Hey guys, new to forum and couldn't find any info online so looking here for help. Long story short I bought a boat with a 98 merc 40 hp . Took it for test run today and when I would put the boat full power it would start a constant loud beep while at full throttle on the lake. I then idled to neutral and it was still a solid beep for maybe 15 seconds and then shut off(the beep would stop, motor stayed running). It's a 3 cylinder. I took out thermostat and put in hot water and it worked . Also when I go to start boat there in no beep when I turn key in ignition(usually common on most boats,but mine doesn't make a sound). I read solid beep s overheat but the motor ran fine for the 20 seconds I had if at full power . What's the deal? Any and all help apppriciated!!!!