This is to help others to fix there problem because i've notice that in many threads people write the symtoms but dont comeback to post how it was fixed. I have a 2003 prestancia, Doral with a couple 5.0 GXI. One of the engine would overheat between 3000 and 4000 RPM under load. On idle and high rev in neutral it would be perfect at 160 deg. It started last year the alarm would go off after a long cruise period, more than 30 min. This year it would do it quicker and more often. I would get the boat on cruise at 3200 rpm the temperature would rise, just before the alarm goes off i would put it over 4000 rpm or full throtlle and the guage would come right down to 160. After changing raw water, circulating pump, thermosthat, looking in the engine in every port and hole with a camera and not solving the problem,.............i ended up changing the seal (not the o-ring) of the raw water pump, a 6$ part at GBS and that fixed it. The engine was sucking air through the seal. To change it u have to take off the crank pully and dismantal the pump in 2 with a puller, and press the bearing back together. Hope that helps someone cause i was scraching my head for 2 years with this.
