Somone i know has a 2004 vnl with d12, when you start the truck cold after about 20 minutes of idling the coolant bootle expends and you can see that top of the is expending looking like buble and the radiator hoses are very hard. From what i understand the blue cap on side is pressure cap and black one is only to fill. The blue cap was replaced with volvo part problem is still there, from mine understanding the cap should relase exesive pressure. Any sugestions?