Questions and answers from the ‘Car Doctor’

Q. I have a question about the tire-pressure indicator light in my 2007 Subaru Outback. After anywhere between 8 and 15 miles of driving, the tire-pressure indicator begins flashing and continues to do so until the trip ends. The pressure in all the tires is fine, and even a short interruption of my trip with the engine turned off the light will stop the flashing, only to have it reappear again a short time later. Any suggestions?

A. There are two types of tire pressure sensors (TPMS) direct and indirect. Indirect sensors use the vehicles anti-lock brake system have no physical component on the wheel. Your Subaru uses the direct type which uses a wheel mounted battery powered sensor. More than likely one or more of the tire pressure sensors is failing. These battery powered sensors have an expected life of 7-10 years. When the battery gets weak it can’t send a signal to the cars computer and the light flashes to indicate the system is malfunctioning. The Subaru TPMS cost about $75 each plus installation. There are also aftermarket sensors which are cheaper. TPMS is a safety system, until you get the system repaired, check the tire pressure monthly.

Q. My 2013 Honda CR-V has 83,000 on it. The car drives fine but has a has a muffled rattling sound, especially on bumpy roads. Last oil change I asked mechanic to check it out. He said there are plastic clips (forgot on what) that get loose, and he tightened them. It still makes the noise.

A. Rattles are a tough issue to diagnose but Honda does have a couple of common rattles. The catalytic convertor heat shield (part of the exhaust) can rattle —this would be a tinny sound coming from under the car. A different bumpy rattle more in the car could be part of the heater called the heater core and Honda’s fix was just to add foam insulation (a fair amount of work). If it were my car, I would check all of the steering and suspension parts, at 8 years old a strut/shock could be getting weak or one of the mounts could be starting to wear. I would just want to rule out any safety issue. I have found in addition to a good road test, the best style shop lift to check for rattles is a drive on lift. On this type of lift, all the car’s weight is sitting on the tires, like it does on the road. There are also specialized tools to find rattles (one is an electronic listening device called a Chassis Ear. Ruling out the easy and obvious doesn’t take much time. After that finding a rattle can get expensive. This may be one of those cases where if it isn’t dangerous, let time be your diagnostic tool and wait until the noise gets a bit more obvious.

Q. Someone told me to when waxing a car leave the wax on for 1218 hours before buffing it off for the best shine. Is this true? Also, what do you think of machine buffers to apply and remove wax, I see the pros using them all the time.

A. When waxing a car apply the wax in about a two-foot square and remove the wax as soon as it turns hazy. Then continue slightly overlapping areas. Where the confusion is, some people want

a deeper richer shine and, in that case, wait a day before applying a second coat of wax. Regarding buffing wheels, the safest and easiest to use are dual-action buffers. High speed buffers work great, but without the proper care you can easily burn through the paint. If investing in a buffer it has been my experience the more you spend the better the results.

Q. I have a new car and the dealer recommend nitrogen in the tires. The cost was about $75 and it was included in the financing, but I said no. Now I’m wondering should I have had the nitrogen put in the tires. Is nitrogen better than using regular air?

A. Air is 78 percent nitrogen and I’m cheap and would never pay $75.00 for something you can get for free. There are specialized applications that nitrogen is preferred since it is a dry gas and compressed air can contain moisture. These specialized application include racing, military and commercial aircraft and even the tires on the space shuttle. So, unless you are racing or taking your car into orbit, say no to nitrogen.

Q. An evergreen tree has left sticky sap on my car’s hood. And I’m having difficulty removing it. What should I be using of does it need professional detailing?

A. Start with washing the car with warm soapy water (use only car wash soap) and once dry use a specialized bug, tar, and sap remover. Once clean re-wax the car to bring the shine back from using the cleaner. If this doesn’t work, then a professional detailer should be able to remove the sap and restore the finish. The longer the sap bakes in the sun the harder it will be to remove.

Q. I’m not sure if you have tested polishes, waxes and ceramic coatings. What consumer ceramic coating do you recommend? I have used Mothers Ceramic with great results (and its pretty easy to apply) but the auto parts stores have an entire shelf of products. Any preferences?

A. The original ceramic coatings were difficult to apply, expensive and best lest to professionals. They also required some tedious paint correction before they could be applied. Some of the original ceramic products were $250 or more for a single use and when applied professionally could cost $750$1000. Quite expensive, although you needed to do it once every four of five years. Today the ceramic spray products are a sort of hybrid, Mothers is a good product reasonably priced as are Chemical Guys and Meguiars.