It started a few years ago and I can't find a way to fix it. I have a 98 m3 4d sedan and when the temperature drops in the fall my car will not idle and runs rough. The first time this happend I called my dealer and they told me that the newer fuel has more alcohol in it and that I would need to lower the octane during the winter. I had a full tank so I drained it and filled it up with 87 octane and it ran fine except for the knocking. Is there a way around this problem?