Overtemp horn at 2500 RPM

I have a 2003 Mercury 150XL SWB. If I throttle up and remain at approx 2500 RPM the overtemp horn will sound and remain on until I shut the motor off. When I restart the horn will initially sound then turns off and all is normal. I am able to throttle through 2500 RPM with no alarm sounding, it only happens at that RPM area and only if I remain there for a minute or two. Last year I replaced both thermostats and impeller. Water is flowing from the tell tale hole and the motor works fine otherwise. Any thoughts?

Are you able to confirm the temp when underway with an infrared?

I just went through a similar issue on my brother's 150 it was the pump housing the tread is on this page,post your serial no so the techs on here know what you got.good luck

I don't have that engine but I do/have had engines with thermostats and higher RPM, higher rates of coolant flow "pop-off" valves...pressure, not temp regulated. ON my 3-4 cyl. 2 strokers, the change over RPM is spec's at 2500 in the manual. Had the reverse happen to me on buying a used 3 cyl on my first outing.

Launched the boat, ran out to the no wake buoys, was putting around for a bit at lower RPMs and all of a sudden the horn sounds.....what to do what to do.....what is it, new engine to me....daaaaaa. So I firewall the throttle for the first time and up and away I went, everything working like one would expect. Low and behold about 30 seconds of that and the horn shut off. It stayed off the whole time I was out on my maiden shake down cruise until I slowed back down to go back to the launch....wasn't but a short time and the horn started sounding again.

In short, troubleshooting at home later, I found my thermostat was stuck shut. I'm guessing that your Tstat is working fine but your pop-off valve is stuck shut.

No, I have not. I'll check that when I can,

Thanks

It's a 2003 Merc Sn# OT 737355, 150XL SWB

Thanks.

Check the poppet as 2500 is about range it unseats

Thank you. When it warms up here I will.


Thanks