Dear Doctor: I own a 2006 Nissan Murano all-wheel drive with only 10,000 miles. A few weeks after I bought it I noticed a high-pitched squeal when I pressed on the brake when the transmission was in reverse. The dealer said this is a normal condition. The brakes don't squeal going forward. Why? Richard

Dear Richard: Some occasional brake squeal is normal, but brakes should not emit high-pitched bus-like sounds. The Nissan dealer should listen for which brake is emitting the squeal and replace the pads and rotor on both sides.

Dear Doctor: I have a 2006 Hyundai Sonata equipped with the V-6 engine. Is there a computer chip that will give this car a little more performance? Will my fuel mileage change? Robert

Dear Robert: Most upgraded performance chips will require the use of premium gasoline. The computer chips alter ignition timing and transmission firmness and shift points. Some chips also offer speedometer recalibration for larger tires or smaller diameter tires. At this time, however, no companies offer performance upgrade computer chips for your Sonata.

Dear Doctor: I own a 1988 Cadillac with only 80,000 miles. I have an ongoing problem with the a/c system leaking Freon. For the last four years I have had to replace six compressors, some hoses and other parts. Within 24 hours, two new compressors have failed. The shop cannot give me any explanations. Teri

Dear Doctor: I recently purchased a 2005 Hyundai Tucson. I change my own oil and noticed that the aftermarket replacement oil filters differ from the factory filters. Some of the aftermarket oil filters are smaller and they do not seem to have an internal spring inside. Are aftermarket oil filters as good as the factory and will the use of an aftermarket oil filter void the factory warranty? What is the spring in the factory filter for? Stephen

Dear Stephen: Some aftermarket oil filters will differ in size and design. I personally use name brand oil filters that meet or exceed the factory recommendations. The spring in the oil filter can act as an anti-drain back support and aid in the by-pass design. As a do-it-yourselfer, you would be well advised to stay with using the factory filter.

Send questions to: Auto Doctor, 3 Court Circle, Lakeville, MA 02347

Dear Teri: The shop should figure out why the compressors are failing. Is it from contamination in the system? Are they putting the proper amount of a/c oil in the compressor before it's installed? As for loss of Freon, a simple dye test is all that is needed. A common leak in your car is the evaporator. The shop should also convert the a/c system over to the less expensive R134A at the time of the compressor replacement.

Dear Doctor: My wife and I just replaced two tires on our 1997 Toyota Corolla. The sales person suggested we put the new tires on the front and the keep the older tires on the rear. However, the clerk suggested we put the new tires on the rear. We're confused. Where should the new tires go? Joe

Dear Joe: We always put the new tires on the front. The front tires do a lot more work than the rear tires, even on rear-wheel drive vehicles. In real world conditions, with proper rotation, all four tires should show the same signs of wear. I recommend on average tires be replaced that are four to five years old. Tire rubber deteriorates over time. Although the tread may still look good, some of the chemical compounds in the tire make-up become exhausted, making the tire unsafe.

Dear Doctor: I own a 1993 Mercedes-Benz 300SD diesel. I have a strange problem with a rusting gas cap. The gas cap seems to rust internally, making it difficult to remove. I have replaced many fuel caps and use only Mercedes fuel caps. Any suggestions? Alan

Dear Alan: There are no bulletins at either Alldata or Identifix on rusty fuel caps. Take a closer look for water or condensation in the fuel cap area. Also make sure if there is a drain at the fuel filler that it is not plugged up.

Dear Doctor: I am interested in purchasing a new pickup truck. I have always owned Chevy pickups, but my complaint has always been poor brakes. Do the new Chevy pickup trucks have brakes that work better than the 2002 model? Should I consider the new Toyota Tundra pickup? Stewart

Dear Stewart: I, too, own a 2004 Chevy 2500 4x4 pickup, and the brakes on my truck are poor. I have not driven a new Chevy or GMC pickup as of yet for evaluation. As for Toyota's new Tundra I am waiting for my test truck to arrive.

Send questions to: Auto Doctor, 3 Court Circle, Lakeville, MA 02347