[dgriffis]

I have a 40 horse Yamaha. When it is idling it runs good, there is good water pressure. When I drive it 1/2 mile to a mile then the Alarm keeps going off. I can throttle back and let it idle for a few minutes and the alarm will quit and then I go another 1/2 mile to a mile and it starts alarming again.

Does anyone have any idea of what this could be? I have been told it could be thermostats, sensors, and/or heads.

Thanks for any help I can get.
Donald

[BaileysBoat]

You might have a 16 year old thermostat in there.

I would change that first, easiest and cheapest thing to do.

[triumphrick]

You don't say where you are, but a motor that old can have considerable salt residue build up and will sound the temp alarm at most anything over idle. Even if flushed regularly, many salt water motors will experience this. I know first hand!

[Tommy in FLL]

First thing to do is pull the temperature sensor and use the ohm setting on a multitester to find out if it is working right. The sensor should show infinite ohms [total resistance] as long as it is cold. If it suddenly shows no resistance between 205-219 degrees fahrenheit [use a meat thermometer in a pan of boiling water], then it works correctly.

If it tests out right, you can pull the thermostat cover, remove the thermostat, lower the engine into the water, and start it up. If exhaust comes out of the thermostat cover along with water then you probably have a blown head gasket and/or a blown gasket in your exhaust. If no smoke, then you need to replace the cover and run the engine without the thermostat. If the siren doesn't come on, then buy a new thermostat. If you install the new thermostat and the horn comes on again...then remove the thermostat. Just operate without one. You will need to remove and clean your plugs now and then, but that is a hell of a lot cheaper than rebuilding the power head~