# Running without thermostat...



## Dillusion (May 21, 2012)

In a low tech engine like a merc 25 2-stroke. I spoke to a mechanic today who said 'we just remove the stock 120 degree mercury thermostat on the mercury 2-strokes'. I asked why, he stated it was less of a part to break, get clogged, etc.

I understand that the thermostat straps the initial water volume so the block can heat UP to the proper temperature (which some people complain about [the water dribbling and not pissing fully] and some people mistake for a broken thermostat when they dont see the tell tale peeing hard enough).

I did some reading. And the census is that in the older 2-strokes it can 'be ok to do in tropical climates'. But 'the manufacturer put it there for a reason' so I'll stick with it.

Seems a cooler running engine can build up carbon faster and foul- but the older 2-stroke are rich engines to begin with so that's kinda moot there too. The block can also not heat up properly causing uneven piston wear, but the tolerances in a 2-stroke are also loose- so again - not a huge factor.


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## AfterHours2 (Aug 2, 2010)

The thermostat is in there for a reason. It regulates the water and without, water will freely flow possibly causing the cylinder to cool off much more than the piston. Not a good combo if you ask me. I have ran without one for a short day trip due to not having a spare but replaced soon after. I would leave it be but I know what your getting at being you fish the same area as I do. Sand is a PITA but its best to check and replace on a regular basis...


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## cutrunner (Jun 8, 2010)

Theres a lot of opinions out there about this subject.
Heres mine,
Running without a thermostat causes overheat, the exact opposite of what most believe is happening.
Heres how I explain it.
Go run to the end of your street and back in the middle of summer. Then instantly go in your house for thirty seconds.
Did you cool down all the way?
Didnt think so..
Water needs time to pull heat from the surfaces inside the cooling jackets, without enough time there is not as great of a transfer.
Yes the head may feel very cool but that is terribly misleading and not a "perfect" indicator of what the sides of the cylinders temps are, (where all the heat really is)
Sorry, this is not easily explained, and im no shakespear


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## Creek Runner (Sep 1, 2011)

1/4SS! Next


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## cutrunner (Jun 8, 2010)

Or that lol


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## jms (Apr 21, 2011)

> Theres a lot of opinions out there about this subject.
> Heres mine,
> Running without a thermostat causes overheat, the exact opposite of what most believe is happening.
> Heres how I explain it.
> ...


what you're saying is:

"heat transfer" - the water passes through too quickly to pick up the heat ...

i've explained this a few times to clients...

thrmostat acts as a restriction of that flow - like creekrunner suggested,a 1/4" ss washer -it restricts the flow...


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## fsae99 (Apr 8, 2010)

Sorry guys,
While the actual effect on the engine, overheating, may occur because of no thermostat. I do not think your explanation is correct.

It is not the duration of water contact provided by thermostat it is the pressure increase in the cooling jacket that is needed to over come the leidenfrost effect (http://en.wikipedia.org/wiki/Leidenfrost_effect). Aluminum is an excellent heat transfer material and the heat in the cylinder is sufficient to cause this effect.


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## GoldSpoon (Jun 14, 2012)

I've seen a 70 2 stroke evinrude that'e been running for years without a thermostat. 

My buddy had a 30 hp 2 stoke that he ran for years without one. 

I don't think it hurts one way or the other but I wouldn't pull it out unless it was broken.


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## cutrunner (Jun 8, 2010)

> Sorry guys,
> While the actual effect on the engine, overheating, may occur because of no thermostat. I do not think your explanation is correct.
> 
> It is not the duration of water contact provided by thermostat it is the pressure increase in the cooling jacket that is needed to over come the leidenfrost effect (http://en.wikipedia.org/wiki/Leidenfrost_effect). Aluminum is an excellent heat transfer material and the heat in the cylinder is sufficient to cause this effect.


Took the words right out of my mouth!


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## DuckNut (Apr 3, 2009)

I know Troutons Rule increases boiling point of a liquid based on pressure.

Water boils at 212 at 1 atmosphere (bar or 14.69psi) and the boiling point rises to 246 at 2 bar or 29 psi.

And with that logic, it makes a big difference in preventing boilover by restricting flow and increasing pressure.


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## Creek Runner (Sep 1, 2011)

Way to much thinking going on this thread! But since you want to debate it.

1st conduction
2nd convection
3rd radiation 

Now for the simple part if you want to run a thermostat install one, if you believe there is no ill effect on it don't run one. With over 80 years combined of certified marine mechanics (me, my dad, & uncle) all certified techs none of us have ever seen a major mechanical failure due a thermostat not being installed, on the other had we have seen many major mechanical failures due to a thermostat failure. 

3 out of my current 5 outboards that I own do not have thermostats in them. However I do not remove them from a customer engine unless they okay it 1st it's their decision not mine no matter what my feelings are on the subject.


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