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MG MGB GT V8 Factory Originals Technical - Radiators 'Closed System'

I have a Ford Falcon Radiator with a 3000CFM fan in my 74 MGB/Rover 3.5 V8.
I have a standard 15# radiator cap and the overflow goes into a container.
However once the radiator cools the contents of the container do not flow back into the radiator.
If I understand the information I have read correctly I am trying to create a ‘Closed System’. As the water in the cooling system heats up and expands it flows into a container where is sits until the system cools down and then the excess water is drawn back into the rad!?
What is the component which allows the water to flow back into the rad when cooled? Is the radiator cap responsible?
As I look at my existing radiator cap I can see how it expands at 15# (I checked it) and then when the cooling system cools down, the cap stay closed.
Any information would be appreciated.



Bruce Mills

It does require a special cap for this type system and the tube from the radiator to the container should reach to the bottom of that container. IIRC the cap should have two seals, the outter one for the radiator neck and an inner one for the return flow. I believe that it also takes a special neck to seal with this type cap as it must seal both the input to the radiator but the overflow area in the neck to the outside as well.
Bill Young

Shouldn't be anything special about the cap, almost all modern pressure caps are made to operate this way and have been for a long time. Even before recovery tanks were common pressure caps had a vent valve to relieve vacuum in the radiator, so all they had to add was a rubber gasket to seal it at the top, and some of them already had that too.

What may be the trouble is a gravity problem if the tank is positioned too low, a small leak that prevents the siphon action, or a pressure leak onto the coolant system from one of the cylinders. This last can be hard to sort out, but if the radiator still has pressure after the car has been parked and sits overnight you can bet that's the problem. Can be caused by several things, such as a cracked head, bad head gasket, porous casting, or, something common in diesels, an electrostatic discharge that eats holes from the water jacket into the cylinders. Pretty rare in a gas engine but can happen. You can have it checked for hydrocarbons in the coolant, that's probably a real good idea, but start at the top of the list and work your way down, maybe it'll be something simple.

Jim Blackwood

When this setup was first introduced, the rad cap was a blank & the pressure cap was on the expansion tank. Barrie E
Barrie Egerton


Thanks for you comments

I took a look at my 2002 Ford Truck today and it has a lower (normal/older style ) gasket and another gasket up under the cap. The expanded fluid goes over to the bottom of the expansion tank via a rubber hose. The plastic cap on the expansion tank is snug fitting.

My MGB rad cap only has 1 rubber gasket and it runs to an expansion tank which might not be sealed tight enough. Also I am about to pull the heater core because I suspect it is leaking.

So as Jim suggests, "small leak that prevents the siphon action". I'll wok on this

I understand the rad cap seals the system until it reaches a point where there is enough pressure to push the rad cap spring up and lets the expanding water out.
But I still don't understand how the cap works in this system when it cools down since the radiator cap's spring is still excreting pressure on the lower rubber gasket sealing the neck of the radiator.
I understand a vacuum is formed, but how does the fluid get drawn past the rad springs lower gasket which is sealing the rad?

Bruce Mills

The cap has a small return valve in it, that's the metal disc in the center of the lower gasket; this allows flow back in to the rad, as opposed to the main valve, which is the one with the big spring and the lower gasket. The expansion tank cap just keeps birds out of the tank, it does not and cannot seal, else no venting of pressure out or vacuum back would take place.
The expansion tank cannot be too low in a car, unless the car approaches 30ft high, since that's how far a vacuum can draw water.
Any pinhole, even if too small for coolant too escape, will break the vacuum and keep the system from working. So the heater core is a prime suspect.
Jim, it is not electrostatic discharge that causes the pinholes, but cavitation. The vibration of the cylinder wall due to combustion pressure and piston thrust causes small gas bubbles to form on the surface of the liner. These are the dissolved air, hence oxygen, which causes a scouring of the metal down to the "chemically clean" point. The metal then oxidizes, and the next bubble repeats the process. It is prevented by coolant system additives, formerly chromates but I don't know what is used now that chromate is recognized as evil. These pits are always on the side walls of the liner, ie on the plane perpendicular to the crank; they are worst on the thrust side. They can quite easily perforate the liner walls.
FR Millmore

Just to reiterate a point made in the previous post - make sure the overflow tank is open to atmospheric pressure ie. a non-pressurized cap.
My particular coolant/overflow system uses the std MGB brass overflow bottle and a std pressure cap with the spring removed and the system works like a charm.
Graham Creswick

Re Graham's post, the original B system functions differently. In this, the expansion tank is inside the pressurized loop. The bleed to the tank comes off the highest point of the system, or should. This causes any air or gases to go into the ex tank, followed by coolant from expansion. The bleed tube goes to the bottom of the tank, so on cooling, coolant only is drawn back into the engine. On this system, there is no pressure valve between tank and engine. The pressure cap is on the tank. In the event of excessive expansion, coolant will escape via the tank overflow. If all is working well, the tank capacity is sufficient to accommodate the expansion without loss, and when cold there will always be enough coolant to cover the bleed tube at the bottom of the tank. This system is better, in that there is no valve to interfere with the expansion/return, but, the tank is subject to pressure flexing, and they are known to crack with age.
If Graham is using a pressure cap on the rad or engine, then his B tank is functioning as the simple plastic bottle does, not as the original B system.
FR Millmore

Thanks all

It sounds like all I have to do is get the proper cap and make sure the main cooling system is sealed (i.e no leaks).
The catch tank is not tightly sealed and the hose to the radiator should be covered with fluid otherwise the vacuum will be disturbed.


Bruce Mills

This thread was discussed between 04/02/2007 and 07/02/2007

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