Strange overheating alarm on 2008 Honda 135

F

Ferg

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C Dory Year
2008
C Dory Model
25 Cruiser
Vessel Name
Here & Now
If I suddenly take the RPMs down from cruising speed (4400rpm for me) to idle speed, I get an immediate overheat alarm. I shut down and restart, and go on my merry way. It's happened twice now.

Engine has been flushed with Salt-Terminator after each use, new thermostats, newish impeller, good flow from the pee.

I'm guessing it's the sensor switch gone bad.

Thoughts?
 
I saw the same thing on a 150 which I had been running fairly hard, and brought down to idle. The flow seemed to be fine--I suspect that there was some localized overheating--and in the case I had, I brought the engine back up to a fast idle.

I always back down, and make sure that I have not caught a piece of kelp or plastic over the intakes when I get an overheat, I had one time at Powell, that there was so much fine debris in the water, (up San Juan arm), that I had an overheat alarm. I stopped, backed down, and then shut down, and cleaned off the intake--resolved the issue--and I turned around and didn't go further into the debris field.
 
Could be sensor(s) or a failing thermostat?

How old are the thermostats?
 
Dr. Bob, there was no debris and the intake was clear, that's why I'm thinking it's a questionable sensor.

Brent, the stats have less than 20 hours on them, and I think that if they would have failed, it would be at high RPM
 
Thanks

Can the OB be connected to computer diagnostic software?

Yamaha has one called YDS
 
It does have a diagnostic port. That's my next step, but I don't know if the history will show these two events. We'll see
 
Are you sure its an overheat warning? My old Suzuki once had an alarm go off and I couldn't figure it out. Then i came to find out it was a reminder for the oil change. The previous owner must have set it to go off at a certain time interval.

Do you have a battery operated temp gun? If you're absolutely sure it's an overheat alarm then I would take off the cowling immediately after it happens again and get different readings from different parts of the block (assuming theres no plastic there). Those guns are very accurate. I bought one and use it on my volvo engine in my new boat.
 
It's defiantly an overheating alarm, but as I mentioned in my initial post, it only happens when I suddenly reduce the RPMs. I shut down the engine and immediately restart with no problem and no alarm. If was truly overheating, it would go back into alarm I think
 
My 2003 200 hp Honda did the same thing 3 yrs ago. After much typing and a-s scratching it turned out to be the thermostats.
Has not done it for 3 yrs now.
 
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