2005 Yamaha 90 hp 2 stroke overheat

Sorry to have to ask this question but after looking at the other posts regarding an overheat alarm I still don't see the same issue. This engine starts, idles for long periods at the dock without any alarm (solid beep). Taking the boat out after about 5 minutes of idling while in gear, the alarm started. Checking the temp the engine was running hot (over 200 F). The waer discharge was good but didn't see to be hot a all. Replaced thermostat and pressure spring/plunger, started engine again, water discharge now warm but not hot at idle with oodflo which increases with rpm. Took the boat out with no problems or alarm at both low and high speeds. Checked temp and in the 160 F range. Problem solved right? Nope! Next time out, alarm went off on way out in the no wake zone. Water pump replaced previously at the end of last season. Sensor tests good. Thinking of looking at the water pump although it has been replaced and seems to be pumping. What else?

My 90 Hp had an "overheat" alarm which would then slow engine to maximum of 1600 RPM.


Didn't seem or feel hot at all, but turned out that it actually was overheating.


I have a 1991 TLRP-90 and the Cylinder Head has two chambers, there was a pinhole leak (corrosion) from combustion chamber into the inner cooling chamber and after running for a while it would force the water out of this inner chamber.


You could hold a finger on the head for three seconds as cooling water was still flowing through that portion (outer chamber), but engine's temperature sensor is within the head against inner chamber and could sense the overheat condition. This failure was through top Cylinder into the head and overheat was localized close to the temperature sensor. This would usually not alarm until after running a short while at high speed, it was truly just a pinhole leak.


Never noticed any steam from pisser as there was still plenty of cooler water flowing through, but had to replace the head anyway.


You could test the temperature sensor and see what temperature closes its contacts, but if it checks OK, then you've got to believe it. I found out that my 'finger test' was not enough and damaged my engine by ignoring it.

Pull the plugs after an alarm and check to see if water in one combustion chamber has really cleaned that sparkplug, don't recall seeing this on mine though, but I was convinced that the sensor was faulty...
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