A gallon and a half a month is not very much. I sold a chevy powered buick to a lady once who began complaining that it was loosing a cup of coolant every day. I of course thought she was crazy but after a couple of months of this I told her to bring the car to my garage. I raised the hood and saw nothing. Removed the air cleaner and saw a dead kitten, hard as a rock. When I removed it and then tried to clean up around the carb(a little hair is all) I felt something on my arm. The car had a pin hole leak in the heater hose spraying on the choke pull off and immediately evaporating. As the car cooled the pressure dropped and so did the leak. It was such a fine stream of water that it could not be seen even when you knew where it was coming from unless you got the light just right. I'm not saying you have a dead cat, I'm saying the other guys are right. The water is going somewhere - keep looking.