(See my comments below for fix) I removed the thermostats form a 1988 ProV150 because the alarm was going off. With both removed, you can see that the water pressure is less on the starboard side. It also runs hotter on that side.
With the advice of my mechanic, I filled a large outdoor garbage can with water and muratic acid. Lowered the motor into the can and ran it for a while to get the solution through the motor. Finished by running clean water to rinse the motor. It worked which is nice because the alternate was to open the heads to clean them. Good luck with yours.
I'm having a similar problem with my 1994 pro v 175. No alarm when running but slow down to about 20-25 and alarm goes off in a couple minutes. I can put in neutral rev up and alarm stops.? I pulled my therm cover off as you did but I have good water flow. Could my thermostat be bad?
i literally just had the same issues with over heating only i found the issue, after back flushing and disassembling the entire disassemble-able parts of the water flows i found my issue was fragmented impeller peices located in the lower unit directly from the impeller pump to the feed tube the pump goes to up about 9inches into the pipe was an entire 6 peices of rubber blades now my engine ran with water during months of running before i just got sick of never going over 2400rpm, back flushing alone couldnt get the fragments out, i ended up taking a long wire and bent a tiny hook in the end and pulled each peice out one at a time. my engine seemed to run water only causing overheating when the water pressure sensor was plugged in, yes in. but this just goes to show that somtimes you really need to look hard for those peices as black in a black pipe can be mistakenly overlooked. hope this helped :)
Update. After MUCH diagnosis, it has turned out to be that the water passages in the block were blocked. I had originally changed the impeller, Poppit (pressure relief) valve and TStats. After clearing the water passages, the alarm disappeared (It would go off after 3 minutes in low speed).
Crappie Head Tv pull the heads off and the water jacket covers that are on the heads and clean them. The water jackets get filled with salt deposits and minerals from the water. You should also pull the exhaust cover off
