I just had my head gaskets replaced, thermostat and sensor, and radiator cap replaced three weeks ago. For the first time I took my car on a long drive that was freeway speed and uphill 3/4 the way. I pulled over after 35 miles and shut off my car. I heard the loud popping/gurgling sounds so I opened the hood. I saw the coolant resivor fluid was boiling and I mean boiling and was all the way to the top of the resivor. Needless to say, I discontinued my trip and returned to the shop all the while I put it in neutral most all the way to keep my engine from burning up. Weird thing is the interior gage did not show my car was overheating. The shop called me and said they drove my car up several hills for 30 miles and the coolant did not boil. They replaced the radiator cap and said that might be the problem as the one they put on may have not been heavy enough.
Can you tell me what would make the coolant in the resivor boil over, and not reflect on the interior gage it was over-heating? Plus, why wouldn't it do it again for the shop technician?