I have an 87 560 SEL (126 body like yours) and had the same exact problem last winter as you are having now. You have a monovalve located near the battery. It is 1 inch square with 4 small screws and a 2 wire connector. Get a new valve diaphram ($38.00 at the dealer) and simply remove the connector, the 4 screws, and the diaphram. Upon close inspection you will find a small tear. It is a simple repair. Good luck.