We have seen this several times. You either have an exhaust seat leak or pcc seal leak which filled your cylinder and found its way into the PCC line via an open pcc valve. Best thing to do is borescope every cylinder and look for the one or multiple ones with leaks (it could be on A or B bank. It might be hard starting after that because the coolant may foul the plugs. You can try to purge the gas line with air to help this, also roll the engine over with start air with the spark plugs removed. This will help purge the coolant out of the lines.
You may also want to sample your oil to make sure it is not contaminated.