If it s not opened up enough the internal o rings dry up and hardens and coolant crusts up the cap and cause the lip that on the piece that is stuck in the reservoir to let go and all you get is the upper part.
Ford coolant cap stuck.
Removed the expansion tank cap on lady targ s 2010 fiesta and the cap came apart with half of it stuck in place in the tank.
How to determine if your radiator cap is bad p2181 duration.
All you have to do is to wait until it is cool down then use your palm to pound on the cap a couple of times to loosen up the sticky cap push down while you are turning it should be coming out easily.
The cap s two retaining ears are probably corroded stuck to the under rim of the filler neck flange.
I normally wet the o rings with a smear of something when i open one up to prevent what you have.
If too much pressure has built up removing the cap can send boiling water and steam up and out in all directions.
You want to turn the radiator cap counterclockwise to open it but it is stuck.
Since it s not turning at all you either need more torque or less resistance.
Less resistance might come from a few squirts of your favorite spray penetrant up under the 2 folded under ears that wrap under the neck flange.