Mystery overflow tank bubbles?? It starts with your overflow tank getting bubbles right when you start up. Will go to over 30 pst quickly before your engine gets warm. When it is warm enough to open the 2 thermostats pressure goes to zero. This went on for two years. I had the overflow tank tested twice for exhaust gas. Using the chemical test. Blue will go to yellow if exhaust gas if present. Both times stayed blue. What a mystery. Rather than going to a head gasket job. I replaced the water pump. I seemed to help a little. So then we though it was a small air leak from the water pump. Replaced the water pump (Again this time bought the pump and housing) Still same issue. Then I got hit on the front end. Repairs replaced just about all cooling related items. When I got my truck back the bubbles were now pressurizing the overflow tank continuously, warm or cold. The truck would loose about a gallon of coolant until the tank was empty. Did not tow anything while this new problem was happening. Truck never overheated after the repair. So took my truck back to a Diesel specialist. They tested the truck for a gasket leak and it tested bad. Chemical test went yellow this time. New head gasket kit with titanium head bolts finally fix my problem. Yeah!