How does an overflow tank work?

[Brad]

Should a radiator overflow tank be vented to atmosphere or should it be sealed up?
Seems so simple, but started thinking about it and now I'm not so sure if I understand how the system is supposed to work.

[67Fleetsidedream]

It is supposed to be vented. When the radiator heats up the water expands past the limits of what the radiator can hold. So the radiator cap is pressure sensitive (usually 15 pounds) and allows the expanded water to flow in to the overflow tank when the radiator pressure reaches 15lbs or over. When the engine cools down the contraction of the once hot water is slowly sucked back in to the radiator. Hope this helps.

[scotts67]

I dont have an overflow tank on my truck I just use a 18 lb cap it dosent allow any fluid to come out

[jorgensensc]

Scotts67,
That doesn't seem like it can be good for the truck. If the water gets hot and expands it has nowhere to go, except through a gasket, or hose.

[Longhorn Man]

An 18 pound cap is not a good idea at all. If all the gaskets are good and hoses are good, it can literally blow out the radiator or heater core. I have seena few come through our shop like this.

[SWEET7T]

On the overflow tank, make sure the inlet is submerged in the coolent, otherwise the coolent wont get sucked back in to the radiator. If you use tank with a hose going in the top, the hose would have to go to the bottom, or if you look on other vehicles, the hose is connected a the bottom.
Just makin sure...

[MikeH]

I am ready to install an overflow tank also, how critical is the overall vertical level of the tank relative to the radiator cap? In my installation, the top of the overflow tank is ~ 2" below the level of the radiator cap. Hose from the radiator filler neck goes to the bottom of the overflow tank, the top of the overflow tank is vented to atmosphere via another hose that travels downward and exits below fender level so if it ever does overflow through the top it will not collect in the fender and radiaotr support , etc. Sound OK? Thanks

[scotts67]

Originally posted by jorgensensc
Scotts67,
That doesn't seem like it can be good for the truck. If the water gets hot and expands it has nowhere to go, except through a gasket, or hose.

I have never thought abot that and hear latley I have seen alittle water on my intake that coul be whats causing it what do ya think

[jorgensensc]

I would check it out!! You don't want to blow a head gasket or a hose over something as silly as a radiator cap.

[scotts67]

Thanks for the info I will be sure to change that

[zyggy72]

This is the over flow bottle that I put together and getting ready to install but I'm not sure what radiator cap I should use. Any suggestion?

[Longhorn Man]

Z, Get a 15 pound cap for a 77 chevy pick up. A 77 should have the puke tank stock. Personally, I always get the Stant cap with the pressure relief lever.
I used the same bottle for a recovery tank, but didn't put a fitting on the bottom like that, I just made sure the hose on the inside went to the bottom of the bottle.

[tom hand]

The cap is what makes it work...if you have a cap that is not designed for a recovery tank, it will never "suck" the coolant out of the tank and back into the radiator. What's the differance? Just turn the cap over and look at it. A cap not designed for a recovery tank will just have rubber on the plunger under the spring. One designed for a tank will have a second rubber gasket along the top. This is what seals it and allows it to create a vacuum to suck the coolant back. A cap for a recovery system will work fine without the tank but not vice-virsa.

[Stocker]

Originally posted by MikeH
I am ready to install an overflow tank also, how critical is the overall vertical level of the tank relative to the radiator cap? In my installation, the top of the overflow tank is ~ 2" below the level of the radiator cap. Hose from the radiator filler neck goes to the bottom of the overflow tank, the top of the overflow tank is vented to atmosphere via another hose that travels downward and exits below fender level so if it ever does overflow through the top it will not collect in the fender and radiaotr support , etc. Sound OK? Thanks

Sounds fine to me - I bought my truck (27 years ago!) with an aftermarket tank mounted several inches lower than the top of the radiator, and a vent hose as you describe - and it works well. I doubt the height makes any difference, as the suckage & drawage of the coolant is what makes it all work.

[MikeH]

Thanks Stocker, I figured it would be OK! I've been running the setup for a few days and it seems to be working well. No more puking coolant all over my gargage floor...

[rambler]

Now I know why my truck blew off its upper raditor hose yesterday,, and spewed antifreeze all over the block! I just put on a 18lb cap on it.. geeze... the water pump failed and the pressure just blew! I'll post the other part of the story on another post, basically not a fun day yesteday!

[67chevemall]

Or run no bottle at all! It will go down about 1 " the first time and then stay that way. Run a hose down past the rad so she drips on the road!
:p