ok, heres the thing, i have a 2004 chev 2500 6.6L. the customer complains of loss of heat while driving and his low coolant light coming on. I think head gasket. i pull it in and there is no combustion gases in the coolant, i tested four times. the coolant , i see is being pushed out the over flow and not being consumed by the engine. he says that he only loses about four liters of coolant at any time. this would be consistant with the coolant loss out of the over flow for that is about what it holds. I pull a few glow plugs and no signs of coolant, i pull the thermostats and test them, they work, plus the customer told me he replaced them not a week ago. they are in the proper position too. i flush the entire system with no visible blockages. when driving at high speed we will get hot air and the temp guage will reach normal. the thing is that the rad is still cool. i shut off the tuck and let it sit for the night, in the morning it still had pressure on the system, before this i had also replaced the rad cap. i pulled the water pump and inspected - all ok, i dont know what else to try?