I have a friend with a mercury 200 and it would not keep cool when running. at idle it was fine. we replaced the water pump, then tried...no luck, we replaced the thermostat, tested it also it was fine but replaced it anyhow. upon removal of the power unit we noticed a valve stem. you cannot access the valve stem when the unit is together. we put a cap on it and it quit running hot. so then we replaced the valve core and put a new cap and some permatex also to seal it in case future leakage....what is it for? mercury dealer has never heard of it and i do not know where else to turn....any suggestions? thanks in advance
