You already found the major clue. How can coolant come out the tail pipe?
All of these are typical symptoms of a leaking cylinder head gasket. When combustion gases get pushed into the cooling system, that can pool under the thermostat and prevent it from opening. Thermostats have to be hit with hot liquid to open. Hot air won't do it.
Next, most cooling systems use a 15 pound radiator cap because the higher pressure makes the water in the coolant boil at a much higher temperature. If yours actually does call for a six pound cap, doubling that increases the maximum pressure that can be reached, and that is going to push coolant into the cylinder even harder and make the problem worse.
There's two tests you can do to verify a leaking head gasket. One is to add a small bottle of dark purple dye to the coolant, then check later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. If the head gasket is leaking, you'll find the dye inside the tail pipe.
The second test involves drawing air from the radiator, while the engine is running, through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow. It's usually less expensive to have your mechanic perform that test. You can borrow that tester from an auto parts store that borrows or rents tools, but they make you buy your own bottle of fluid. That's because it will be rendered inactive if it is allowed to freeze or if it gets contaminated with antifreeze. It's easy to suck up antifreeze if you aren't careful, and since they don't want to risk borrowing you this tool with contaminated fluid, they made the last person buy their own too.
Monday, June 8th, 2015 AT 11:12 PM