Engine overheat question

[Boont Boater]

We were out yesterday going across San Pablo Bay. I turned off the 150 hp Mercury Verado and ran the kicker for about 15 minutes. When I restarted the main engine and gave it the gun to get up on plane the "Smart" sensor shut me down to idle and said that the engine was overheated. After shutting down the engine and scratching my head I restarted it and checked that water was flowing out. I let it idle then went back up on plane with no more alarms. However I did monitor the engine temperature which I really haven't done before. The temperature swung back and forth from the `150's up to the 190's for quite awhile. It finally stopped and stayed at 155 degrees. On the way back it repeated the up and down temperatures for most of the trip.

I'm not a great mechanic and I don't know if this is an engine specific question or a general one. In a car you might think that there was a problem with a thermostat or maybe the "smart" sensor. I wish that I'd paid more attention to the temperature before this happened. Should temperature in an outboard, run at a constant speed, stay at the same level or does it make sense that it would go up and down? When the problem occurred I shut the engine down after it had been working hard and restarted it taking it to full power. I wonder if not letting it idle and cool down could have been my problem. Any thoughts you Brats have would be welcome. I'm afraid that after I talk to a mechanic that I'll end up pulling the boat out and taking it down to Oakland.

I'm glad this didn't happen on the Straits of Georgia

Jeff

[Larry H]

Outboards can be subject to 'heat soak'. This is a condition where a large outboard is run hard, then shut off. The heat in the block is trapped under the cover and heats up the entire powerhead including the fuel system, electrical system, and the sensors.

This condition can make restarting difficult and is hard on the engine components.

Best plan is to idle the large motors to cool off the powerhead.

[hardee]

Larry,

Thanks for your input here. I am guessing that the same operating technique, (idle for a while to cool things down), would be good for the smaller (40hp Yami's), as well?

Harvey
SleepyC

[Chester]

IMO all engines that have been working ought to be run at low throttle settings for a bit before shut down. The bigger the engine the more important this is. Driving your car at 25 mph or idling up to the dock are obviously low throttle situations.

[Boont Boater]

Thanks for the "heat soak" info Larry. That makes sense.

How about the temperature going up and down? Anybody have any sense of how these outboards normally run? I would think that at a constant speed the temperature shouldn't vary 30+ degrees. Maybe the "heat soak" damaged a sensor.

Jeff