Mercury 150 overheating at full throtle

[Tomg11757]

I have a 2003 Trophy with a Mercury 150 Salt Water Series outboard. I took it for a ride this morning. Ran fine at idle set out to the bay. Once on plane within a minute or two the overheat alarm goes off. I immediately shut down and threw the anchor. I had just changed the impeller last year. Thinking that it was the thermostats I removed them both. I started back to the dock once on plane the alarm sounded again. There was a steady stream coming from the motor so i assume the impeller is working good. I stopped again and let it cool off. Started again and limped back to the dock and the temp stayed normal. Reading some other threads seems like it the Poppet valve. My question is - Could it be something else? How difficult is it to replace? Can it be done on the water or do I have to pull it out? Thanks

 

[ondarvr]

Things like this like this can be done on the water, but it can be frustrating and expensive when you drop something.

​You're looking in the correct direction, that or corrosion build up in the water jacket.

 

[Dukedog]

pull it out.. replace "complete water pump".. base, ride plate, housing, impeller and gaskets.. replace poppet assembly.. both are fairly inexpensive compared to motor.. seems age has no meaning on "new parts" anymore............jmo

 

[racerone]

Checked compression and torque on the head bolts ?

 

[Tomg11757]

Went down to take a look at it. I see the hose that goes from the top to the bottom of the power head. Doesn't look accessible without removing the bottom cowl. Look like a job out of my expertise. Gonna stop by the mechanic next to where I keep the boat. Thanks for the responses.