your water shouldn't affect that, if u have str8 water that boils at 212* F, and then u add a 7# rad cap ( standard for most anything older ), for every 1 PSI of preassure, the boiling point raises by 3* F so that's 21* more so your looking at 233* F, which if you get to 230* F as is, your almost overheating, because when the coolant boils, that's what overheating is. ither change all hoses and go with a 10-13# cap or get a cooler thermostat. if you have a 180* stat, it will be fully open at 195*, a 160 will be open at 185*