a tak zeby wszycy juz wiedzieli zacytuje (po angielsku) manuala (z ASO):
"...The radiator caps are pressure type caps which seal the radiator and, resulting in pressurization of the radiator as the engine coolant expands. The pressurization prevents the engine coolant from boiling even when the engine coolant temperature exceeds 100°C (212°F). A relief valve (pressurization valve) and a vacuum valve (negative pressure valve) are built into the radiator cap. The relief valve opens and lets steam escape through the overflow pipe when the pressure generated inside the cooling system exceeds the limit (
engine coolant temperature: 110–120°C (230–248°F), pressure; 69–113 kPa (0.7–1.15 kgf/cm2,
10.0–16.4 psi). " ......
"...To avoid the danger of being burned, do not remove it while the engine and radiator are still hot, as fluid and steam can be blown out under pressure..."
a wiec widac ze jezeli po nagrzaniu sie silnika i odkreceniu korka zacznie wyrzucac wode (płyn chłodniczy) to nie jest to nic nadzwyczajnego)

dodam ze w normalnie napompowanej oponie jest cisnienieok 2.0 kgf/cm2 a tu mamy połowe z tego - czyli jest to w miare duze cisnienie