radiator cap

[long1776]

Cars with an expansion tank uses two different caps, one on the exp tank which opens to let coolant return to the rad. On the rad there is a cap which has an opening pressure, to let coolant go to the exp tank. This pressure is often 15 - 16 lbs. My concern, irrespectively of car, isn't that a lot on an old car with an old radiator?

[Gunslinger]

There's expansion tanks and there's overflow tanks...they're two different things and do two different jobs.

An expansion tank is where there's room for the coolant to expand...fill up without going through the overflow tube. It's pressurized. An overflow tank is unpressurized and is to keep from losing coolant that would otherwise puke on the ground when the engine runs too hot and over pressurizes the radiator, and will return the coolant back to the radiator as the temperature cools.

An expansion tank uses a pressure cap that is for what's called an open system as it vents to the ground when it opens. A pressure cap for an overflow tank is for a closed system...it also allow coolant to return....a cap for an open system won't do that.

As far as the rated pressure, 15-16 psi is fine as long as the system is in good condition. If there's any physical problem with the radiator or hoses or heater core, etc., then the pressure can be a problem.

Depending on the year of the car, it may require a pressure cap in the 7 psi region. Back in the early 1950's, due to shortages of brass during the Korean War, radiators were often made much thinner and required far less pressure to operate as they couldn't safely withstand the higher pressures.

[long1776]

Perhaps I'm a little slow understanding, but aren't you contradicting yourself when you say;" an overflow tank is unpressurized......a pressure cap for an overflow tank is for a closed system" If it's unpressurized, how could it need a pressure cap? Or could we say that a radiator + tank becomes either a expansion system or a overflow system depending where, or if, you put the caps?

[junior]

hey, I'm confused here...I thought overflow tanks needed no cap (other than something to keep crud from getting in the tank), and expansion tanks and radiators need a pressure cap that vent to the atmosphere when the coolant reaches a predetermined pressure, yet will let liquid or air return into the system when a vacuum is created during the cooling period. I had no idea that expansion tanks and radiators had different styles of caps....can someone please clarify? Thanks, Junior. Also, when I had a shop rebuild my 54's rad, they said I needed no more than a 7psi cap, although they said the rad could take 15psi they felt the rest of the system was old and should not be subjected to any more pressure, makes no difference to me as it never really gets hot enough here to make the car run hot, the temp gauge has never gone over 200F, even in rush hour traffic.

[studegary]

It would help to know the year and make of car that you are asking about (referring to).

No Studebaker was built with either a recovery tank or overflow catch container. Therefore, Studebakers only need a cap that handles fluid in one direction, out. The cap does need to let air back in when the coolant cools. Different Studebakers used different pressure caps of varying pressure (0 up).

[Gunslinger]

Perhaps I'm a little slow understanding, but aren't you contradicting yourself when you say;" an overflow tank is unpressurized......a pressure cap for an overflow tank is for a closed system" If it's unpressurized, how could it need a pressure cap? Or could we say that a radiator + tank becomes either a expansion system or a overflow system depending where, or if, you put the caps?

OK...I misspoke...it was clear in my mind but obviously it wasn't to anyone else....I apologize for that. When I said pressure cap for a closed system, I meant to say the the cap on the radiator, not the overflow tank itself. That was really wrong of me. The cap on the overflow is simply to keep the crud out, and provide a place to check the coolant level and to add coolant if needed.

I don't see where the age of the system has anything to do with the pressure cap if its in good condition. Use whatever pressure cap it's designed for. If a cast iron block and heads can't take 15 psi you have more problems than what cap to use. If you have good hoses clamped properly and a good radiator tested to take the pressure, there's no reason not to use its rated pressure cap. My Avanti is a 1970 and when it was rebuilt, I had the radiator cleaned and tested...it held 30 psi...same with the heater core. Forty years should qualify as old and there's no reason why I should use a 15 psi cap. The system can take it. If you have a system designed only for a 7 psi cap, that's what you use.

Again, if a 15 psi system can't safely handle 15 psi pressure, then there's more serious problems to be addressed. Just don't mix types of caps...a cap for a closed system where one for an open system is called for, and vice-versa.