Dear readers: A friend who was driving me around last week complained that the windows were taking too long to clear, so I simply pushed the button on his dash to use fresh air instead of recirculated air.

That didn’t make any sense, he said, because the defroster uses the air conditioning to dry out the air blowing onto the dash. He maintained that it would do a better job in recirculate mode than it would by letting in lots of fresh, ultrahumid air from the rainstorm through which we were driving.

About the time he finished saying that, the windscreen cleared up, almost as if by magic. So much for his theory.

On damp, rainy days, there’s probably enough moisture permeating the interior fabrics, the occupants’ clothes and the carpets to keep the humidity in the car near 100 percent for a long time. But cool, fresh air, even at 100 percent relative humidity outside the car, will have far less relative humidity once it has been heated up in the heater core and then cooled off in the A/C evaporator.

Yes, the recirculated air goes through the same heating, cooling and moisture-removal process in the heater, but it picks up plenty of moisture inside the car before it gets sucked back in for another round. That’s why some cars automatically use fresh air in defrost mode, giving you no choice.

Q: I have a 2002 Dodge Intrepid. My heater only works when my RPMs are above 2,500. I think it has to do with the pressure through the heater, but I am not sure about that — or how to fix it.

A: First off, many modern cars don’t really have good heater performance at idle, especially if you live in a chilly place like Michigan, where I’m from. The coolant flow through the heater core simply isn’t fast enough to bring in sufficient heat from the engine.

That’s because the water pump isn’t spinning very fast at idle. Pumps have been made smaller and underdriven in recent years to save gasoline — a smaller pump that isn’t spinning as fast doesn’t need as much power to move coolant through the block and radiator. Unfortunately, heat output to the heater core at idle suffers.

There really is no way to bump up the computer-controlled idle speed, aside from opening the hood and putting a nickel under the idle-stop screw. I’ve done that, if I really needed to sit in an idling car awhile when the temperature was near zero, but I don’t recommend it on a regular basis.

Has your car always had marginal heat at idle? If this is a new phenomenon, check the thermostat. It may be stuck open, or some misinformed mechanic may have replaced it with a lower-temp one to keep the engine cooler.

Or you might have a water pump with eroded blades, but that’s rare and would tend to make the car run very hot in warm weather.

Q: I recently changed the spark plugs and wires in my 1998 Mazda B4000 pickup, and now it wants to spit and sputter. I checked the gap twice before installing them. What could be causing the sputtering?

A: Dollars to doughnuts you’ve crossed two of the spark-plug wires. That’s why I always recommend that you remove only one wire at a time, change the plug and reinstall that wire before going on to the next plug.