VW would have issues cranking over in the cold temperatures and have very rough idle once started. Dealership said could be battery, had that changed, continued to have issue, but wouldn't occur when taken to dealership. Read some forums online that said US diesel fuel is not regulated like in Europe. Could possibly be lower quality diesel fuel around where I live. Began to add Power Service Winter diesel addictive. After about a day or two of commuting, has not done it since. I have added it every time I fill up, now using the regular additive.