I have a 98 Olds calais with a 2.5 liter 4cyl. Engine that is overheating. After driving under 50 miles, the coolant blows out of the overflow and the car over heats. I have replaced the water pump, radiator cap, thermostat and finally the radiator itself. It still blows coolant out of the overflow. There are no (other) signs that would indicate a head problem, (water in the oil, blue or white smoke from the exhaust, ect.). Any ideas of what might be wrong that I haven't thought of? The car has 153,000 miles on it.