Notice the radiator has two distinct sealing lands. The outer keeps the complete system from letting coolant out and letting air in. The inner landing
should be inspected for debris or damage. If paint, rust, some form of debris is keeping the cap from making a good seal it can spew over before reaching pressure.
Also a visual inspection of the cap may reveal a gouge, cut, debris stuck to the rubber that prevents the inner land from sealing. That includes the valve built into the cap. Pull it down and inspect for debris.
Here are a couple of pictures; next time its cool inspect the inner landing for any flaws. A simple chunk of debris on this landing can lead to venting before set pressure is reached. Also, it prevent it from refilling as the engine cools which ultimately leads to it pushing water out as it has MORE ROOM FOR STEAM. Once the block makes steam inside it tends to push more and more water out.
Back to the inner landing: As the engine heats the coolant expands and goes past the inner landing into the coolant overflow container. This is normal. So that inner landing lifts and lets engine coolant out at 16 psi (caps rated pressure). Once the engine is off and cool down begins, the coolant in the block contracts. Here is is where that little disc in the Radiator cap comes into play, vacuum internal to the blocks coolant system pulls it open and s u c k s fluid back into the block.
So, if the inner landing is flawed, that throws a wrench in the hole process. Also if the tube in the overflow tank has a leak it won't pull the fluid from the bottom of the tank. Instead it just sucks air.
Confirm the inner landing is good.
Confirm the flapper on the cap has no debris in it.
Confirm the tube from the neck to the tank has no holes in it and it reaches the tanks bottom.
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