Yamaha 2.5 thermostat

[Mykidslazy]

I have a Yamaha 2.5 outboard. Boat would start and run good for about 7 minuets and then slowly lose power and stall out. Would not restart. Motor seems super hot. Pulled the gas tank off and the checked the thermostat. It was corroded and stuck in what looks to me the open position. I put a new thermostat in it and the first sea trial it did the exact same thing. I pulled the new thermostat out and now this one was stuck open as well.

Could it just have have been a faulty new thermostat, or would something else Be blowing the thermostat. The impeller works and water flows strong from the tell tale is is warm coming out

[Mykidslazy]

[99yam40]

my thoughts are that if the motor gets so hot that it binds up and dies,
then that much heat could damage the stat and other parts of the motor.

and I see no way an open stat would cause an overheat condition. you have other problems

[robert graham]

Do you have fresh gas with proper amount of outboard motor oil in it?......the " slow loss of power and then stall" could be a seizing of piston caused by lack of oil....this is a 2 stroke motor?... Right?

[Mykidslazy]

4 stroke. Has oil

[TownsendsFJR1300]

How old is the WP impeller (and engine itself) ?

If equipped (we don't have a model or year of your engine) with a pressure relief valve, was that checked?


*I gather you checked for the "issue" after replacing the thermostat and it's still there?*

[Mykidslazy]

The impeller is about a year old. When I got the motor the impeller was shot so I replaced that first. Motor is a 2011 f2.5 msh. I did not check the pressure relief valve. Not sure what that is.

[TownsendsFJR1300]

The impeller is about a year old. When I got the motor the impeller was shot so I replaced that first. Motor is a 2011 f2.5 msh. I did not check the pressure relief valve. Not sure what that is.

The pressure relief valve (PCV) bleeds off excesses water pressure(usually at higher RPMs). A smallish plastic piece (often mushroom shaped) with a spring holding it closed (if it's good). If crudded up with salt/debris, it can stick open and water bleeds off faster than it can be pumped in..


Re the old impeller, was it missing ANY VANES and if so, did you retrieve all of them? It's not un-common for the smaller engines to loose the vanes and clog up water jackets, etc..



*Looking at three parts diagrams, I don't see a PVC valve, (usually by the thermostat):
https://www.boats.net/catalog/yamaha...er-crankcase-1

[Mykidslazy]

The old impeller was totally destroyed. I thought I cleaned out all of the little pieces. Would this cause the motor to get hit or blow the second thermostat I changed? The tell tale is spitting good

[99yam40]

I do not think any small motor has a PRV or PCV

tell tale flowing just means the pump is pumping, it does not mean water is circulating thru the motor properly

[Mykidslazy]

Okay thanks. So what moves the water from the pump to allow it to circulate? What would be my next thing to check?

[panasonic]

You got pieces of the impeller up in the engine block cooling passages...that's why it's running so hot....no water.

New water pump is working as it is getting as far as the telltale. Take the lower unit off, put the leg in a bucket of water as if you where going to run it. Blow compressed air back through the engine block through the thermostat hole and see what comes out the water tube into the bucket. May have to blow air up through the water tube and see what comes out the thermostat hole. Could be jammed in there good.

Be careful there may be external cooling lines that you could blow off if too much pressure is applied to the system. I cleared out a couple 9.9. this way last year or two with impellers destroyed.

Please post the exact model/year on the transom bracket so we can know what exact engine is in question here...that is very important.

[Mykidslazy]

I just want to say thanks for helping me guys I really appreciate the input

[99yam40]

I never did anything on a small 4 stroke water cooled motor before, but on larger 2 strokes I dropped the lower unit , hooked up a water hose to the pipe up to the powerhead, and pulled the stat out.
turned on the water full blast and shot compressed air into the water hose every so often to help move things around inside.( i used a home made air injector I just poked the tip into the side of the water hose )

good luck, and capture in a tube the water and debris that comes out of the stat opening and the regular out flow of cooling water to see if and what comes out.
you may have to take the motor apart to get it all out

hope you have not damaged the motor already from overheat