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My 200 HP Merc 2 stroke is running fine...but when it sits at idle the temp guage after a bit soars to way hot. It is flowing water out the water ejection port the whole time. It does this after about 1-2 min at idle.

If I then ramp it up and run it fast in gear the temp goes down right away. When I then slow down, it stays cool just fine for a few min, then the guage starts going up again and it gets hot.

Once again, the water is flowing out of the hot water ejection port at a steady stream the whole time.

So, could use some help as to what is happening and why can't it sit at idle and stay cool?

Thank you,

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When was the water pump changed out last ? When was the thermostat's replaced last ? Good places to "Start".

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The impeller act as a positive displacement pump at low speed and a centrifical pump at higher RPM. When the fins wear there is not sufficient displacement of water to cool, but when rpm goes up there is still enough from the centrifical force to cool. You're impeller is begining to fail. Replace impeller.

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I had a V6 merc that would behave like that at times. I had the change the water pump regularly every other season, and thermostats 2 or 3 times during the 20 years I had it. The last water pump change (I did it) the impeller looked perfect... But apparently the veins were weak. As soon as I changed the water pump the temp was fine...

Do you use it in salt water? If so even more important to keep an eye on the thermostats (2 of them) How old is you engine?

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also the metal "wear plates" around the impeller get worn down leaving a space between the impeller and the plates. This can significantly reduce water pressure, if you take the old plates out and put them together and hold them up to a light you can see how worn they get (light passes through between them), It's best to get the whole water pump replacement kit. Do the "Thermostat's" too. A friend has a V6 too and it's cooling system behave's like "Jon's" former engine (same maint. sched.).