How does the Coolant Overflow Work?
 

We haven't had a coolant problem for quite a while, now all of a suden the overflow resorvoir is filling up and overflowing.

I can't remember how the coolant moves "back" to the AST. Is it just a syphon type effect when the AST is low and it pulls from the resorvoir?

More importantly, what would cause that much coolant to be trying to move to the overflow in the first place. Rebuild is not that old -- it is stupid hot here, but still?

It's the same route back to the AST as from it right? So it wouldn't be a blockage.

Right now the Resorvoir is full and a puddle on the floor.
Any help would be appreciated.

As the engine heats the coolant it expands. When it expands some of it travels to the overflow bottle. On shut down the coolant begins to cool and in a sealed system a natural vacuum forms which draws the coolant back out of the overflow and returns it to the system.
Your symtoms could be anything that allows a loss of that natural vacuum....bad cap(s), bad hoses, bad hose connections or worst case senario, bad coolant seal. Any of those individually or in combination would allow coolant to be stranded in the overflow tank on each successive cool-down cycle. Eventually that would cause it to...overflow.
I guess it could also conceivably be a blockage of the hose from AST to the overflow....though I've never seen it. IIRC it's wall thickness isn't that of the other hoses and it might be able to collapse under vacuum.

Here's the entire (stock) system if that's any help.....
http://i1126.photobucket.com/albums/...antdiagram.jpg

Good post Jim.

The most frequent culprit is a bad seal from the AST to the AST cap. This is not always the fault of the cap. Some AST's are flimsy (like the Pettit), and they warp. Check the top of the AST with a very flat object, like an iron, to make sure it is completely flat.

The problem is one of three things. Bad cap, The line from the ast to the overflow is bad or your coolant rings are shot.

Personally, Get a new cap and replace that whole line. This has happened to me twice now, both times, the line was bad. well, once it actually came out of the overflow.

yes, it does make a vacuum when it cools off sucking the coolant back into the pressurized part of the cooling system.

Thermo-siphon effect, to be technical.
Great advice and spot-on in the above replies.

If replacing the cap doesn't fix it, rent a cooling system pressure tester from an auto parts store.

Thanks!
 

We'll try the caps first, Seals are so new -- we have to hope it's a leak somewhere else.
Great Advice, we did recently change the battery and I know the one AST line was crunched, might tell us something.

I can see coolant stains around the engine bay, so somewhere it is pushing out.

Thanks again.