Yesterday my 1996 Mercedes C220 would not start. It cranked like crazy but would not fire. Some things I suspect contributed: The gas tank was below 1/4 full. It was cold ... about 25F. The night before I had put 140 lb. of sand in the trunk just before driving it about 15 miles home and parking it. My theory was that because of the weight of the sand and the low gas level, gas was not able to get to the fuel pump. So this morning I was going to put in a couple gallons of gas from my garage gas can. When I opened the gas cap there was a serious whoosh of air into the tank; there had been a substantial vacuum inside the tank. I put the cap back on, without adding any gas (the sand was still in the trunk), and the car started right up. Here's my question(s): 1) Did the sand contribute in any way to the problem?; 2) Did the low gas level contribute?; 3) Did the cold weather contribute?; 4) Do I have a mechanical problem I need to take care of? Thanks.