Conversion to open system with overflow

[Guest sharkeymark]

I am sure this has been discussed a billion times but many may need to know. When converting from a closed system with no overflow to an open system with an overflow what needs to be done. Just change the cap and add a tank? Doesn't the change in pressure affect anything 22 psi vs 15psi. Wouldn't that lower the boiling point and require you to change your ratio of coolant and water? Are the closed style radiators able to suck the overflow back in with a proper cap upon cooling?

Do the downflow radiators require an airgap in the top of the radiator to increase efficiency? With all the eco terrorists out there, they have a fit when an overflow works properly on a hot day after stop and go and using AC. They never seem to overflow while running only when you park the car and turn off engine. Then the boiling commences and then your sparkling car hikes its leg. Yes a totally correctly working system and doing what its supposed to. People under 40 don't have a clue or remember this and think the world is coming to an end when it happens. What about placement and level of the overflow tank? Doesn't it have to be just the right height in relation to the cap? I was thinking about adding a 70's style, large and high capacity, to my '66. They are fender well mount and there is nothing on that adjacent fender and you can buy them brand new. What about hose collapse, I have seen cars literally suck there radiator hoses flat, that obviously isn't a properlly set up system.

Edited May 26, 2012 by sharkeymark (see edit history)

[Don McClair]

Mark, the windshield washer pump did not work on my 65 Riviera and I wanted the added coolant in the engine. I ran a longer washer bottle hose back behind the hood hinge and brought it out beside the battery. That hose with a little coaxing will slide into the overflow hose from the radiator. I installed a 15 lb cap from a 75 Riviera which has the overflow bottle from the factory and topped off the cooling system and added a little coolant into the washer bottle. The cooling system works great I feel more comfortable knowing the system is full.

[Jim Cannon]

Mark, come over and look at my '63 Riv and I can answer all your questions. Which vehicle are you doing this to?

[1965rivgs]

Whoaa...quite a few misconceptions in your question but a great question.

First, the original system is an "open" system and the later model systems with a remote overflow reservoir are called a "closed" system.

A 15 pound cap is typically the highest pressure cap one will find, especially in a `60`s car. The higher the pressure in the system the higher it will raise the boiling point.

The dead air space at the top of the rad in an open system is to allow for expansion of the coolant as it increases in temp. In a closed system, this coolant is diverted into the remote overflow reservoir, instead of filling that air space or dumping it out of the overflow hose, as the engine comes up to operating temp.

When the engine cools and the coolant contracts in volume there is a draw or vacuum created in the system. This is the draw which can result in a collapsed hose which you referred to (there are other reasons for a collapsed hose...ever notice a spring inside the lower rad hose?). This is also why coolant, which has expanded into the coolant overflow tank when hot, can be drawn back into the radiator as it cools thus varying the level of coolant in the overflow reservoir. A second gasket in the rad cap (as compared to the single gasket in the original open system cap) allows this drawback to be applied to the coolant in the overflow tank. This is the reason converting to a closed cooling system requires rad cap replacement. Generally the replacement rad cap has the same pressure rating as the original for the reason stated above.

A closed system offers many advantages, environmental and otherwise. One of the nicest features is that checking the cooling system level (assuming the system is operating as intended) simply requires viewing the level of coolant in the overflow tank as opposed to having to remove the rad cap and peer down into the rad.

For a closed system to operate properly the pickup hose in the overflow reservoir must be immersed in coolant so never let the overflow tank go completely dry.

Dont be concerned about the install level of the overflow tank as compared to the radiator. If the system is tight and leak free the "water seeks it`s own level" guideline doesn`t apply.

Good luck and great questions!

Tom Mooney

Edited May 26, 2012 by 1965rivgs (see edit history)