93 Black Max Overheat alarm

I have a pair of 93 V150 Merc Black Max's. I recently replace the swivel brackets. Both motors are running good, only problem is that when I'm running @ around 18 to 20 knots, about 3200 - 3400 RPM the warning alarm goes off. When I shut them down to an idle the alarm stops. I can run the boat @ 30- 45 mph and higher RPM's w/ no problems, but if I slow down to the lower RPMs off it goes.I have replaced the therms and impellers and the water flow and tell tale are good, I don't get it. I think that I may have a tiny hairline crack in the exhaust plate.Don't know if that would have any thing to do w/ it. Does anyone have any thoughts or experience on this situation. Some insight would be appreciated

Re: 93 Black Max Overheat alarm

Check the poppet valve on the lower right side of the block, it will look like a figure 8. It normally open about 3200 rpm, dumping excess water down the exhaust hole. This is one of those times you need a water pressure gauge, then you can tell what the water pressure is doing and you can see where the poppet opens.