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Discussion Starter · #1 ·
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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Discussion Starter · #2 ·
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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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... :cool:
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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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 :cool:
 

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Discussion Starter · #5 ·
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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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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RK stop worrying. Just run it like you stole it! ;D
 

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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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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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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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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 :cool:
 

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Discussion Starter · #13 ·
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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Discussion Starter · #15 ·
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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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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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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Discussion Starter · #18 ·
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.
If what is going to happen?
 

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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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Discussion Starter · #20 ·
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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