# New Thermostat Opening and Staying Open



## rkmurphy (Nov 2, 2008)

I just replaced all of the gaskets on my '98 Mercury 25 2 stroke, did a complete water pump job, and replaced the thermostat. In the process, I removed some corrosion from the powerhead at the thermostat housing bolt holes. I had to used a gasket maker in conjunction with the dry gasket to seal the water cover.

The motor is pumping water good but now it doesn't open and close like it used to. It is initially closed, opens, and stays open. I don't know why it's doing this now. It doesn't seem to be running hot but, then again, I haven't checked with a temp gun.

I just pulled and tested the thermostat in boiling water on the stove and it opened just fine. I took it out, threw it in some luke warm water and it closed fine.

Not too worried but just curious.

Any thoughts?


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## rkmurphy (Nov 2, 2008)

Also, is it normal for the compression to fall fairly quickly when doing a compression test.  It'll go up to 115 at 5 pulls and start falling when I go to look at it.  I feel like I can't get an accurate reading when it falls this quick.  Although, it is even compression on both cylinders.

Could it be a malfunctioning tester? Or something with the engine internals?


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## Brett (Jul 16, 2008)

Max compression is what the gauge reads.
If the valve that is part of the gauge doesn't seal completely
then you'll be able to watch the pressure in the gauge bleed off.

You fixed the problem in the water jacket... 
Now you're just looking for an excuse to keep taking the thermostat apart.
I guess that's one way to keep the bolts from corroding in place.


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## Kemo (Jun 22, 2008)

You may need a new o-ring on your tester on the part that screws into the spark plug hole. They can only be compressed so many times before they need to be replaced. Do you just screw it in tight by hand or do you use a wrench to tighten it? If only tightening by hand, try using a wrench on it (but not too tight). If it improves, replace the o-ring.

Kemo


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## rkmurphy (Nov 2, 2008)

Haha well I guess I'll just stop worrying about the thermostat. Haha.

I guess I need to return my compression tester. I tried to tighten it with a wrench and no improvement.

Could internal engine damage cause the compression to act like that during a test? Or would the compression not even reach that level if there were damage?


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## Brett (Jul 16, 2008)

If the rings were worn or the cylinder walls scarred
you wouldn't be getting 115 psi. Just the valve in the stem
of the compression gauge needs cleaning.


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## Gramps (Nov 1, 2007)

RK stop worrying. Just run it like you stole it! ;D


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## Tom_C (Dec 10, 2006)

> If the rings were worn or the cylinder walls scarred
> you wouldn't be getting 115 psi. Just the valve in the stem
> of the compression gauge needs cleaning.


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## Tom_C (Dec 10, 2006)

A good compression gauge will hold the max reading untill you release it. Even if you removed it from the cylinder.


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## Brett (Jul 16, 2008)

Like most tools a compression gauge requires some maintenance.
The valve core can get dirty, loosen or worn
and will require either cleaning, tightening or replacing.


standard instructions for use of compression gauge:

http://www.iequus.com/downloads/manuals/compress.pdf


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## mark_gardner (Mar 18, 2009)

> I guess I need to return my compression tester.  I tried to tighten it with a wrench and no improvement.


 let me guess, harbor freight??? :


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## Kemo (Jun 22, 2008)

Yeah, I forgot about the release valve. Never had a problem with mine, but did have to replace the o-ring once. What Brett and Tom C said, my guess. Also, a cheap tester may not be a bargain.

Kemo


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## rkmurphy (Nov 2, 2008)

Ok well I did a little experiment today. I first pulled the LU and checked the impeller. Looked brand new. I then removed the thermostat. Pees like a race horse with the stat out. So it looks like it's a faulty thermostat. I'll call the company on Monday and request a replacement.

In the meantime, is it okay to run the motor without it? It could probably use some super cool running for a while. The motor would probably be refreshed...although it wasn't really getting hot.


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## Tom_C (Dec 10, 2006)

I'm confused, [smiley=1-crazy-eyes.gif] why do you think the thermostat is faulty.


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## rkmurphy (Nov 2, 2008)

> I'm confused, [smiley=1-crazy-eyes.gif] why do you think the thermostat is faulty.


It's supposed to open and close so you can visibly see the pee stream stopping and starting. My stat will open but stays open and lets a weak stream out. The stream gets a little stronger when the RPM go up. It doesn't get too hot but hotter than I like. When I pulled the stat and ran it without, you could definitely tell that there was an issue. Without the stat, it pees better than it ever has. Keeps the motor nice and cool. I may just leave it out...we'll see.


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## Brett (Jul 16, 2008)

With an electric start outboard, it's easy enough to install
a sensor and temperature gauge to monitor your outboard.
Or if you just want to spot check, get a meat thermometer
to check the temp of the water coming out of the whizzer tube.


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## Tom_C (Dec 10, 2006)

> > I'm confused, [smiley=1-crazy-eyes.gif] why do you think the thermostat is faulty.
> 
> 
> It's supposed to open and close so you can visibly see the pee stream stopping and starting.  My stat will open but stays open and lets a weak stream out.  The stream gets a little stronger when the RPM go up.  It doesn't get too hot but hotter than I like.  When I pulled the stat and ran it without, you could definitely tell that there was an issue.  Without the stat, it pees better than it ever has.  Keeps the motor nice and cool.  I may just leave it out...we'll see.


I'm not a motor expert, but in florida warm water I don't know if that is going happen.


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## rkmurphy (Nov 2, 2008)

> > > I'm confused, [smiley=1-crazy-eyes.gif] why do you think the thermostat is faulty.
> >
> >
> > It's supposed to open and close so you can visibly see the pee stream stopping and starting.  My stat will open but stays open and lets a weak stream out.  The stream gets a little stronger when the RPM go up.  It doesn't get too hot but hotter than I like.  When I pulled the stat and ran it without, you could definitely tell that there was an issue.  Without the stat, it pees better than it ever has.  Keeps the motor nice and cool.  I may just leave it out...we'll see.
> ...


If what is going to happen?


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## mark_gardner (Mar 18, 2009)

every outboard i have ever had will start peeing once the thermostat opens and continues to pee untill i turn the motor off. i couldnt tell you if i have a thermostat in my 25 merc or not but she pees real good and starts doing so within a minute of me starting it


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## rkmurphy (Nov 2, 2008)

Mine opened and closed before I had the water pump job. After I pulled the stat, it pees freakin' GOOD. It never peed that good with the stat in, even at WOT when it peed hard. It wasn't getting hot (to my knowledge) but I definitely feel better about it with the stat out.


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## Tom_C (Dec 10, 2006)

> > > > I'm confused, [smiley=1-crazy-eyes.gif] why do you think the thermostat is faulty.
> > >
> > >
> > > It's supposed to open and close so you can visibly see the pee stream stopping and starting.  My stat will open but stays open and lets a weak stream out.  The stream gets a little stronger when the RPM go up.  It doesn't get too hot but hotter than I like.  When I pulled the stat and ran it without, you could definitely tell that there was an issue.  Without the stat, it pees better than it ever has.  Keeps the motor nice and cool.  I may just leave it out...we'll see.
> ...



Open and close. As southboundchicken said once open will stay open.


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## mark_gardner (Mar 18, 2009)

> Mine opened and closed before I had the water pump job.



maybe it wasnt the thermostat opening and closing but rather a bad impeller or blockage of some sort? like i said, when mine starts peeing it pees till i turn it off or unless i've sucked up some weeds or such


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## rkmurphy (Nov 2, 2008)

No blockage or bad impeller. Pulled the LU and checked the impeller and it looked brand new. Then I removed the thermostat and it peed like crazy. Great solid flow that kept the motor nice and cool. Flow never got hot. I dunno...maybe it was fine before. We have the same motor, right?


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## DuckNut (Apr 3, 2009)

Maybe I missed it but by virtue of having a thermostat in place you automatically create a restriction.

If it pees hard in the driveway without a stat in place and then slows when it is in place you have restriction.

Also, if you are using ear muffs you are not getting a true representation of the effectiveness of the impeller. Use a bucket/cooler and use the hose to keep the water at adequate level in the bucket. By using ear muffs from your house hose you are forcing water through the impeller and not letting the impeller do what it is suppose to do.

The purpose of the stat is exactly the same for your car. It is designed to keep the engine warm but not hot. Once the engine is warmed the stat will open and allow the hot water to pass at a slow rate. Replacing hot water with cold water equals warm water. The cycle continues.

When the engine first starts there is no hot water to open the stat and the engine pees very little. The stat has bypass capillaries to allow the pump to fill with water when first started.

If you ever have the chance to run a small outboard when the water temp is 35 degrees and the air is 5 the function of the thermostat is very pronounced.

Engines run better when warm and it sounds like there is nothing wrong with your engine. It is just the hypocondriac showing through.


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## mark_gardner (Mar 18, 2009)

> No blockage or bad impeller.  Pulled the LU and checked the impeller and it looked brand new.  Then I removed the thermostat and it peed like crazy.  Great solid flow that kept the motor nice and cool.  Flow never got hot.  I dunno...maybe it was fine before.  We have the same motor, right?


yep i think so? 2003 25 hp mercury 2 stroke tiller steer short shaft w/ electric start


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## rkmurphy (Nov 2, 2008)

Hmmmm...so...you think that it would be bad for it to run cool? I just figured with it's little overheating experience and it running hot for that period of time it may be better for it to run cooler for a while. But I'm pretty clueless when it comes to this...still learning.

I have no problem putting the stat back in. It's only 2 bolts and a new gasket that costs $1-$2. But would it be better for it to run cool for a while is my question? It did definitely pee pretty good as the RPM went up. But with the stat out it pees REALLY good...

southboundchicken, do you maybe have a vid of yours on the hose or in a bucket? (BTW, my motor pees pretty much the same on either)


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## mark_gardner (Mar 18, 2009)

unfortunetly i dont have video capabilities but i can tell you that mine pees a solid stream of water, if i were to drink a gallon of water straight i dont think i could pee as hard as my merc does  i'll try to post a pic or two this afternoon of it peeing if the weather holds out


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## mark_gardner (Mar 18, 2009)

heres a couple of pics for you, the motor is hooked up to the ears and idleing








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## rkmurphy (Nov 2, 2008)

Thanks man! Yeah from the pics yours looks to be a stronger flow than mine with the thermostat. Without the thermostat mine looks stronger. Weird...oh well. I'll leave it out and see how it runs.


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## mark_gardner (Mar 18, 2009)

like i said earlier, i'm not sure if its got a thermostat in it or not as i bought this thing used a few months ago and havent done a thing to it other than use it, and clean the carb of course LOL


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## DuckNut (Apr 3, 2009)

RK-
I would not be so quick to say it is overheating. Find out what the temp on hte stat is rated and then find out how hot the water is. The palm of your hand can tolerate 130 dergrees for a short tim but will hurt at 140. If the stat is 140 and the water temp is 190 then there is a problem somewhere. 

You can test the stat in a pan of water on the stove and a themometer. The stat should open at a temp very near the rating +/- a few degrees. Hold the stat with pliers while heating the water and note the temp when it opens.

It should not hurt your engine to run without the stat but it will run more efficiently when warm.


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## johnhumid (Sep 4, 2009)

The stat should open at a temperature very near the rating +/- a few degrees. so that would be a thing to do...


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