Just replaced the radiator cap by one that I bought from The Fierostore (locking)
Now the cooling system emties itself in the overflow tank.
Has anybody experienced this as well?

Model and brand, if not a Stant, cap?

Picture will help.

Likely? TFS send you the wrong model.

See my cave. Radiator Cap in coolant section.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

To answer your question - Yes this is a known issue.

It's well documented in Ogre's Cave.

The little relief valve on the cap must be spring-loaded. If it flops around loose, you will have problems. Look up the Stant number recommended for your car on their website, and I think the one you want is one number lower than what they recommend (if I remember correctly). In any case, it will be right next to it on the parts store rack.

Many thanks for your valuable info. The flap is just loose and not spring loaded.