Boiling over is a common trait of HGF but don't panic yet replace the expansion cap first they are well known for failure giving the symptoms you are experiencing but make sure you get a genuine replacement as pattern parts are known to be useless, good idea to bleed the system, start at the rad then heater matrix with the heater valve fully open then the engine coolant rails, some say to raise the rear to do this but I have found it un necessary as yhe coolant both is the highest part, afterwards top up with OAT coolant but Don't mix with normal coolant if that's what's in there if it is this points to possible previous owner bodging so change and flush, if a ok run engine to operating temp, can take around 20 mins from idle or longer in this temp after the fans engine and rad should operate at approx 106C and off around 98 and continue to cycle like this, if not and it still overheats then check both rad pipes are hot if not then stat is no good, over than that it could be air getting in so looks for leaks especially on the cool down cycle, air locks, bleed again HGF, sniff test the list goes on, good luck hopefully just the cap.
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