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1999 yamaha 115 two stroke cooling problem

J

john stevens

New member
I have owned this engine for 3 years and it runs fine other than this. At full throttle 5600rpm after about 10 minutes the overheat sensor comes on. If you run 5300 rpm, no issues. After some investigation I found someone had removed the thermostats. I installed a new set of OEM stats and the overheat sensor now comes on in 2 minutes at 3000 rpm , removed stats and back to original problem. Now I verified the pressure control valve is in good shape which it is. Next replaced impeller and found the wear plate previously installed has a much larger feed area than the kit I have. I installed the new kit with the smaller feed hole no thermostats and the problem is still there as listed above. I can see at some time the heads have been removed as well. Its a fresh water engine and I don,t see any corrosion or obstructions up the feed tube when the gear case is removed. Any ideas?
 
Sounds like a high temp issue is the root cause, but don't allow the stats to throw you off. Whether stats are incorporated into the cooling system or not, the only way installed stats can exacerbate an already hot motor is by not opening at their predetermined temp. Your engine is running hot with and without stats, so excluded stats as being a cause.

Using a water pump kit out of model spec shud absolutely be considered as the cause, or at least considered to be contributing to the cause. And just because an engine is freshwater run does not imply it's excluded from corrosion. It's a 20 yr old engine. Bound to corrosion built up in the water passages if never addressed in the past.
 
This will be my 1000th post, hope I can help.
Maybe the temp sensor is off parameters. Did you check the actual temp with an infrared digital thermometer? Secondly, could there be a partial obstruction leading INTO the pump?
 
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