Hello i own a 2005 Mustang GT and recently my coolant resevoir started to boil and steam. I've checked the radiator fan and it is working fine, the radiator itself is fairly clean. I am not sure if the cap is faulty or its out of coolant? The thermostat seems to be working fine, the A/C shoots the proper heat and cold air. I really don't know. Money is tight right now, i won't be able to get it to the shop until my next pay check : (. I thought it would be good idea to ask this question, incase someone has dealt with this particular problem before. Hopefully it's not a blown head gasket.