1999 Mercury 115 alarm

[Kebmo]

I have a 1999 Mercury 115. Twice now after long wide open runs, a constant alarm sounds just after slowing to trolling speed. When I shut it down the alarm goes off and 2-3 minutes later I can start the engine and the alarm will stay off. Motor is peeing and oil tank is full. Any ideas?

 

[Texasmark]

There are 2 systems controlling engine block temperature: The thermostat and the pop-off valve. At lower speeds there isn't enough water pressure (from the water pump) to open the pop-off valve and the thermostat controls block temperature. At about 2500 RPM there is enough pressure from the water pump to force the pop-off valve open, much larger opening and takes over the cooling process.....The internal temperature drops from around 140F to a few degrees over ambient water surface temperature.

When you slow down, the water pressure drops and the pop-off closes putting you back under the control of the thermostat. With what you said, the only thing that makes sense to me is that the thermostat isn't opening back up after the pop-off, allowing cooler water into the engine block caused the thermostat to close.......its stuck shut as I see it.

I'd get a new thermostat and while you are at it a new pop off valve. The two are located under a (roughly) 1.5 x 3" cover on the upper left rear water jacket cover on the rear of the block.

 

[Kebmo]

There are 2 systems controlling engine block temperature: The thermostat and the pop-off valve. At lower speeds there isn't enough water pressure (from the water pump) to open the pop-off valve and the thermostat controls block temperature. At about 2500 RPM there is enough pressure from the water pump to force the pop-off valve open, much larger opening and takes over the cooling process.....The internal temperature drops from around 140F to a few degrees over ambient water surface temperature.

When you slow down, the water pressure drops and the pop-off closes putting you back under the control of the thermostat. With what you said, the only thing that makes sense to me is that the thermostat isn't opening back up after the pop-off, allowing cooler water into the engine block caused the thermostat to close.......its stuck shut as I see it.

I'd get a new thermostat and while you are at it a new pop off valve. The two are located under a (roughly) 1.5 x 3" cover on the upper left rear water jacket cover on the rear of the block.

Thanks!