overflo tank

i have added an overflo tank to the cooling system, do i need a special radiator cap to have the "overflo" siphon back to the radiator? tho its not much,its all staying in the tank.
thanks
charlie

Just the regular pressure cap. Hose should either connect at the bottom or reach to the bottom of the tank if routed thru the top. When hot, level will rise and then suck back in when cold. Usually "about" a half inch difference.

You want to mount the overflow tank as high as possible. It will help the radiator suck the juice back in.

Charlie-

The correct cap should actually have two seals. The first seal is on the spring-loaded plunger which seals down in the filler neck. The other one, usually a simple gasket, seals the cap to the TOP of the filler neck.

The radiator's overflow outlet is located between these two seals. When system pressure builds, the plunger is pushed open and coolant goes past the lower seal and out the overflow tube. When the system cools down, low pressure pulls the coolant back into the radiator. Without the upper gasket, fluid will still go out the overflow tube since it is lower than the top of the filler neck, but when it cools back down, air is easily sucked through this connection since it isn't sealed. That's why with a recovery system, you don't open the cap to check the level but instead check it in the overflow tank - you don't want to break that seal and allow air into the radiator.

I'm not sure when Detroit started putting overflow tanks on cars, but back in the late 60's and early 70's, coolant recovery tank kits were a popular aftermarket item. These kits consisted of a tank and a gasket to add to your old radiator cap. Without the gasket, it will do exactly what you are describing. Once everything is in place, it will takes a few heat/cool cycles to totally purge all the air from your cooling system.

-Stu

I feel sure you are correct, but I'm using a plain old Stant cap with a pressure release handle and it is working correctly. Please don't think I'm arguing that you're wrong, just that mine is working.

I think the 'vented' caps are the ones that were used without a tank. When I have had those with a tank they dont ever seem to suck it back. Looking at them they look identical so I am not sure where the vent parts is.

I wonder why you want the fill/refill overflow option. I had an overflowing problem until I managed to purge almost all the air out of the system. Of course it took about six months. But it is the air blowing the water out. Once I got it down to just the bubble under the cap, the runout stopped.

I guess the proper name would be expansion tank.

It's on the unpressurised side; it seems manufacturers now just pressurise the reserve tank and run a simpler cap.

That which the OP needs, is sold (here, at least) as a "recovery type cap".

I think one reason for the tanks being part of the system now is the radiators are often no where near the top of the system anymore.

addo":39q6m2rw said:

That which the OP needs, is sold (here, at least) as a "recovery type cap".

Click to expand...

I've seen that designation here too...depending on who's site you''re checking (want to say napa or autozone used the 'recovery type' designation for caps.

fordconvert":39q6m2rw said:

I guess the proper name would be expansion tank.

Click to expand...

IMO That may be more appropriate. The expanding coolant pressure (part water vapor) under high heat has to vent somewhere...stock was to let it run out a tube to the ground...recovery allows just that, and also think it increases capacity slightly by always insuring full/purged system.

I just bought a cap from a later model vehicle known to have a recovery system and in the poundage desired.
Good luck!