Q My friend doesn’t run the air conditioning, insisting that the air conditioning in the car uses more gas. I was told by my dealer that the new cars use a different system and running the air doesn’t affect the gas mileage.

Also, how do you get rid of the stinky smell that comes from the vents? Please help. I’m about to melt!

I.M. Sweating-Buckets, San Jose

A Two timely questions, Ms. Buckets, as that season is upon us.

Operating the air conditioning (regardless of a new or older system) does reduce fuel economy by a small amount, as the engine must rotate a rather stiff-to-turn refrigerant compressor and generate the electricity (via the alternator) to operate the blower fan and compressor clutch.

On the other hand, driving a modern vehicle with one or more windows lowered taxes the engine to a greater degree because of disrupted aerodynamics. So if the choice is windows down or A/C, flip that switch on!

The musty odor you’ve described is a common ailment on many vehicles.

It’s caused by microbial growth on and around the air conditioning system’s evaporator core – a finned heat exchanger located within a plastic housing behind/below the glove box.

When the A/C system is operating, condensation accumulates on the evaporator and drains from the evaporator housing to the outside of the vehicle via a hose or tube.

When not in use, some condensation might remain, and when exposed to airborne pollutants, can result in fungi growth.

Here’s how to do battle with the stinky growth: Locate the housing drain (near the right-center of the engine/passenger compartment bulkhead) by looking for dripping condensation after an A/C-assisted trip.

Swab the drain clean of debris with a Q-tip or pipe cleaner. Next, operate the heating-A/C system in “vent” mode (engine running), “outside air” (not MAX) mode, “high” blower speed, temperature set to full-hot, for 15 minutes with the doors closed and windows partially lowered.

Next, follow up with a treatment of Pure Air, an over-the-counter aerosol A/C cleaner/deodorizer, available at many auto-parts stores, via the system’s fresh air inlet screen at the base of the windshield.

With the blower on high, exhaust the container into the inlet, and continue running the blower for 10 additional minutes (windows fully lowered, no occupants within).

During normal A/C use, microbial growth can be minimized by turning the A/C system to “vent” during the last mile or two of each trip as this helps dry the evaporator fins of moisture.

To really get the job done – perhaps once or twice a week – during the last moment of operation, move the temperature selector to the medium/hot (not full hot) setting, and leave it in this position when exiting the vehicle.

The heat supplied by the adjacent heater core might cook emerging fungi to extinction. This tip comes from fellow automotive instructor and A/C guru David Ames.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif.

E-mail him at under-the-hoodjuno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. He cannot make personal replies.)

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AP-NY-05-21-04 0622EDT