Q. Since the cold weather began, the windows (and sunroof) in my 2007 VW GTI 5-door keep fogging up. It was taken to the dealership and after they spouted some nonsense about “off-gassing” mentioning that I left some rags in the hatchback, they departed from those avenues and were able to recreate the problem. Basically after the car has been running if the windows start to fog the only way to see usually is to turn on the defroster. The problem is when I turn on the defroster, it almost instantly becomes worse to the point I can barely see out of the front window and almost not at all out the side windows. The car has been checked for leaks and the sunroof collection tray was not full. The dealership says next step they say is to leave the car overnight while they install a heater . . . not sure what that will do. Can a defroster actually be introducing moisture into the car? Might I have bad window seals? There is a rust spot forming above the “b” pillar on the passenger side, but the VW district manager has to approve repair of this problem. I know this sounds complicated, but do you have any advice?

A. The rust on the pillar could be a clue to the moisture problem. The sunroof drains through hoses at both the “A” and “C” pillars. If the sunroof is not draining and collecting at the “B” pillar, this could certainly be the cause of the problem. The other possibility is the tail-light assembly could be causing moisture in the car. Volkswagen issued a technical service bulletin (number: 9408-02) that describes water intrusion through the tail light assembly.

Rotation and pressure

Q. I own a 2005 Cadillac DeVille that has, as standard equipment, a tire pressure monitoring system. I’ve been trying to follow the manual’s suggested rotation schedule but got a little concerned when the last rotation (at the dealer) resulted in a charge of $49.95! I do understand that special equipment is required to reset the system after rotation but can’t help but wonder why, given the costs involved, I shouldn’t skip the whole thing and save the money for a new set of tires. On the other hand, I did have a car that had 85,000 miles on the tires when traded and were still good in good shape due to rotating the tires every 8,000 miles. Additionally, the tire monitoring system seems to be “wacky” at times. I’ll inflate the tires to 30 pounds of pressure but, in checking the monitoring system, the pressures vary, anywhere from 27 psi to 32 psi just a few days later. I do recognize that temperature does have a bearing on air pressure. Any thoughts or ideas?

A. You make a very good point considering the cost of the tire rotation. Although expensive, when the tires are rotated; in addition to the benefits of longer life, the tires also get checked for damage and uneven wear. Regarding the variation of the tire pressures, you are correct that temperature has a bearing on pressure. A 10 degree change in temperature will change the air pressure by one pound. For example, if you check the tires on your car in a 70 degree shop, drove home and the temperature dropped to 30 degrees, the tire pressure could change by four to five pounds of pressure. Add the effects of the sun on one side of the car and the pressure could easily vary a few pounds from left to right.

Gas mileage varies

Q. I’m one of those people who calculate my car’s gas mileage every time I fill the tank. I’ve noticed that with both of my last two cars (both Hondas) that when they were new, my mileage didn’t change much between summer and winter driving. But, as both cars got older, my winter mileage is substantially worse (up to 6-8 mpg less) than my summer mileage. Why does this happen? Is there something that can/should be done about it?

Tom, Barrington

A. As any vehicle starts to age fuel economy will slightly decrease. As your Honda ages, one area that is often overlooked is valve adjustment. The combination of improper valve adjustment, sensors slightly out of range, a lazy thermostat, and being slightly out of tune could certainly have a dramatic effect on fuel economy during cold weather operation.

Bucking Jimmy

Q. I own a 2000 GMC Jimmy with a 4.3L engine. When I first leave for work in the morning (after “warming up” for minimum 10 minutes) the truck wants to “buck” during acceleration. This problem only occurs after the first starting of the truck. I drive no more than 5 miles and the truck is restarted no sooner than 1 1/2 hours later and it runs great. If the first trip is a highway trip, the truck wants to “buck” on the highway. I have had the truck tuned up and have replaced the catalytic converter as well as the fuel pump and filter. My next guess is an oxygen sensor, what do you think?

A. A technician with a scan tool can check the oxygen sensor to avoid guessing. If I had to guess I would look more at a faulty mass air flow sensor or an EGR valve that is coming on a little too early.

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