Blown head Gasket or head.../ olds cutlass?

It is entirely possible you have a pinhole head gasket leak between a cylinder and a water jacket, Engine compression forces pressure into the coolant passages. The guage they use tests cooling system pressure, it is not a detector in the sense that they can tell you what the pressure might be caused from.You can do a compression test, but many times in a case like this you may not see significant results. instead try a reverse pressure test, you will need a coolant pressure tester and an engine compression test guage. Connect the engine compression guage to a cylinder, (one at a time until you test all six) then pressurize the cooling system. a leak will be evident if the engine compression guage starts reading pressure.