Overheating outboard

I have a 2000 90hp Johnson and my hot alarm goes off only when under HEAVY load. I can drive around all day at half to 3/4 throttle and no issues but if I take it up to WOT then the alarm goes off. One other time last summer I was a little overweight and trying to even get on plane, the alarm went off.

I am not sure if it is actually overheating, because all I have to do is idle down, turn off the motor and an immediate restart will will reset the alarm and then right back up to cruising speed no alarm.

The water pump is brand new and no wear, (lower unit pulled last fall) new t-stats installed along with a good back flushing.

What else could be causing this? Any thoughts would be helpful, thanks

my first thought is the impeller. Might be a new pump, but doesn’t mean the impeller is in perfect working order. I’ve seen newly installed pumps fail. Pumps enough water at low idle, but can not keep up with the demand as RPMs are increased. But, this is only a guess

Could possibly be cavitation at the intake not allowing enough water to flow through the cooling system.

I’m with munchy… look for air, bubbles coming out of the water stream out of the motor. If you have a good steady stream of water going through at WOT, you should be ok and not have damaged the motor. New impeller, tstats and a backflush would indicate a problem with the temp sending unit.

If it only does it under heavy loads, I would assume it’s cavitating at that load/speed. Check for a solid stream at all times. Other thinking is they used the wrong waterpump/faulty pump and it’s not working at high RPMs like it should.

Since the alarms in both heads are just a grounded fault, if it was impellor, cavitation, etc, wouldn’t the alarm for overheat still be there immediately after the turn off back on?

I’ve overheated mine couple times before different instances and it took sometime for the alarm not to be there when turned back on. Takes awhile for those heads to cool down and “untrip” the sensor.

I’m still guessing faulty sensors man

You gonna fix this by Wednesday?

heres a way to check it yourself,you will need a volt/ohm meter and an infrared temp gun.
check the resistance on both sensors,if one is reading a ways off from the other one you may have a failed sensor.
check all of the connections for corrosion all the way back to the helm,any corrosion can set off a false alarm due to high resistances.
look for places a wire may have rubbed through and are grounding out while you are checking connections.

if everything checks out good there,run the motor until the temp alarm goes off.
use the temp gun and check both cylinder banks at the top of the heads.
if you havent used a temp gun before,the closer you hold it to the head the more accurate your reading will be.

if it is running hot,you may have the wrong thermostats in it or for whatever reason it is not getting enough water under load.
if the old impeller was missing pieces it could be that part of the old one is stuck in the water inlet tube where it enters the motor.

a weak fuel pump or gummed jets could be causing it to run lean at the upper end of the rpm range.
if it hasnt been done lately,a good carb cleaning is in order on a eighteen year old motor.

if all checks out good and you dont find anything out of the ordinary I would suspect the over temp alarm module is failing but that would be the last thing I would blame,check every thing over and let us know what you find.

water pump.

Could be a vacuum leak in the fuel system. Results in a lean situation which causes the motor to run hot enough to trip the overheat alarm but not actually warm enough to overheat the engine. Thus the nearly immediate cool down and alarm reset. Inspect the fuel lines and use an IR thermometer to check real temp at alarm. I have an ’89 Johnson 40hp that did that once. Fuel line was seepy near the buld. A new line and fittings fixed it.