1987 175 alarm

[<stony>]

hey folks,
i have a 1987 175 yamaha outboard and my overheat alarm has gone off twice in the last 5 outings, i have checked the oil and the oil is always full. the next thing i am going to do is replace the water pump, thermostats and gaskets. i have heard of something called the popit valve but i don't know where it is located at ? any ideas would be appreciated.

thanks stony

[<boats.net>]

It is located at the bottom of exhaust cover at rear of engine, It has 1 inch hose on it going around to starboard side of engine.