Motor is mounted on a 1983 Ranger 335 Bass Boat. Last week I was out fishing and running at WOT, fully trimmed out when the overheat alarm went off. I stopped immediately and idled for a couple of minutes and the alarm stopped.
I proceeded as before, but after a few minutes the alarm started again. I trailered the boat, took it home and inspected it and replaced the water pump (kit).
Yesterday, I took it out again to make sure the pump was working correctly and, when I found it was, ran the boat a little. Made a stop to fish a few minutes and started back to the ramp. I decided to try WOT and after a few minutes, the alarm went off again. I stopped and let it idle and the alarm stopped. Ran it the rest of the way to the ramp at 3/4 and had no alarm.
After I loaded the boat, I removed the cowling and felt the heads. They were hot, but no so hot that I could not hold my hand on the heads for several seconds.
This is the first instance of any overheating alarm on this old motor. The fact that it runs fine (even at WOT) and the alarm does not sound at 3/4 throttle has me confused.
Your motor has only two temperature switches in the cylinder heads that can sound the alarm. A false signal indicates a short in the wiring or a failing temperature switch. The temperature switches rarely fail.
You can have an overheat condition from the water pump weakening or failing, from the thermostats failing, or from the water deflectors moving within the passages.
Overheat alarms indicate that the temperature has already reached the boiling point and that your engine is at risk; it should be shut down immediately and not run again until the source of the alarm is corrected.
When the engine overheats, it is good shop practice to replace the cylinder head gaskets and inspect the cylinders.