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The boiling point is raised by increasing the pressure of the liquid. I do not have pressure tables for ethelyene glycol-water mix, so I will use that for water to illustrate the change in boiling point with pressure. The following table was extracted from the Handbook of Chemistry.WATERDeg F Pressure, psia (absolute pressure)212 14.696213.8 15.228215.6 15.776..221.0 17.521.249.8 29.717Water boils at 212 F at one standard atmosphere of pressure. or about 14.7 psi. If a pressure cap of a radiator adds 15 psi (or 29.7 psia), then the boiling point is raised to about 250 F. In other words, a radiator with a 15 psi cap will not boil until the water temperature raises to 250F. At 250 F, that system is dangerous. If the cap is suddently removed, most of the liquid will flash to steam and gush out of the opening scalding anything it touches.A closed, non-leaking automotive coolant system filled with water with a 15 psi cap will not boil until it gets to 250 F. A leaky system can boil as low as 212 F. However, the coolant does expand while it is heating.Here’s what supposed to happen. As the coolant expands, it squirts by the pressure regulator in the radiator cap and goes to the overflow bottle. The cap should have an second seal under the very top of the cap. This gasket is supposed to seal the overflow tube from the atmosphere. When the engine cools down, liquid from the bottom of the overflow bottle is sucked back into the radiator, that is, if the radiator cap’s outer seal is working. If the overflow bottle is not completely emptied, then no air gets back into the coolant system. This is critical. It is the admission of oxygen into the coolant system that causes most of the corrosion and crud. After a few heating and cooling cycles, most of the air in the coolant system should be flushed out and no more will get in until the system is opened again, for example, opening the radiator cap or allowing the overflow bottle to go dry.This system fails when there is a leak somewhere from the radiator cap to the overflow bottle. One typical cause is a wrong radiator cap. A cap from an older system without an oveflow bottle may fit but it does not have the second seal under the cap. It has only the pressure regulator. A second cause is a proper cap with a second seal that no longer works. Another cause that I have seen is a leak in the hose that goes from the radiator to the overflow bottle. I had a hose that had a slit in its underside that could not be seen from above.From what you have written, your fan may not be turning on soon enough. For example, the original equipment temperature sensor for ’97 Saturns was notorious for failing after about 3 years and not turning on the fan. After replacing one twice in our Saturn, I found an after market sensor with a metal sensor nose that lasted for years. The original equipment sensor had a plastic nose that eventually dissolved in the hot coolant. The head cracked on this engine at 130,000 miles probably due to over-aging of its aluminum alloy head.

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