OT: Radiator cap question

Back in my youth, when pressurized cooling systems with overflow tanks were introduced, it was sometimes said the radiator caps had "thermostatic valves" that opened when cold, to expel air, but closed when hot, to control boiling.

I've been looking at caps ever since, hoping to discern just how this trick was accomplished. I've never seen anything resembling a bimetal or wax capsule but admisttedly have never dissected a cap. There just didn't seem any space for such a gadget. All I could find was an inlet check valve to refill the cooling system as it cooled after shutdown and the spring loaded relief valve that vented pressure at setpoint.

Nonetheless, every liquid-cooled engine I've dealt with has kept its radiator full to the absolute top of the radiator filler neck, so the scheme somehow manages to expel air very effectively. This seems to work even when the coolant doesn't get hot enough to come near the relief pressure.

Does anybody happen know how this is accomplished? Does mere expansion of the trapped air and coolant produce enough pressure to force the relief valve open? Clearly, expansion of the liquid coolant can accomplish the task, but that would allow an air pocket to persist at sufficient volume, at least till the system started to boil. At that point steam would expel the air and the system would refill next time it cooled.

Or, is there some invisible heat-sensitive element hidden in the radiator cap?

Thanks for reading,

bob prohaska

Reply to
bob prohaska
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It is a simple 2 way valve with different pressures each direction The cap is also double sealed, unlike the older pressure caps. (basically the only difference required for a recovery system). When the pressure in the rad excedes the rating (14 or 17 psi, or whatever) it forces the main valve off the sear (which is part of the radiator neck) and the water and air are expelled into the overflow tank. When the system cools down the pressure in the rad drops below atmospheric pressure (develops a "vacuum") and it draws coolant in from the bottom of the overflow / recovery tank through the small valve in the center of the main valve (which is helf closed against the main valve by a very light spring)

IF a system does not stay full and the recovery tank fills up there are several possible failure modes. The hose could be leaking, the top seal could be leaking, or there could be a minor leak near the top (generally) of the coolant system preventing it from "drawing down" The pressure vlve could also be leaking. A rad cap tester checks both valves and the main top seal.

Reply to
Clare Snyder

Ok, that's the information I was looking for. There's no temperature sensitive device in the cap, it's just a brute-force relief valve going out and a low-differential check valve going in.

Thanks for writing!

bob prohaska

Reply to
bob prohaska

You are welcome

Reply to
Clare Snyder

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