Your engine is running fine. The t-stats open and close when you engine is operating under 2000 rpm (or close), once they are above that the Tstats/popit valve are full open and allow straight lake water to run through the engine to allow for cooling, thus why your seeing lower temps in the winter, then you would in the summer. Dont be concerned until your temps fail to reach 120 at idle, if they wont come up to temp at idle, then you would have a sticking t-stat, and DO NOT accelerate if your engine wont come up to temps at idle. Also its recommened that the first take off of the day should be done gradually. . Also remember to let your engine idle for 30-45 seconds in cold water after you come down off plane in cold water, and try not totrim it out of the water immediately after shutdown in cold weather as, you will pour all the cooling water out of the block. Some really good info here on cold water and temperatures regarding the optis:http://www.bbcboards.net/mercury-motors … timax.html
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