: I have a 3306 I top off coolant it blows out 2 gallons but don’t overheat steam comes out of overflow is that a head gasket problem

 

A test with an electronic sniffer for combustion gas in the radiator should answer that question......Ben

 

You can also use ph test strips on the coolant- take a reading from fresh coolant and compare with used coolant at low hours- more acidic usually tells you that CO2 is getting into the coolant.

On the Cats I saw some bad T-Stats overheating the engine in the head, the steam in the head pushed the coolant out of the overflow, while the temp gauge read low..