Q. My wife's 2005 Ford Focus ZXW SES (station wagon) purchased new and has provided yeoman service with never a problem. As the mileage just passed 75,000 I purchased a K&N replacement air filter element on-line. After receiving the air filter I found out that it didn’t fit (UK focus only). The vendor won’t refund my money and only suggested I purchase an air filter box and sensor from Ford. I have looked on line and only found dead ends. Any ideas would be appreciated. 

A. I was a little shocked to find out that the air filter on your Focus is a non-serviceable part that is supposed to last 150,000 miles. I verified this with local auto parts stores as well as Ford through AllData online. Regarding the vendor, perhaps cancel the purchase with your credit card company and request a refund.

Q. I hope you can help me with a shift problem in my 2003 Toyota Highlander. The car is a four-cylinder engine and front wheel drive. Shifting into gear works fine until the cold weather arrives. In the cold weather I have to almost stand on the brake to get it into drive. I’ve had it checked and no one can find any issues. After warm-up (cold days) there is no problem. Any help would be appreciated.

Advertisement - Continue Reading Below

A. Since stepping on the brake pedal helps it sounds as if the problem is related to the brake/shifter interlock. There are very specific tests to find the problem with the shift interlock. I would suggest waiting until the weather get cooler and leave the vehicle with the shop overnight to they can test the component while they are more likely to act up.

Q. I have Nissan with a push button start. The other night I came home and thought I shut the car off, only to find out in the morning the engine had been running all night. Did I do any damage to the car?

A. You are not the only person to do this with the style ignition. As long as the car didn’t overheat I would say your car is fine. What I would do is have the oil and filter changed. Excessive idling can cause engine oil to break down.

Q. I had a fuel pump replaced on my 2000 Mercury Grand Marquis for $650. Is this within the proper amount to pay for this type of repair?

A. According to AllData, the data base I use, a fuel pump from Ford would cost nearly $400.00 and it would take two hours to remove the fuel tank and replace the fuel pump. In addition there would an additional charge to drain and refill the fuel tank. You could certainly save some money with an aftermarket fuel pump. I have seen aftermarket fuel pumps for your model car run from $129.00 to $299.00. Keep in mind that is the cost of the pump and you should expect some “mark-up” to list price.

Tom, from Milton writes in response to my theory of tire pressure and weight: I am taking you up on the invitation you made in your August 5 column in the Boston Globe, asking for physicists to weigh in on the question of tire pressure with and without the tire supporting the vehicle. I am a high school physics teacher, so I guess close enough to qualify. Your explanation was correct.

 This is a result of pressure being defined as force divided by area. The weight of the car is a downward force which must be cancelled by the upward force of the ground on the tires. That force is equal to the pressure in the tires times the area of contact. When you added 500 pounds to the car, the system compensated by “squatting” a little bit, which flattened the tires somewhat, thereby increasing surface area, as you wrote in your column. Since that area was larger, the product of pressure and area (equal to upward force) could increase without any change in pressure, allowing equilibrium to be restored.

This is commonly demonstrated in science classes by laying four sheets of newspaper out in a parking lot and driving a car onto them. Students trace the footprints of all four tires onto the paper and then measure the tires’ pressures. They then take the sheets back to the classroom and compute the areas of the four rectangles in square inches. Multiplying by each pressure gives a pretty accurate estimate of the weight of the car.  

By the way, I liked the homespun experiment you did to check your theory. As a science teacher, I know that conveying the knowledge of science to kids is less important than getting them to believe that they can themselves be everyday scientists, meaning get answers to questions by simple experiments. Once they do, it is then just a matter of giving them a little training in how to do it. Your piece was a great example of doing just that.