You are treating a symptom by opening the 1 1/2 inch pipe. There is a major problem, either the septic tank, if that is what you have, is full and you are getting an air lock, or the sewer system is obstructed with the same result. You will eventually reach a point with your 1 1/2″ solution where the pipe will be higher than the house plumbing and then it will overflow in the house.