AT least 50 cars in Ballarat have undergone LPG tank repairs in the past week after a gas contamination at a Shell outlet.Shell yesterday confirmed its Coles Express outlet at the corner of Pleasant and Sturt streets had been supplied with an affected LPG batch from its Lara depot.The LPG contained higher-than-normal residue levels, which may have led to operating problems in some vehicles.One Ballarat dealership estimated between 50 and 100 cars had been towed into their service department over the past week, half with gas-related issues.Most of the cars had been found to have a build-up of a wax-type substance in the gas that was causing the valves to stick.Scarsdale resident Jenny Ellis, whose car runs on LPG gas, put about $30 worth of gas in her vehicle early on Sunday morning at the Shell outlet. When she next went to drive her car, it would not start and her mechanic later confirmed the gas was contaminated.Mrs Ellis said having the problem was an inconvenience and another expense."It's not only the inconvenience of having the car towed and repaired, but also the expense - there would be a lot of people who would be put out with the cost of the fuel and a repair job," she said.A Shell spokeswoman said some motorists who bought the affected LPG had experienced operating problems with their vehicles, including difficulties cold-starting and rough running.She said as soon as the problem was found, Shell stopped supplying LPG from Lara while extensive testing was conducted."Some retail sites identified as containing LPG with a high level of residue were closed while actions to fix the problem were undertaken," she said."As such, motorists can be confident of the quality of Shell LPG currently selling at service stations."Those who have found operating problems with their vehicles and who bought Shell LPG in Victoria between June 17 and 28 should call 131 618.