accordingly, it might be wise to install a proper thermostat. You never know when you might need to run the motor for an extended period.

FWIW...


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## hellosailor (Apr 11, 2006)

"Do I even need a thermostat? "
If the engine didn't benefit from having a thermostat, the manufacturer would't have thrown away their profits and wasted money on installing one.

Do you NEED one? Probably not, at least not all the time. But the thermostat keeps the engine at the optimum temperature for proper operation, and that means better economy and better power. You may be able to find a thermostat at a NAPA store which is the same part, for 1/4 of the price, under someone else's name. If not...yeah, buckle down and buy one. Or try to clean that one out and test it in a pot of water on the stove, see if it will work once cleaned up. Sometimes even brand new thermostats are defective--so i won't use one unless I stove test it first.


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## LakeSuperiorGeezer (Oct 8, 2010)

*Bypass Thermostat*



svHyLyte said:


> Let me first say I am not an engine guy, But...
> 
> I had a problem with overheating in an outboard some years age. Plenty of water discharge but still over heating. It turned out that the thermostat had been removed at some point and not replaced. The addition of a proper thermostat resulted in the cooling system functioning normally. The mechanic explained that the thermostat helped regulate the speed of the water flow which was necessary to allow for heat transfer to cool the motor. Accordingly, it might be wise to install a proper thermostat. You never know when you might need to run the motor for an extended period.
> 
> FWIW...


Some thermostat housings have a bypass re-circulation circuit. If there are three water passages that come together at the thermostat housing, then that is a concern. Better get a thermostat. One thing about an engine is that the metal parts wear faster when cold. That's just one of those things about metallurgy. Your piston rings and cylinders will wear faster when the engine is cold. With a two cycle, there is no oil to change so no acid buildup in water from blow by as in a four cycle. The condensate in the crankcase and the cold running needed to prevent calcium carbonate buildup at hot spots in the four cycle engine are a problem with raw water cooling (no heat exchanger) not to mention corrosion. Bypass re-circulation in an engine without heat exchanger is necessary to prevent hot spots and calcium carbonate precipitation, which causes more heat in the metal as the calcium carbonate insulates the metal from cooling water. The result is cracks in the engine head around the exhaust valves.


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## LakeSuperiorGeezer (Oct 8, 2010)

One final thought is flush the outboard after use in saltwater as there does appear to be an electrolysis problem between the engine aluminum and the metal in the thermostat


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## Nautichthys (May 2, 2010)

That gunk you're describing sounds a lot like aluminum hydroxide - aluminum corrosion caused by extended contact with water. The snot will eventually dry to a white / grey powder.


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## donlofland (Dec 8, 2008)

My boat came with a late 80's vintage Merury 2 stroke 9.9.

No thermostat. As of last week, it was starting and running fine in ~upper 40's/50 degree salt water.

So I don't think whether there is a thermostat on these motors is a huge deal.

If I found myself in your fix, I'd probably run it without the thermostat until I could get one, and then install it.

How's about used parts places near you? Chance of getting a used thermostat from some guy who does repairs, has a motor with a blown head/lower until/etc. but a working thermostat?


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## hellosailor (Apr 11, 2006)

"So I don't think whether there is a thermostat on these motors is a huge deal."
Depends on your definition of "huge", a highly subjective term.

If you were losing 5% of your power and burning 5% more fuel, would you call that huge? 10%? 20%?

There's no way to tell, unless you run measured tests in a controlled environment. Or ask Mercury about it. Engines don't "need" a thermostat, they just run better when the temperature of the combustion chamber can be fixed and regulated. Which means a thermostat and cooling system. Or, an ideal outside temperature and load.

Combustion physics.


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## sealover (Jun 27, 2009)

OK an update: I soaked the thermostat in white vinegar which it quickly turned blue/green. Put it in a little jelly jar with a lid and shook it every few hours. After sitting all night in that I physically forced it closed. Then I put it in CLR. Then scrubbed it. Then buffed it on a buffing wheel (just for kicks, really) Several tests in a mason jar of boiled water proved it now operates properly without human interaction. And it's shiny. 

Also, I just picked up a Honda 2HP (air cooled, 4 stroke) for sailboat use, so the Yamaha 4 (the subject of this thread) is now relegated to crab boat duty. 6 hours of alternating between half throttle and idle every 60 seconds. Quite a different operating scenario than 2 minutes to get off the dock and 2 minutes to get back on the trailer (the sailboat routine) 

The boat has oars and oarlocks so I think I'll just take it out on the lake once it warms up and see if it runs hot (or cold, or whatever) 

If there's problems I'll put the thermostat back in. 5% less efficiency and 5% worse emissions is not any concern to me in a motor that burned a whole 4 gallons of gas last year. I just don't want to kill my little engine.


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