coolant overflow tank - how it works

Expansion of the hot antifreeze forces it out into the overflow bottle. As the engine cools, the antifreeze contracts, creating a vacuum effect to pull antifreeze back into the radiator from the bottle.

Correct, and also the radiator cap (on many vehicles) has a one way valve to allow that back flow of coolant when the vacuum develops in the cooling system. One reason to always replace with the correct cap!

You've gotten some good answers to your question but my question would be, why is it important?

I spent a couple thousand dollars 4 years ago for repairs due to an overheating problem. So about a year ago, when my Jeep began overheating at idle (stoplights) but not at speed, I took it straight in to the shop expecting another huge bill. The tech raised the hood, noticed the "bypass hose" had come loose from the overflow tank, shoved it back on, and charged me $85 which is outrageous but I was so relieved it wasn't in the thousands, I was a happy camper. It was later that I was kicking myself in the butt for not at least just having a looksee before taking it to the shop.

I still don't understand why a disconnected hose to the overflow tank can cause overheating at idle and only at idle.

Rick

. HeBeJeepN '99 XJ Sport, Selec-Trac, Trac-Lok

The reason for the tank, as I understand it, is just what it seems, a holding tank for coolant, allowing the radiator tank to be smaller. Years ago, radiator tanks were huge, then when aerodynamics came into play, hood lines dropped down, it was found that shrinking the top tank and making an external tank worked well.

As to your problem what was happening was that coolant was forced out by expansion, but none was getting back in, and the coolant level was dropping too much.

Then why would my temp go back down when I pulled away from a stoplight and got back up to speed?? It would only heat up at idle. I thought for sure it would be a fan problem. So I was surprised to learn what the problem turned out to be.

Rick

Low coolant? Almost the only reason I can think of.

At idle, the pump impeller wasn't turning fast enough to slosh what coolant you had through the radiator, so the engine got hot.

Budd