Cooling Systems

First chance I have had to respond to my detractor regarding my post on why the coolant level raises in a 5.3 coolant tank when the pressure cap is removed on a hot system. It occurred to me that some individuals may not understand what is happening in a closed cooling system when the coolant is heated under pressure. The average system has a 16psi cap, which effectively makes the pressure on the system 30.7psi, give or take (14.7 atmospheric + 16psi). This is done because raising the pressure on an antifreeze/water mixture raises its boiling point. We all know that a heated coolant mix expands. The pressure exerted on the coolant by the sealing of the system also controls the coolant from expanding as much as it would at atmospheric pressure. So, when the cap is removed while hot, the coolant immediately expands to its natural state at atmospheric pressure based on its temperature, and will also immediately boil if the coolant is near its boiling point at atmospheric pressure. This is why it is not recommended to remove the cap from a hot system, as the expansion and boiling can cause scalding. Hope this helps, and saves someone from severe burns. Had a friend that had major burns from a radiator incident, it's not pretty.

I got burned pretty bad that way when I was about 13-14 years old. We had a Farmall H that liked to run hot if you worked it hard. The darned thing was hot,I ran the front of it in the garage and loosened the cap. It blew up,hit the overhead door and rained steaming water down on me. All I had on was a T shirt. It blistered me up some.

Been there done that. My dad poured a bucket of water that had straw floating around in it. Actually more like chaff. Plugged a lot of the cores. When it really overheated guess who uncapped the problem. Had very bad second degree all of the way up my arm. Thought it had cooled enough and this radiator didn't have those stops on the filler neck. It just unlocks. Never ever again. Even if you throw a heavy towel or blanket over the cap and Slowly unlock it i would still say NO. You are playing with a disaster.

Its a wonder more folks don't get a scalding than they do. I put a transmission in a PT cruiser I had ran it a good 30 min. in the shop making sure the fluid level was full no leaks and performing a quick learn when I backed it out I heard a rumble/rattle. I popped the hood and the coolant tank was boiling so shut the hood by the time I went to shut it off the radiator tank exploded it sounded like a gun going off and shot coolant out if I had been under the hood I would not have liked it. The radiator cooling fan was bad.

"The pressure exerted on the coolant by the sealing of the system also controls the coolant from expanding as much as it would at atmospheric pressure."


Unless the system is at the atmospheric boiling point of the coolant at the time the cap is removed, you are trying to apply the "Ideal Gas Law" to a liquid, IMHO.

Below that temperature I don't believe there'd be enough change in volume of a non compressible liquid to explain the change in coolant level.

I'm sure the hoses expand and contract some with the change of pressure and there's probably some pockets of air trapped in the system that expand with the release of pressure.

For all practical purposes, liquids are mostly not compressible. I have worked with large vessels in hydro testing at pressures up to 10,000psig. I can tell you first hand that a 5 cubic foot vessel pressurized to 10,000psig using plain water will not take more than a pint of water added to a full vessel to reach that pressure.

Your coolant at 16psig (or 30.7psia if you prefer) will not compress by any measurable amount. If your level is going down, it indicates that something besides the coolant is causing that. Perhaps a radiator hose is weak and expanding more than normal.

In any case, coolant will expand as the temperature rises. That is just a fact of life. Some laws can be broken, but the laws of physics cannot be broken.