How does radiator cap work?

[Coldwaves]

As I understand, the radiator cap has a spring so it can hold pressure. When the car heat up and the pressure exceed the pressure the spring can hold, the coolant will flow to the reservoir. When the car cools down, the fluid contract and create a vacuum. Then coolant can be sucked back through a vacuum valve in the cap. This is what I understand.

Here is a thing that confuses me. When the car is cold, I can squeeze the upper or low radiator hose. When I squeeze it, I can see coolant goes in and out of the reservoir. This is pretty confusing to me. The radiator cap should withstand 15psi pressure. So if I light squeeze the hose, The fluid should not flow out to the reservoir because a light squeeze should not create too much pressure. Since no fluid is out, when I release my hand from the hose, there should not be any vacuum and fluid should not come back. So when the car is cold, I should not be able to see the fluid go in and out of reservoir if I squeeze the radiator hose.

So why did I see this happening? Is my understanding of how radiator cap work wrong or is it because my radiator cap is faulty?

 

[Fivefingerdeathpunch]

Look at the bottom of your radiator cap. There are two seals. One that seals at the top neck and one that seals at the bottom of the filler neck. As the fluid heats up it expands. It eventually over powers the primary spring in the radiator cap.

The bottom seal is broken as the pressure exceeds the spring force of the cap and the cap rises. Fluid then enters the overflow tank until enough pressure is relieved to allow the spring to force the bottom seal back into place.

You'll also find on the bottom center of the cap that there's a brass or stainless circle plate. That circle plate actually creates a one way check valve. When the engine cools back down the coolant shrinks in volume. As that happens the pressure in the coolant system becomes a vacuum pulling down on the bottom circle plate and thus opens the check valve.

Once the valve opens the fluid is drawn in through the tube that leads to the bottom of the reservoir and refills the system. The system is actually self burping in that it evacuates air on each temperature cycle. Air is pumped out when the system gets hot and fluid is then drawn in when it cools.

Does that help?

 

[Coldwaves]

Does that help?

Thanks! That does help. It confirms that my understanding of how radiator work is correct. Then, my confusion still persist. When cold, I can squeeze the upper or lower radiator hose and saw coolant being pushed into reservoir. How does the fluid go there? The fluid can only go out of radiator to reservoir when there is big pressure to push against the spring in the cap. I don't think the light squeeze create enough pressure to push up the spring.

 

[Rob97gtp]

I would say your cap is no good