Radiator Cap pressure/expansion bottle

[NotAGolfCart]

Hello everyone, is there a way to use an expansion bottle to catch the excess radiator fluid when hot and then have it suck it back into the radiator when it cools on my 47' CJ2A? My brain just doesn't want me to let it leak so I see it when I come back to the Jeep.

Thanks for the help.

[bmorgil]

Yes, just run the radiator overflow to a catch bottle. Make sure the hose from the radiator goes all the way to the bottom of the catch bottle so the cooling radiator can suck it back out. Fill the bottle about a third of the way when cold.

https://www.summitracing.com/search/part-type/overflow-and-recovery-tanks/department/cooling-heating

[NotAGolfCart]

ooohhh and it's a shiny recovery bottle! Thanks!

[gmwillys]

If you don't like a bottle with a shine to it, then you could use any bottle of your favorite beverage. A common rat rod catch bottle would be a bourbon bottle or etc.

[bmorgil]

Like gm said, you can use anything you can attach for the catch can. If you scroll down on that link page, there are plastic ones for $15.

[LarrBeard]

" ... then have it suck it back into the radiator when it cools .."

A nickel's worth of comment here:

On the '48, the pressure cap seals against a lip at the bottom of the fill well. When coolant expands, it lifts the spring loaded seal and dumps excess coolant out the overflow tube. The tube inlet is above the sealed cap once it reseats - effectively cutting off the overflow path. The catch bottle will keep the Jeep from piddling on the floor, but it won't let it suck back excess coolant.

Old Jeeps tend to tell us where they want fluid levels; transmission lube, differential lube and even engine oil. The radiator isn't much different. I have found, by making several messes on the car barn floor, that the '48 wants coolant just about halfway between the top of the cores and the bottom of the fill neck. If I try to top it off, it lets me know by spitting it back out. If I'm lucky it dumps it on the road somewhere - if not - there goes another puddle on the floor.

[bmorgil]

Good point Larry. The radiator cap needs to be a Bi-Flow design to allow the water to be drawn back in to the radiator when the pressure drops.

This company does a good job explaining the caps. https://www.tridon.com.au/databank/documents/LR-092-99-intro-Radiator-Caps.pdf

[LarrBeard]

"Bi-Flow.."

Another new word we've learned. Sounds like the medicine the Doc just gave me

[bmorgil]

Hahaha! Bi-Flow meds are popular in us seasoned fellas!

Better known as a "Recovery System Cap". Most caps are this type. They allow air or water to re-enter the sealed cooling system. As you pointed out if you fill the radiator to the top, when it warms it will push water out. This is why as you suggested you should not fill a radiator all the way to the top if it is not a recovery type system. As the water warms it expands and if the radiator is full with no room for expansion, it will push some out to make room. When it cools if air cannot get back in the hoses can collapse in the sealed system. The "Bi-Flow" allows the air or water that was pushed out to return preventing the hoses from collapsing.

In the original owners manual for the CJ2A it is explained this way.

"RADIATOR FILLER CAP:
This cap is of the pressure type, which prevents evaporation and loss of cooling solution. A pressure up to 4 1/2 pounds makes the engine more efficient by permitting a slightly higher operating temperature.Vacuum in the radiator is relieved by a valve in the cap which opens at 1/2 to 1 pound vacuum"


If you have the original cap or a direct replacement it should be a Bi-Flow. It should draw the water back in. If it does not, it is an indication the gasket at the top of the tank is leaking or there is a leak somewhere in the cooling system preventing vacuum.

There's more than you ever wanted to know about a radiator cap!