Coolant starts to overflow along with steam from expansion tank...at about 210 F temp. upper radiator hose is hard....I believe system is having excessive pressure... recently change of thermostat gasket ...checked thermostat and it opened at the right temp...change of timing belt...brand new cap.... I don't see any obvious leaks... Car does jerk when cold start ....but it smooth out when it warms or hot... I checked with combustion leak tester ...the fluid does not change color...I waited until 180 F ..I cant wait any further because coolant fill start flowing out (without cap) can vacuum leak cause this ( I don't think so ....) I understand head gasket can cause this...but why blue liquid does not change color to yellow or anything else... Also, faulty expansion tank can cause this?