Mechanic may have left lights on too long

Dear Mike: I took my 2-year-old car into the shop last week for diagnosis of a bad alternator. Both headlights looked fine when I dropped it off, but when I picked it up the plastic headlamp lens on the driver’s side was hazed over and cracked, like they had spilled some sort of solvent over it.

It’s not water inside, it’s a pattern of slightly yellow cracks toward the top part of the lens. I can’t see any evidence of the solvent on the paint or trim around the lens, and the service manager says that it couldn’t be the shop’s fault, because the mechanic who did the work didn’t use any solvents while working on my car.

Can I prove what he used?

— T.O’G., Okemos, Mich.

A: It probably wasn’t a solvent. Your headlamp lens is made of polycarbonate, which is not soluble in any product that the mechanic would have used.

My hunch is that he used a fender cushion to keep his belt buckle from scratching your car. Then he left your headlights on for a long time, resulting in the fender cover collecting enough heat to crack the plastic.

Hold your hand in front of your headlamp while it’s on, and you’ll appreciate how much heat it creates. Normally that heat will dissipate harmlessly, but any covering will make it build up.

General Motors has issued a service bulletin about this — but good luck proving it and getting any satisfaction from the “stealership.”

Dear Mike: For the past few weeks a local tire dealer has been running radio ads touting the use of nitrogen to inflate car and truck tires. Now, I know that nitrogen is used in aircraft tires and race-car tires to keep temperatures down. The ads say that the nitrogen-filled tires will last longer, drive better and hold pressure longer.

At $5 per tire, it sounds like a scam.

— R.McK., via e-mail

A: I used to use nitrogen to fill my race-car tires, because it was the cheapest bottled gas I could buy in big cylinders at the welding shop. Twenty bucks’ worth of nitrogen would fill enough tires for a whole weekend of endurance racing, and also would run the air wrenches that I used to change the tires.

Remember, air is about 80 percent nitrogen to start with, so the difference between air and pure nitrogen is not profound. The balance of the volume of air is mostly oxygen, however, which is bad for your tires. Oxygen promotes breakdown of the rubber, so your tires should last longer with pure nitrogen. Furthermore, nitrogen molecules migrate through rubber more slowly than oxygen, so your tires should lose pressure more slowly — enough that you should be able to notice it, assuming that you check your tire pressure on a regular schedule.

Another advantage to nitrogen, one that’s perhaps more important, is that the nitrogen delivered from a welding cylinder or nitrogen generator is desiccated and clean.

Moisture inside a tire is bad, because it causes pressure fluctuations and corrodes rims. And I’ve seen lots of water come out of service-station air pumps. I generally will check the line for moisture before I use it by depressing the inflater pin with my thumbnail. If my thumb gets wet, I try to purge the line for a few seconds.

There’s also the potential for the compressor to force lubricating oil and debris from the inside of the tank into the tire. Oil will further accelerate breakdown of the rubber, and dirt can become trapped in the valve core, starting a leak. Worse, some shops use automatic oilers to lubricate their air tools, adding even more oily mist to the mix.

So nitrogen is indeed useful for prolonging tire life. To properly use nitrogen in your tires, however, all of the air from inside has to be purged, generally by the time-honored tradition of alternately filling and venting the tire. Unless the tire is broken off the rim, cleaned of moisture and debris, and then remounted with a waterfree rim lubricant before purging with nitrogen, you’ll miss most of the benefits.

Bottom line: I’d fill new tires with nitrogen, if the tire shop will do it for free or at least at a discount. Most will. But simply cruising in and topping off from a nitrogen hose for 20 bucks? Save your money.