1998 Yamaha C 150

[MEM1971]

I have a 1998 Yamaha C150 I bought brand new may of 98. It has been an excellent motor and will be hard not to go back with Yamaha when I finally upgrade. Only thing to ever be done to this motor is change water pump impeller, lower unit fluid, and spark plugs. Recently I have been getting high temp alarm sometimes shutting down cylinders. Once I bring it back down to idle after a minute or so it goes off and all is good. Water is coming out great and last time it went off I hurried and jumped back there to feel the water and it was cool. A little warm, Normal. It only does it trimmed up and almost wide open. Once did it pulling kids on a tube and last time it did it I was not trimmed all the way up or wide open but I had a lot of ice, tackle, another person on board. I truly do not think its actually getting hot but it has all the symptoms. I don't run wide open anymore but I got caught in the worst storm of my life a few weeks ago and was following someone back to launch in the rain and dark after waiting on lightning to stop for over 3 hours. Gauges are not working yet but do have a rpm gauge to put in it. Just guessing id say somewhere around 40-45 mph is where it starts. Prop needs fixing but boat used to run 55mph. I do not believe this motor has a computer and I add my own oil to the gas , 2 stoke, which is something I wanted back then when a lot were having trouble with their oil pumps going out. Are there sensors or thermostats that can go bad? I have a new water pump impeller im putting in this weekend but since its stream is great I doubt this is the cause. Anyone out there with knowledge of this particular motor I would sure appreciate some guidance.

[B-Stealth]

Sounds like your losing water pump suction, and heating up or RPM's are too high.

[Lil Silver]

Built up with scale inside the water jacket areas not allowing heat transfer to the water?

[MEM1971]

but water is cool coming out when this happens. I wondered about the RPMs cause prop is in bad shape and just not willing to except the month downtime to fix it just yet. But they tell me I only have a heat alarm only and no computer to for anything else.

[redaddiction]

Originally Posted by MEM1971

but water is cool coming out when this happens. I wondered about the RPMs cause prop is in bad shape and just not willing to except the month downtime to fix it just yet. But they tell me I only have a heat alarm only and no computer to for anything else.

The water shouldn't be cool. It should be pretty hot. Not scalding but hot to the touch. The heat has to leave he motor some kind of way.

[MEM1971]

It is cool. That's why I'm curious if there are sensors or thermostats that can go bad . I do not think its really getting hot

[Lil Silver]

Take your water jacket plate off to have a look inside.

[MEM1971]

Ok going to check it all out next week. Only getting Sunday off so going fishing. Does anyone know what tells the heat alarm to go off? And/or cylinders to shut down? I'm thinking this may be the problem. Like I said water is too cool coming out to be getting hot. I think. Idk and why I'm seeking advice. Thanks

[T-Bone]

Not sure if you have one or two thermostats on that motor but it sounds like the thermostats are are not opening and the water is flowing through the poppit valve.

[Feesherman]

Originally Posted by T-Bone

Not sure if you have one or two thermostats on that motor but it sounds like the thermostats are are not opening and the water is flowing through the poppit valve.

I've seen thermostats completely clogged with salt. I'd check those ASAP.

[perchman]

Pop it valve or thermostats. If alarm is going off it is getting hot. Had to pull my power head off and it was full of shell and other stuff had water restriction so a higher rpm it would go off but as soon as your got off plan with boat and back to idle it would go off.

[Gerald]

A thermostat can get "trash" caught in it.

Only way I could get all the trash out was to put in a pot and boil the water. This opens the valve and lets the trash get washed away.

Pictures show trash from running shallow bayous.
.

[tgsx750f2000]

I had same problem with alarm going off only at higher rpm. Changed every part it could have been and still went off. After six months of trouble shooting finally figured out it was a bad battery shorting out at high rpm!


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[perchman]

In salt water you order stainless thermostats. That will help a lot. Fill your water hose with dawn dish soap and then put on the water muffs on your motor to flush after running in salt water. Want fix the problem but will help a lot

[lil bubba]

I got an 08' 70...With 200 hours it did the same. Half throttle you could run all day. Open it up and temp sensor alarm goes off and cylinders cut out. Water was pumping fine and not hot. Changed impeller and thermostat and same. Pulled head and water jacket around cylinder and block was clogged. Water was bypassing block so it wasn't coming out hot but was tripping sensor in head. I know 3 other people had same problem with 3 cyl. yammies.

[B-Stealth]

Pull the thermostat and run without, and see what happens.

[MEM1971]

Will try guys thanks. Put a new prop on, water pump impeller and took out the autolite spark plugs I put in during a flood and put the NJ 2K back in. The plugs and impeller along with seafoam I added for first time has the Yamaha running smooth as new. It came with 19 pitch prop new but would not jump out of water in west cove one winter and had to idle against north wind forever to get out. So I went down to 17. Well I was looking fwd to opening her up with the much faster prop but alarm still goes off. And I will go back down on this prop too once I get it running right and check rpm.

[smooth move]

Originally Posted by T-Bone

Not sure if you have one or two thermostats on that motor but it sounds like the thermostats are are not opening and the water is flowing through the poppit valve.

X2----poppit valve was my first thought. by the age of the engine it would be time for the corrision build up to clog the poppit or thermostats. if doing the work yourself, be very careful taking the bolts out of the thermostats. easy to snap off after so long in the case.

[Gerald]

Originally Posted by smooth move

X2----poppit valve was my first thought. by the age of the engine it would be time for the corrision build up to clog the poppit or thermostats. if doing the work yourself, be very careful taking the bolts out of the thermostats. easy to snap off after so long in the case.

Take a hammer and hit on the head of the bolts. Hopefully this will loosen the threads some.

OR... if you have another person to help, put a wrench on the bolt and put some force on it while the other person hits on the bolt head. The combination of vibration and twisting force....might get the bolts out.

[Goooh]

Soak those old bolts and nuts overnight with PB Blaster, it'll work it's way through the threads and loosen anything that may have them seized