"I have a 2003 150 Optimax and
"I have a 2003 150 Optimax and have the same problem. It started in my first season and it always occurred when going through a long, no wake zone. The alarm would sound so I would shut it off the engine and let it sit for a few seconds. I would keep turning the key back on until the alarm did not sound. It never happened when running on plane. The water flow appeared to be strong and the water pressure gauge got up to about 15 PSI at top end speed. Afraid of voiding my warranty, I took it in for service. Of course they could not simulate the problem so they changed the impeller and the thermostats at my expense and to no avail. The next time I was in a long, no wake zone, the alarm went off. I took it back at the end of the season and guess what? They changed the impeller again, and again it did not solve anything. Finally, on the third attempt a year later, the technician found something that seemed to be the real cause of the problem. The dealership had just recieved a new piece of diagnostic equipment. A voltage reading that correlates to the water pressure was low. The spec called for a reading between 1.5 and 3, but the reading on my engine was .8. The technician took a harder look and found a clogged screen under the cow. It definitely seemed to solve the problem for a while. This past season the problem returned, probably because I am a Bass fisherman and am always in very, shallow water. If the problem is that simple, I want to fix it myself, but when I lift the cow off, I can't tell where such a screen would be. My guess is if I could remove the lower cow, it will reveal the plumbing that makes up the water path. I tried to remove the lower cow by unfastening the screws that appear to hold it togehter, but it won't separate and I am afraid to force it. My neighbor works for the shop that worked on the engine, so I asked him to find out from the technician how to correct the problem, but apparently the technician has amnesia and doesn't recall my particular problem. Guess it's time to spring for a service manual."