Leaderboard

Depending on answers to above, the thermostat may need replacing if it is not opening - is the radiator top hose getting hot once the engine has warmed up showing the thermostat is opening ? If the temperature is going high also the radiator fan should be running. If the coolant level is low or the air is not bled out then be careful since if the engine does overheat the cylinder head is at risk of warping. Is the cabin heater working as normal showing circulation of the coolant ? Does the engine run as normal when warming up showing the temp sensor may be ok - these can fail but the other causes of high temp should be checked first due to possible damage if the temp is high. Obviously top up using anti freeze since there is still a risk of freezing weather.

Sounds as though internal pressure is forcing the cap off the bleed hole - it's only a weak plastic thread. Simple problems first!! Has the coolant been refilled recently? Has the system been bled properly?