I have a 1993 2500 4 wheel drive with a 6.5 turbo diesel pickup. My problem started with a complete loss of power 3 months ago. I took it to a reputable diesel mechanic and he rebuilt the injection pump. Truck worked great for 2 weeks. All of a sudden there was bubbling in the coolant tank. I can take the radiator cap off and it looks like a pot boiling. Took it back to the reputable diesel mechanic. The mechanic thought it was a blown head gasket. He took the heads off and inspected it and reassembled. Still bubbled. He got re-manufactured heads and replaced one and then the other. Still bubbled. He tried 2 new radiators, several new thermostats, replaced the water pump and fan clutch. Still bubbles. There is no water getting in the oil. Does anyone have any thoughts on where to go next? The mechanic is at a loss. I love this old truck and do not want to get rid of it. It has been well taken care of. HELP!!!!