1999 sx150txrx

Hey guys, hope all is well with everyone..

I had my boat out last weekend and every time I got to about 30-31 mph (by gps) I would get the overheat alarm..I think it's the overheat alarm but my gauge is not readable anymore.
I back off throttle to neutral, wait about 5 secs then it clears.

I changed the water pump a few days ago, and I go get a little stronger stream of water now. I also tested temp sensors (good) and pressure relief valve was cleaned. I also flushed the engine with some vinegar since I don't have any saltaway etc...

Took it back out today and the first few times it did it again...I cruised around at 29 mph for about 30 min or so, randomly speeding up and getting alarm.

A little while later I got on it and ran at full throttle for about 20 minutes with NO alarms?? Then, the alarm came back, slowed down, cooled off, slow to full throttle again, no alarm.. Then I noticed that if I trim the engine up a good bit, it seems to not happen as often, but it does periodically happen.

This is just weird but I believe running in the lake is slowly cleaning out the saltwater buildup increasing water cooling ability..But it's hard to pinpoint the real issue..
The engine was ran in saltwater by previous owner and it appears that he took good care with the engine and kept it really clean.

Any suggestions other that removing exhaust box covers, cyl head cooling water covers etc and cleaning?

I had the same problem with my 2000, 115 HP. The problem was salt build up in the cylinder head water cooling jackets. Even if you flush the motor everytime you use it, salt build up will occur over time. By the time you flush the motor, time has lasped and the heat of the motor will create dried on salt residue.

I was thinking of taking them off and cleaning them to see if I can clear the problem..Thanks for your input

Cleaned the cylinder cooling water jackets and removed all the scale buildup. Running on the hose feels the stbd head feels hotter than port..

Are you sure you cleaned out ALL water ports on the stbd engine? Have you water tested it? If you still have problems, test the top the heads with an infra red temp gun. Normal operating temp should be 140-165 degrees.

Took her out this afternoon, ran around a few hours with no overheating at all...Scraping all the scale off the cylinders did the trick..