I've got a 1982 30 HP Mariner. Took a run in San Diego Bay this a.m. All seemed normal. Good flow from tell-tale. Didn't notice the exact head temperature, but didn't recall it being excessive (normally shows in the 145-150 degree range, again, on my home made head gauge). Shut the motor down to do some fishing, and, a few seconds later my home made overheat alarm went off. Since the motor was shut down, I figured the thermistor had crapped out so I pulled the fuse to shut it off.
Started the motor and began running at low speed to change location. Noticed the head temperature reading was about 180, but, still strong flow from tell-tale. When I speeded up, the temperature began to drop and got back to the 145-150 degree range.
We fished some more, then decided to cruise by the Spanish Landing area, about 1/2 mile down bay. Same thing with the head temperature, except after a few minutes it wouldn't drop even going at a higher speed. Still strong flow through tell-tale.
Limped back to the launch ramp with a run/shut down/run shut down sequence. Got to the dock with head temperature showing about 200. Running OK, no steam or smoke, still good flow from tell-tale, but definitely way too hot. Let it cool for awhile, then pulled the thermostat. Housing was bone dry, and thermostat was wide open.
Buttoned it up w/o thermostat. Head temperature was still showing 140 degrees, but dropped like a rock as soon as the water started pumping with out the thermostat.
Ran OK to put back on trailer and to flush back home.
Any ideas on problem? New thermostat, last I looked, was about $140. Cheap for peace of mind, but, I'd like to make sure I've zero'd in on the problem.
