How to test a thermostat

halfmoa

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Aug 19, 2011
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I asked this question in my current thread about overheating on a 1965 Johnson 65hp outboard but since it's only gotten 10 additional views in 24 hours I'll ask this here....

Is the process for testing a thermostat from an outboard motor the same as testing a standard motor vehicle's thermostat? i.e. place in a pan of water on stove, gently heat, measure opening heat and fully open heat.

Thanks,
half

EDIT: well now that I posted this I see the related posts at the bottom that've answered my question...:facepalm:
 
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5150abf

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Re: How to test a thermostat

No more than they cost just get a new one and be done with it, then you know it works and you aren't going to fry your engine over a $12 part.
 

Silvertip

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Re: How to test a thermostat

You do not set the stat in a pan of water. You "suspend" the stat in a pan of water. The bottom of the pan is much hotter than the water so a false temp reading can be obtained.
 

halfmoa

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Re: How to test a thermostat

No more than they cost just get a new one and be done with it, then you know it works and you aren't going to fry your engine over a $12 part.

Interesting that you read "change the impeller" over and over yet rarely have I heard about replacing the thermostat but yes, I understand your point. And I believe I already have fried it....

You do not set the stat in a pan of water. You "suspend" the stat in a pan of water. The bottom of the pan is much hotter than the water so a false temp reading can be obtained.

Yes, I should've been more specific. Thank you kindly!
 

Texasmark

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Re: How to test a thermostat

You do not set the stat in a pan of water. You "suspend" the stat in a pan of water. The bottom of the pan is much hotter than the water so a false temp reading can be obtained.

Right out of the manual. I hear them and know what and why, but I'm not that specific and there isn't that much physical contact with the bottom for conduction. So I just toss mine in. I do turn the fire down to slow the heat rise and give the pellet time to follow the temp and use a candy thermometer for a monitor.
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On Mercs that I know, 143 is degrees F, stamped on the bottom of the pellet. Opening temp within +/- a few degrees (tolerance), fully open in another 10 or so degrees. Surely OMC isn't far from that.
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I had a '75 70 hp looper rude I bought used and lost the powerhead due to a stuck shut stat. They didn't have an overheat alarm back then....would have saved me $1000 and a lot of work. The guy I bought it from said it was hot. Yes it was but it wasn't performance!

Mark
 

5150abf

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Re: How to test a thermostat

Changing the stat isn't pushed maybe as much as it should be but I would bet failed stats cause as many problems as failed pumps, just less obvious symptoms.

They do fail though and if your engine is 47 years old there is a goodly chance your stat is too, my point was as cheap as they are just change it and you have it apart anyhow.

Since you need it right now check with Napa, they have a real nice selection of outboard parts and can probably have it tommorow.
 

halfmoa

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Re: How to test a thermostat

Thanks guys. I turned the flywheel and heard two thumps through the leg and one "pffft" from the bottom cylinder so I'll be needing more than just a stat. Hopefully no more than a head gasket. :mad:
 
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