Hello, all.I've been chasing this for awhile, So I have Good water flow but no pressure, After about 10 minutes on plane, the overheat alarm goes off.My coolant temp is right at 140ish Oil temp is right at 180, I physically cleaned out the oil cooler , New sea pump. Not sure what i'm missing, But ever since I have no water pressure anymore.I've been getting this alarm,

It wouldn't be the overheat alarm sounding if your coolant temp is only 140... Perhaps you are getting a code for low water pressure? The stock merc sensors can trigger an alarm if they get sand or crap in them and read incorrectly.

It's a carb motor I don't think it has all that computer stuff, The water gauge is mechanical, so no sensor on that,

do you have a power steering cooler?if its before the oil cooler,back flush that.
where do you get you water from,,drive or transom mounted pick up?

Power steering cooler is after the oil cooler, I do have a drive pick up, gimble housing was replaced last year. Fresh waster only, engine block was also replaced last year,

Also forgot to mention, If I take the thermostat completely out, I still saw no water pressure but my alarm doesn't come on anymore, And the water temperature fluctuates pretty good from 100 to 190

are you sure you put the thermostat in the housing the right way,,you have to look at the diagram to see where the thermostat goes.
did the problem show up after the gimble housing?

Do you mean that you have ZERO psi of water pressure???
That is nearly impossible if you have some flow. My 525SC had around 1-3psi at idle and 20psi at WOT
First thing to do it verify that the gauge is working and the hose to it is not clogged. Where is the hose connected?? If its at a block drain, its probably clogged in the first few inches or it has a kink somewhere

I did an impeller on a 700 SC last year. It had dry exhaust, and at idle, after the new impeller, the water pressure gauge was zero, but still had a solid stream coming out the back. I immediately shut down and called the owner, he confirmed the gauge was always at zero at idle.

Can you refer me to a diagrams do I can double check