Yes sir t IS possible that you can drive 220 miles and NOT overheat. WHen you added the additional load you got the higher temps. The rad cap has several purposes, one to obviously keep the coolant IN the radiator. It also seals your cooling system which due to physics RAISES the boiling point of your coolant. You suffered a triple or quadruple whammy when you didnt have your cap on… You probably lost a amount of coolant due to it being off, after that you had less coolant thus lowering your cooling capacity of the rad and also the cap being off lowered the boiling point of your coolant. This coolant loss can also create an air pocket around the water pump not allowing the pump to properly circulate the coolant further exaserbating your cooling woes. Let me know if you need more info. But yes if they left the cap off you will be able to go a while with NO problems. It will take a while and then you will see precisely what you have seen. Hope you didnt warp your cylinder heads.