Q: Whenever I have wheel or tire work done, the shop always makes a note on the repair order to return after 50-100 miles to have the lug nuts retightened. Is this necessary, or are the tire dealers just covering their butts in case of a screw-up? It is hard to believe that lug nuts properly tightened with a torque wrench would work loose. If they did, why wouldn't they work loose again in the next 50-100 miles? Wouldn't continually checking them be ludicrous? - S.P., McGraw, NY.

A: Only new wheels (or wheel studs) need to be re-torqued once more after 50-100 miles. Compression of the wheel's metal or elongation of the new studs could occur. This would only happen once. By the way, when the nuts are re-torqued, the wheels should be cool and the nuts loosened and then torqued to the manufacturer's specification. As to whether your tire dealer is covering his butt is a question we are not prepared to answer.

Q: I drive a 2013 Mercedes GL350. The SUV requires diesel fuel. You recently referred a reader to www.pure-gas.org to find ethanol-free gasoline. I am looking for pure diesel fuel for my vehicle and cannot find any in my area. I am concerned that the biodiesel available at my local gas stations is damaging my vehicle. Do you know of any stations in my area that provide pure diesel fuel? - M.M., Glen Ellyn, Ill.

A: Mercedes-Benz's published statements say, "Diesel fuel with up to B5 biodiesel content according to ULSD (ultra low sulfur diesel) specification ASTM D975 meets Mercedes-Benz approved fuel standards and will not void coverage under the Mercedes-Benz New Vehicle Limited Warranty. Diesel fuels between B6 and B20 or higher pose risks of engine and fuel system damage, and are not approved by Mercedes-Benz. Please refer to the following recommendations to help avoid engine and fuel system damage if you do not have the chance to refuel your vehicle continuously with ULSD, maximum B5: Fuel with biodiesel content greater than 20 percent, including B100, is not approved by Mercedes-Benz due to the risk of severe engine damage. Any damage caused by the use of such non-approved fuels will not be covered by the Mercedes-Benz New Vehicle Limited Warranty." The TDI Club (http://www.tdiclub.com maintains a forum where you will find a link to locations selling non-biodiesel fuel . Truck stops are also often a good bet. Always check the pump for accurate fuel labeling prior to refueling.

(Bob Weber is a writer and mechanic who became an ASE-certified Master Automobile Technician in 1976. Send questions along with name and town to Motormouth, Rides, Chicago Tribune, 435 N. Michigan Ave., Fifth Floor, Chicago IL 60611 or motormouth.trib@verizon.net)

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