I have a 1990 Sea Ray 180 with a 4.3 Chevy and a Mercruiser outdrive. It has overheated at idle ever since I have owned it. I have replaced the outdrive water pump, the engine water pump and the thermostat and have removed and cleaned the exhaust risers (not too bad) and installed new gaskets on same. At speed it runs fine and does not overheat. At idle it will climb steadily to the hot side of the gauge but will come down steadily as I increase engine speed in neutral. Nothing I have done to date has changed the situation.
I have looked at the check balls in the T-stat housing outlet and am wondering at their function. If they are not opening as designed will it run hotter or cooler? Or, if they are not sealing as designed will it cause a hotter or cooler condition? I can't tell from looking what the water path is supposed to be or what those check balls do. I thought about removing them thinking they restrict flow out of the engine but they obviously have a purpose or they wouldn't be there.
It's an open system using only lake water and of course the only salt water in my area comes from one's pores in this kind of weather.
Any help is appreciated.
Thanks in advance.
Chuck.

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1983 M-B 240D-Gone too.
1976 M-B 300D-Departed.

"Good" is the worst enemy of "Great".